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Q:
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A:
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An abstract class can have instance
methods that implement a default behavior. An Interface can only declare
constants and instance methods, but cannot implement default behavior and all
methods are implicitly abstract. An interface has all public members and no
implementation. An abstract class is a class which may have the usual flavors
of class members (private, protected, etc.), but has some abstract methods.
. |
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Q:
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A:
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The purpose of garbage collection is to
identify and discard objects that are no longer needed by a program so that
their resources can be reclaimed and reused. A Java object is subject to
garbage collection when it becomes unreachable to the program in which it is
used.
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Q:
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A:
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With respect to multithreading,
synchronization is the capability to control the access of multiple threads
to shared resources. Without synchonization, it is possible for one thread to
modify a shared variable while another thread is in the process of using or
updating same shared variable. This usually leads to significant
errors.
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|
Q:
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A:
|
The thread could be implemented by using
runnable interface or by inheriting from the Thread class. The former is more
advantageous, 'cause when you are going for multiple inheritance..the only
interface can help.
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Q:
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A:
|
Pass By Reference means the passing
the address itself rather than passing the value. Passby Value
means passing a copy of the value to be passed.
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Q:
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A:
|
Map is Interface and Hashmap is class that
implements that.
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Q:
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A:
|
The HashMap class is roughly equivalent
to Hashtable, except that it is unsynchronized and permits nulls. (HashMap
allows null values as key and value whereas Hashtable doesnt allow). HashMap
does not guarantee that the order of the map will remain constant over time.
HashMap is unsynchronized and Hashtable is synchronized.
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Q:
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A:
|
Vector is synchronized whereas arraylist
is not.
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Q:
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A:
|
AWT are heavy-weight componenets. Swings
are light-weight components. Hence swing works faster than AWT.
|
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|
Q:
|
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|
A:
|
A constructor is a member function of a
class that is used to create objects of that class. It has the same name as
the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator. |
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Q:
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|
A:
|
Some of the collection classes provide
traversal of their contents via a java.util.Iterator interface. This
interface allows you to walk through a collection of objects, operating on
each object in turn. Remember when using Iterators that they contain a
snapshot of the collection at the time the Iterator was obtained; generally
it is not advisable to modify the collection itself while traversing an
Iterator.
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|
Q:
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A:
|
public : Public class is
visible in other packages, field is visible everywhere (class must be public
too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package. |
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|
Q:
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|
A:
|
Abstract class must be
extended/subclassed (to be useful). It serves as a template. A class that is
abstract may not be instantiated (ie, you may not call its constructor),
abstract class may contain static data. Any class with an abstract method is
automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated. |
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|
Q:
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|
A:
|
Static means one per class, not one for
each object no matter how many instance of a class might exist. This means
that you can use them without creating an instance of a class.Static methods
are implicitly final, because overriding is done based on the type of the
object, and static methods are attached to a class, not an object. A static
method in a superclass can be shadowed by another static method in a
subclass, as long as the original method was not declared final. However, you
can't override a static method with a nonstatic method. In other words, you
can't change a static method into an instance method in a subclass.
|
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|
Q:
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||
|
A:
|
A final class can't be extended ie., final
class may not be subclassed. A final method can't be overridden when its
class is inherited. You can't change value of a final variable (is a
constant).
|
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|
Q:
|
What if the main method is declared as
private?
|
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|
A:
|
The program compiles properly but at
runtime it will give "Main method not public." message.
|
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|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What if the static modifier is removed
from the signature of the main method?
|
|
|
A:
|
Program compiles. But at runtime throws an
error "NoSuchMethodError".
|
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[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What if I write static public void
instead of public static void?
|
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|
A:
|
Program compiles and runs properly.
|
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[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What if I do not provide the String array
as the argument to the method?
|
|
|
A:
|
Program compiles but throws a runtime
error "NoSuchMethodError".
|
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|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the first argument of the String
array in main method?
|
|
|
A:
|
The String array is empty. It does not
have any element. This is unlike C/C++ where the first element by default is
the program name.
|
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|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
If I do not provide any arguments on the
command line, then the String array of Main method will be empty or null?
|
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|
A:
|
It is empty. But not null.
|
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|
[
Received from Sandesh Sadhale]
|
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|
Q:
|
How can one prove that the array is not
null but empty using one line of code?
|
|
|
A:
|
Print args.length. It will print 0. That
means it is empty. But if it would have been null then it would have thrown a
NullPointerException on attempting to print args.length.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What environment variables do I need to
set on my machine in order to be able to run Java programs?
|
|
|
A:
|
CLASSPATH and PATH are the two variables.
|
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|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Can an application have multiple classes
having main method?
|
|
|
A:
|
Yes it is possible. While starting the
application we mention the class name to be run. The JVM will look for the
Main method only in the class whose name you have mentioned. Hence there is
not conflict amongst the multiple classes having main method.
|
|
|
|
[ Received
from Sandesh Sadhale]
|
|
|
Q:
|
Can I have multiple main methods in the
same class?
|
|
|
A:
|
No the program fails to compile. The
compiler says that the main method is already defined in the class.
|
|
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|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Do I need to import java.lang package any
time? Why ?
|
|
|
A:
|
No. It is by default loaded internally by
the JVM.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Can I import same package/class twice?
Will the JVM load the package twice at runtime?
|
|
|
A:
|
One can import the same package or same
class multiple times. Neither compiler nor JVM complains abt it. And the JVM
will internally load the class only once no matter how many times you import
the same class.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What are Checked and UnChecked Exception?
|
|
A:
|
A checked
exception is some subclass of Exception (or Exception itself), excluding
class RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. |
|
|
|
Q:
|
What is Overriding?
|
|
A:
|
When a class defines a method using the
same name, return type, and arguments as a method in its superclass, the
method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private. |
|
|
|
Q:
|
What are different types of inner
classes?
|
|
A:
|
Nested top-level classes, Member classes, Local classes,
Anonymous classes
Nested top-level classes- If you declare
a class within a class and specify the static modifier, the compiler treats
the class just like any other top-level class.
Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor. |
|
Q:
|
Are the imports checked for validity at
compile time? e.g. will the code containing an import such as java.lang.ABCD
compile?
|
|
|
A:
|
Yes the imports are checked for the
semantic validity at compile time. The code containing above line of import
will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD location: package io import java.io.ABCD; |
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Does importing a package imports the
subpackages as well? e.g. Does importing com.MyTest.* also import
com.MyTest.UnitTests.*?
|
|
|
A:
|
No you will have to import the
subpackages explicitly. Importing com.MyTest.* will import classes in the
package MyTest only. It will not import any class in any of it's subpackage.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the difference between declaring
a variable and defining a variable?
|
|
|
A:
|
In declaration we just mention the type
of the variable and it's name. We do not initialize it. But defining means
declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. |
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the default value of an object
reference declared as an instance variable?
|
|
|
A:
|
null unless we define it explicitly.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Can a top level class be private or
protected?
|
|
|
A:
|
No. A top level class can not be private
or protected. It can have either "public" or no modifier. If it
does not have a modifier it is supposed to have a default access.If a top
level class is declared as private the compiler will complain that the
"modifier private is not allowed here". This means that a top level
class can not be private. Same is the case with protected.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What type of parameter passing does Java
support?
|
|
|
A:
|
In Java the arguments are always passed
by value .
|
|
|
|
[ Update from Eki and Jyothish Venu]
|
|
|
Q:
|
Primitive data types are passed by
reference or pass by value?
|
|
|
A:
|
Primitive data types are passed by value.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Objects are passed by value or by
reference?
|
|
|
A:
|
Java only supports pass by value. With
objects, the object reference itself is passed by value and so both the
original reference and parameter copy both refer to the same object .
|
|
|
|
[ Update from Eki and Jyothish Venu]
|
|
|
Q:
|
What is serialization?
|
|
|
A:
|
Serialization is a mechanism by which you
can save the state of an object by converting it to a byte stream.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
How do I serialize an object to a file?
|
|
|
A:
|
The class whose instances are to be
serialized should implement an interface Serializable. Then you pass the
instance to the ObjectOutputStream which is connected to a fileoutputstream.
This will save the object to a file.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Which methods of Serializable interface
should I implement?
|
|
|
A:
|
The serializable interface is an empty
interface, it does not contain any methods. So we do not implement any
methods.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
How can I customize the seralization
process? i.e. how can one have a control over the serialization process?
|
|
|
A:
|
Yes it is possible to have control over
serialization process. The class should implement Externalizable interface.
This interface contains two methods namely readExternal and writeExternal.
You should implement these methods and write the logic for customizing the
serialization process.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the common usage of
serialization?
|
|
|
A:
|
Whenever an object is to be sent over the
network, objects need to be serialized. Moreover if the state of an object is
to be saved, objects need to be serilazed.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is Externalizable interface?
|
||
|
A:
|
Externalizable is an interface which
contains two methods readExternal and writeExternal. These methods give you a
control over the serialization mechanism. Thus if your class implements this
interface, you can customize the serialization process by implementing these
methods.
|
||
|
|
[
Received from Sandesh Sadhale]
|
||
|
Q:
|
When you serialize an object, what
happens to the object references included in the object?
|
|
|
A:
|
The serialization mechanism generates an
object graph for serialization. Thus it determines whether the included
object references are serializable or not. This is a recursive process. Thus
when an object is serialized, all the included objects are also serialized
alongwith the original obect.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
||
|
A:
|
One should make sure that all the
included objects are also serializable. If any of the objects is not serializable
then it throws a NotSerializableException.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
||
|
A:
|
There
are three exceptions in which serialization doesnot necessarily read and
write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state. 2. Base class fields are only hendled if the base class itself is serializable. 3. Transient fields. |
|
|
Q:
|
Does Java provide any construct to find
out the size of an object?
|
|
|
A:
|
No there is not sizeof operator in Java.
So there is not direct way to determine the size of an object directly in
Java.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Give a simplest way to find out the time
a method takes for execution without using any profiling tool?
|
|
|
A:
|
Read the system time just before the
method is invoked and immediately after method returns. Take the time
difference, which will give you the time taken by a method for execution.
To put it in
code...
long
start = System.currentTimeMillis ();
method (); long end = System.currentTimeMillis ();
System.out.println
("Time taken for execution is " + (end - start));
Remember
that if the time taken for execution is too small, it might show that it is
taking zero milliseconds for execution. Try it on a method which is big
enough, in the sense the one which is doing considerable amout of processing.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What are wrapper classes?
|
|
|
A:
|
Java provides specialized classes
corresponding to each of the primitive data types. These are called wrapper
classes. They are e.g. Integer, Character, Double etc.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Why do we need wrapper classes?
|
|
|
A:
|
It is sometimes easier to deal with
primitives as objects. Moreover most of the collection classes store objects
and not primitive data types. And also the wrapper classes provide many
utility methods also. Because of these resons we need wrapper classes. And
since we create instances of these classes we can store them in any of the
collection classes and pass them around as a collection. Also we can pass
them around as method parameters where a method expects an object.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What are checked exceptions?
|
|
|
A:
|
Checked exception are those which the
Java compiler forces you to catch. e.g. IOException are checked Exceptions.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What are runtime exceptions?
|
|
|
A:
|
Runtime exceptions are those exceptions
that are thrown at runtime because of either wrong input data or because of
wrong business logic etc. These are not checked by the compiler at compile
time.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the difference between error and
an exception?
|
|
|
A:
|
An error is an irrecoverable condition
occurring at runtime. Such as OutOfMemory error. These JVM errors and you can
not repair them at runtime. While exceptions are conditions that occur
because of bad input etc. e.g. FileNotFoundException will be thrown if the
specified file does not exist. Or a NullPointerException will take place if
you try using a null reference. In most of the cases it is possible to
recover from an exception (probably by giving user a feedback for entering
proper values etc.).
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
How to create custom exceptions?
|
|
|
A:
|
Your class should extend class Exception,
or some more specific type thereof.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
If I want an object of my class to be
thrown as an exception object, what should I do?
|
|
|
A:
|
The class should extend from Exception
class. Or you can extend your class from some more precise exception type
also.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
If my class already extends from some
other class what should I do if I want an instance of my class to be thrown
as an exception object?
|
|
|
A:
|
One can not do anytihng in this
scenarion. Because Java does not allow multiple inheritance and does not
provide any exception interface as well.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
How does an exception permeate through
the code?
|
|
|
A:
|
An unhandled exception moves up the
method stack in search of a matching When an exception is thrown from a code
which is wrapped in a try block followed by one or more catch blocks, a
search is made for matching catch block. If a matching type is found then
that block will be invoked. If a matching type is not found then the
exception moves up the method stack and reaches the caller method. Same
procedure is repeated if the caller method is included in a try catch block.
This process continues until a catch block handling the appropriate type of
exception is found. If it does not find such a block then finally the program
terminates.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What are the different ways to handle
exceptions?
|
|
|
A:
|
There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and 2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions. |
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the basic difference between the
2 approaches to exception handling.
1> try catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach? |
|
|
A:
|
In the first approach as a programmer of
the method, you urself are dealing with the exception. This is fine if you
are in a best position to decide should be done in case of an exception.
Whereas if it is not the responsibility of the method to deal with it's own
exceptions, then do not use this approach. In this case use the second
approach. In the second approach we are forcing the caller of the method to
catch the exceptions, that the method is likely to throw. This is often the approach
library creators use. They list the exception in the throws clause and we
must catch them. You will find the same approach throughout the java
libraries we use.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Is it necessary that each try block must
be followed by a catch block?
|
|
|
A:
|
It is not necessary that each try block
must be followed by a catch block. It should be followed by either a catch
block OR a finally block. And whatever exceptions are likely to be thrown
should be declared in the throws clause of the method.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
If I write return at the end of the try
block, will the finally block still execute?
|
|
|
A:
|
Yes even if you write return as the last
statement in the try block and no exception occurs, the finally block will
execute. The finally block will execute and then the control return.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
If I write System.exit (0); at the end of
the try block, will the finally block still execute?
|
|
|
A:
|
No in this case the finally block will
not execute because when you say System.exit (0); the control immediately
goes out of the program, and thus finally never executes.
|
|
|
Q:
|
How are Observer and Observable used?
|
|
|
A:
|
Objects that subclass the Observable
class maintain a list of observers. When an Observable object is updated it
invokes the update() method of each of its observers to notify the observers
that it has changed state. The Observer interface is implemented by objects
that observe Observable objects.
|
|
|
|
[Received from Venkateswara Manam]
|
|
|
Q:
|
What is synchronization and why is it
important?
|
||
|
A:
|
With respect to multithreading,
synchronization is the capability to control
the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors. |
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[
Received from Venkateswara
Manam]
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Q:
|
How does Java handle integer overflows
and underflows?
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A:
|
It uses those low order bytes of the
result that can fit into the size of the type allowed by the operation.
|
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[
Received from Venkateswara
Manam]
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|
Q:
|
Does garbage collection guarantee that a
program will not run out of memory?
|
|
|
A:
|
Garbage collection does not guarantee
that a program will not run out of memory. It is possible for programs to use
up memory resources faster than they are garbage collected. It is also
possible for programs to create objects that are not subject to garbage
collection
. |
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[
Received from Venkateswara
Manam]
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|
Q:
|
What is the difference between preemptive
scheduling and time slicing?
|
|
|
A:
|
Under preemptive scheduling, the highest
priority task executes until it enters the waiting or dead states or a higher
priority task comes into existence. Under time slicing, a task executes for a
predefined slice of time and then reenters the pool of ready tasks. The
scheduler then determines which task should execute next, based on priority
and other factors.
|
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[
Received from Venkateswara
Manam]
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Q:
|
When a thread is created and started,
what is its initial state?
|
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A:
|
A thread is in the ready state after it
has been created and started.
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[
Received from Venkateswara
Manam]
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Q:
|
What is the purpose of finalization?
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A:
|
The purpose of finalization is to give an
unreachable object the opportunity to perform any cleanup processing before
the object is garbage collected.
|
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[
Received from Venkateswara
Manam]
|
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|
Q:
|
What is the Locale class?
|
|
|
A:
|
The Locale class is used to tailor
program output to the conventions of a particular geographic, political, or
cultural region.
|
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|
[
Received from Venkateswara
Manam]
|
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|
Q:
|
What is the difference between a while
statement and a do statement?
|
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|
A:
|
A while statement checks at the beginning
of a loop to see whether the next loop iteration should occur. A do statement
checks at the end of a loop to see whether the next iteration of a loop
should occur. The do statement will always execute the body of a loop at
least once.
|
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[
Received from Venkateswara
Manam]
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|
Q:
|
What is the difference between static and
non-static variables?
|
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A:
|
A static variable is associated with the
class as a whole rather than with specific instances of a class. Non-static
variables take on unique values with each object instance.
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[
Received from Venkateswara
Manam]
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Q:
|
How are this() and super() used with
constructors?
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A:
|
This() is used to invoke a constructor of
the same class. super() is used to invoke a superclass constructor.
|
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[
Received from Venkateswara
Manam]
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|
Q:
|
What are synchronized methods and
synchronized statements?
|
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|
A:
|
Synchronized methods are methods that are
used to control access to an object. A thread only executes a synchronized
method after it has acquired the lock for the method's object or class.
Synchronized statements are similar to synchronized methods. A synchronized
statement can only be executed after a thread has acquired the lock for the
object or class referenced in the synchronized statement.
|
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[
Received from Venkateswara
Manam]
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Q:
|
What is daemon thread and which method is
used to create the daemon thread?
|
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|
A:
|
Daemon thread is a low priority thread
which runs intermittently in the back ground doing the garbage collection
operation for the java runtime system. setDaemon method is used to create a
daemon thread.
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[
Received from Shipra
Kamra]
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Q:
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Can applets communicate with each other?
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A:
|
At this point in time applets may
communicate with other applets running in the same virtual machine. If the
applets are of the same class, they can communicate via shared static
variables. If the applets are of different classes, then each will need a
reference to the same class with static variables. In any case the basic idea
is to pass the information back and forth through a static variable.
An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members. It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial. |
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[
Received from Krishna
Kumar ]
|
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|
Q:
|
What are the steps in the JDBC
connection?
|
|
|
A:
|
While making a JDBC connection we
go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\"
driver classs for that specific database\" );
Step
2 : Now create a database connection using :
Connection
con = DriverManager.getConnection(url,username,password);
Step
3: Now Create a query using :
Statement
stmt = Connection.Statement(\"select * from TABLE NAME\");
Step
4 : Exceute the query :
stmt.exceuteUpdate();
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[
Received from Shri Prakash Kunwar]
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|
Q:
|
How does a try statement determine which
catch clause should be used to handle an exception?
|
|
A:
|
When
an exception is thrown within the body of a try statement, the catch clauses
of the try statement are examined in the order in which they appear. The
first catch clause that is capable of handling the exceptionis executed. The
remaining catch clauses are ignored.
|
|
Q:
|
Can an unreachable object become
reachable again?
|
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|
A:
|
An unreachable object may become
reachable again. This can happen when the object's finalize() method is
invoked and the object performs an operation which causes it to become
accessible to reachable objects.
|
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[Received from P Rajesh]
|
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|
Q:
|
What method must be implemented by all
threads?
|
|
|
A:
|
All tasks must implement the run()
method, whether they are a subclass of Thread or implement the Runnable
interface.
|
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|
[
Received from P
Rajesh]
|
|
|
Q:
|
What are synchronized methods and
synchronized statements?
|
|
|
A:
|
Synchronized methods are methods that are
used to control access to an object. A thread only executes a synchronized
method after it has acquired the lock for the method's object or class.
Synchronized statements are similar to synchronized methods. A synchronized
statement can only be executed after a thread has acquired the lock for the
object or class referenced in the synchronized statement.
|
|
|
|
[
Received from P
Rajesh]
|
|
|
Q:
|
What is Externalizable?
|
|
|
A:
|
Externalizable is an Interface that
extends Serializable Interface. And sends data into Streams in Compressed
Format. It has two methods, writeExternal(ObjectOuput out) and
readExternal(ObjectInput in)
|
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|
[
Received from Venkateswara
Manam]
|
|
|
Q:
|
What modifiers are allowed for methods in
an Interface?
|
|
|
A:
|
Only public and abstract modifiers are
allowed for methods in interfaces.
|
|
|
|
[
Received from P
Rajesh]
|
|
|
Q:
|
What are some alternatives to
inheritance?
|
|
|
A:
|
Delegation is an alternative to
inheritance. Delegation means that you include an instance of another class
as an instance variable, and forward messages to the instance. It is often
safer than inheritance because it forces you to think about each message you
forward, because the instance is of a known class, rather than a new class,
and because it doesn't force you to accept all the methods of the super
class: you can provide only the methods that really make sense. On the other
hand, it makes you write more code, and it is harder to re-use (because it is
not a subclass).
|
|
|
|
[
Received from P
Rajesh]
|
|
|
Q:
|
What does it mean that a method or field
is "static"?
|
|
|
A:
|
Static variables and methods are
instantiated only once per class. In other words they are class variables,
not instance variables. If you change the value of a static variable in a
particular object, the value of that variable changes for all instances of
that class.
Static methods can be referenced
with the name of the class rather than the name of a particular object of the
class (though that works too). That's how library methods like
System.out.println() work out is a static field in the java.lang.System
class.
|
|
|
|
[
Received from P
Rajesh]
|
|
|
Q:
|
What is the difference between preemptive
scheduling and time slicing?
|
|
|
A:
|
Under preemptive scheduling, the highest
priority task executes until it enters the waiting or dead states or a higher
priority task comes into existence. Under time slicing, a task executes for a
predefined slice of time and then reenters the pool of ready tasks. The
scheduler then determines which task should execute next, based on priority
and other factors.
|
|
|
|
[
Received from P
Rajesh]
|
|
|
Q:
|
What is the catch or declare rule for
method declarations?
|
|
|
A:
|
If a checked exception may be thrown
within the body of a method, the method must either catch the exception or
declare it in its throws clause.
|
|
|
|
[
Received from P
Rajesh
|
|
|
Q:
|
Is Empty .java file a valid source file?
|
|
|
A:
|
Yes, an empty .java file is a perfectly
valid source file.
|
|
|
|
[Received from Sandesh Sadhale]
|
|
|
Q:
|
Can a .java file contain more than one
java classes?
|
|
|
A:
|
Yes, a .java file contain more than one
java classes, provided at the most one of them is a public class.
|
|
|
|
[ Received
from Sandesh Sadhale]
|
|
|
Q:
|
Is String a primitive data type in Java?
|
|
|
A:
|
No String is not a primitive data type in
Java, even though it is one of the most extensively used object. Strings in
Java are instances of String class defined in java.lang package.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Is main a keyword in Java?
|
|
|
A:
|
No, main is not a keyword in Java.
|
|
|
|
[
Received from Sandesh
Sadhale]
|
|
|
Q:
|
Is next a keyword in Java?
|
|
|
A:
|
No, next is not a keyword.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Is delete a keyword in Java?
|
|
|
A:
|
No, delete is not a keyword in Java. Java
does not make use of explicit destructors the way C++ does.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Is exit a keyword in Java?
|
|
|
A:
|
No. To exit a program explicitly you use
exit method in System object.
|
|
|
|
[
Received from Sandesh
Sadhale]
|
|
|
Q:
|
What happens if you dont initialize an
instance variable of any of the primitive types in Java?
|
|
|
A:
|
Java by default initializes it to the
default value for that primitive type. Thus an int will be initialized to 0,
a boolean will be initialized to false.
|
|
|
|
[
Received from Sandesh
Sadhale]
|
|
|
Q:
|
What will be the initial value of an
object reference which is defined as an instance variable?
|
|
|
A:
|
The object references are all initialized
to null in Java. However in order to do anything useful with these
references, you must set them to a valid object, else you will get
NullPointerExceptions everywhere you try to use such default initialized
references.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What are the different scopes for Java
variables?
|
|
|
A:
|
The scope of a Java variable is
determined by the context in which the variable is declared. Thus a java
variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible. 2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope. 3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance. |
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the default value of the local
variables?
|
|
|
A:
|
The local variables are not initialized
to any default value, neither primitives nor object references. If you try to
use these variables without initializing them explicitly, the java compiler
will not compile the code. It will complain abt the local varaible not being
initilized..
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
How many objects are created in the
following piece of code?
MyClass c1, c2, c3; c1 = new MyClass (); c3 = new MyClass (); |
|
|
A:
|
Only 2 objects are created, c1 and c3.
The reference c2 is only declared and not initialized.
|
|
|
|
[ Received
from Sandesh
Sadhale]
|
|
|
Q:
|
Can a public class MyClass be defined in
a source file named YourClass.java?
|
|
|
A:
|
No the source file name, if it contains a
public class, must be the same as the public class name itself with a .java
extension.
|
|
|
|
[
Received from Sandesh
Sadhale]
|
|
|
Q:
|
Can main method be declared final?
|
|
|
A:
|
Yes, the main method can be declared
final, in addition to being public static.
|
|
|
|
[
Received fromSandesh
Sadhale]
|
|
|
Q:
|
What will be the output of the following
statement?
System.out.println ("1" + 3); |
|
|
A:
|
It will print 13.
|
|
|
|
[
Received from Sandesh
Sadhale]
|
|
|
Q:
|
What will be the default values of all
the elements of an array defined as an instance variable?
|
|
|
A:
|
If the array is an array of primitive
types, then all the elements of the array will be initialized to the default
value corresponding to that primitive type. e.g. All the elements of an array
of int will be initialized to 0, while that of boolean type will be
initialized to false. Whereas if the array is an array of references (of any
type), all the elements will be initialized to null.
|
|
|
|
[
Received from Sandesh
Sadhale]
|
|
|
Q:
|
What is the Collections API?
|
|
|
A:
|
The Collections API is a set of classes
and interfaces that support operations on collections of objects.
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the List interface?
|
|
|
A:
|
The List interface provides support for
ordered collections of objects.
|
|
|
|
[
Received from SPrasanna Inamanamelluri]
|
|
|
Q:
|
What is the Vector class?
|
|
|
A:
|
The Vector class provides the capability
to implement a growable array of objects.
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is an Iterator interface?
|
|
|
A:
|
The Iterator interface is used to step
through the elements of a Collection .
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
Which java.util classes and interfaces
support event handling?
|
|
|
A:
|
The EventObject class and the
EventListener interface support event processing.
|
|
|
|
||
|
Q:
|
What is the GregorianCalendar class?
|
|
|
A:
|
The GregorianCalendar provides support
for traditional Western calendars
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the Locale class?
|
|
|
A:
|
The Locale class is used to tailor
program output to the conventions of a particular geographic, political, or
cultural region .
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the SimpleTimeZone class?
|
|
|
A:
|
The SimpleTimeZone class provides support
for a Gregorian calendar .
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the Map interface?
|
|
|
A:
|
The Map interface replaces the JDK 1.1
Dictionary class and is used associate keys with values.
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the highest-level event class of
the event-delegation model?
|
|
|
A:
|
The java.util.EventObject class is the
highest-level class in the event-delegation class hierarchy.
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the Collection interface?
|
|
|
A:
|
The Collection interface provides support
for the implementation of a mathematical bag - an unordered collection of
objects that may contain duplicates.
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the Set interface?
|
|
|
A:
|
The Set interface provides methods for
accessing the elements of a finite mathematical set. Sets do not allow
duplicate elements.
|
|
|
|
[
Received from Prasanna Inamanamelluri]
|
|
|
Q:
|
What is the typical use of Hashtable?
|
|
|
A:
|
Whenever a program wants to store a key
value pair, one can use Hashtable.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
I am trying to store an object using a
key in a Hashtable. And some other object already exists in that location,
then what will happen? The existing object will be overwritten? Or the new
object will be stored elsewhere?
|
|
|
A:
|
The existing object will be overwritten
and thus it will be lost.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
What is the difference between the size
and capacity of a Vector?
|
|
|
A:
|
The size is the number of elements
actually stored in the vector, while capacity is the maximum number of
elements it can store at a given instance of time.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Can a vector contain heterogenous
objects?
|
|
|
A:
|
Yes a Vector can contain heterogenous
objects. Because a Vector stores everything in terms of Object.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Can a ArrayList contain heterogenous
objects?
|
|
|
A:
|
Yes a ArrayList can contain heterogenous
objects. Because a ArrayList stores everything in terms of Object.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
||
|
A:
|
An enumeration is an interface containing
methods for accessing the underlying data structure from which the
enumeration is obtained. It is a construct which collection classes return
when you request a collection of all the objects stored in the collection. It
allows sequential access to all the elements stored in the collection.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Considering the basic properties of
Vector and ArrayList, where will you use Vector and where will you use
ArrayList?
|
|
|
A:
|
The basic difference between a Vector and
an ArrayList is that, vector is synchronized while ArrayList is not. Thus
whenever there is a possibility of multiple threads accessing the same
instance, one should use Vector. While if not multiple threads are going to
access the same instance then use ArrayList. Non synchronized data structure
will give better performance than the synchronized one.
|
|
|
|
[
Received from Sandesh Sadhale]
|
|
|
Q:
|
Can a vector contain heterogenous
objects?
|
|
A:
|
Yes a Vector can contain heterogenous
objects. Because a Vector stores everything in terms of Object.
|
|
Java Interview Questions site discussing
core java IT technical interview questions in detail. These are some of the
java job interview questions me and my friends have faced regularly in campus
interviews and off campuses. I have consolidated all of them from different
people and communities into a vast resource of core java interview questions
all in one place. So I am giving you a chance to prepare well by going
through each of the java interview faqs organized by java topics, before
attending a technical interview on java.
|
The program compiles properly but at runtime it will give "Main method not public." message. What is meant by pass by reference and pass by value in Java? Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value. If you’re overriding the method equals() of an object, which other method you might also consider? hashCode() What is Byte Code? Or What gives java it’s “write once and run anywhere” nature? All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent. Expain the reason for each keyword of public static void main(String args[])? public- main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public. static: Java environment should be able to call this method without creating an instance of the class , so this method must be declared as static. void: main does not return anything so the return type must be void The argument String indicates the argument type which is given at the command line and arg is an array for string given during command line. What are the differences between == and .equals() ? Or what is difference between == and equals Or Difference between == and equals method Or What would you use to compare two String variables - the operator == or the method equals()? Or How is it possible for two String objects with identical values not to be equal under the == operator? The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory. == compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal. public class EqualsTest{
public static void main(String[] args){
String s1 = "abc";
String s2 = s1;
String s5 = "abc";
String s3 = new String("abc");
String s4 = new String("abc");
// if we remove the brackets around "s1 == s5' it gives a different result.
System.out.println("== comparison : " +(s1 == s5));
System.out.println("== comparison : " +(s1 == s2));
System.out.println("Using equals method : " +s1.equals(s2));
System.out.println("== comparison : " +s3 == s4);
System.out.println("Using equals method : " +s3.equals(s4));
}
}
Output
== comparison : true
== comparison : true
Using equals method : true
false
Using equals method : true
What if the static modifier is removed from the signature of the main method?Or What if I do not provide the String array as the argument to the method? Program compiles. But at runtime throws an error "NoSuchMethodError". Why oracle Type 4 driver is named as oracle thin driver? Oracle provides a Type 4 JDBC driver, referred to as the Oracle “thin” driver. This driver includes its own implementation of a TCP/IP version of Oracle’s Net8 written entirely in Java, so it is platform independent, can be downloaded to a browser at runtime, and does not require any Oracle software on the client side. This driver requires a TCP/IP listener on the server side, and the client connection string uses the TCP/IP port address, not the TNSNAMES entry for the database name. What is the difference between final, finally and finalize? What do you understand by the java final keyword? Or What is final, finalize() and finally? Or What is finalize() method? Or What is the difference between final, finally and finalize? Or What does it mean that a class or member is final? o final - declare constant o finally - handles exception o finalize - helps in garbage collection Variables defined in an interface are implicitly final. A final class can't be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method. What is the Java API? The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets. What is the GregorianCalendar class? The GregorianCalendar provides support for traditional Western calendars. What is the ResourceBundle class? The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run. Why there are no global variables in Java? Global variables are globally accessible. Java does not support globally accessible variables due to following reasons: * The global variables breaks the referential transparency * Global variables creates collisions in namespace. How to convert String to Number in java program? The valueOf() function of Integer class is is used to convert string to Number. Here is the code example: String numString = "1000"; int id=Integer.valueOf(numString); What is the SimpleTimeZone class? The SimpleTimeZone class provides support for a Gregorian calendar. What is the difference between a while statement and a do statement? A while statement (pre test) checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement (post test) checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the loop body at least once. What is the Locale class? The Locale class is used to tailor a program output to the conventions of a particular geographic, political, or cultural region. Describe the principles of OOPS. There are three main principals of oops which are called Abstraction, Polymorphism, Inheritance and Encapsulation. Explain the Inheritance principle. Inheritance is the process by which one object acquires the properties of another object. Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places What is implicit casting? Implicit casting is the process of simply assigning one entity to another without any transformation guidance to the compiler. This type of casting is not permitted in all kinds of transformations and may not work for all scenarios. Example int i = 1000; long j = i; //Implicit casting Is sizeof a keyword in java? The sizeof operator is not a keyword. What is a native method? A native method is a method that is implemented in a language other than Java. In System.out.println(), what is System, out and println? System is a predefined final class, out is a PrintStream object and println is a built-in overloaded method in the out object. What are Encapsulation, Inheritance and Polymorphism Or Explain the Polymorphism principle. Explain the different forms of Polymorphism. Polymorphism in simple terms means one name many forms. polymorphism refers to a programming language's ability to process objects differently depending on their data type or class. More specifically, it is the ability to redefine methods for derived classes. For example, given a base class shape, polymorphism enables the programmer to define different area methods for any number of derived classes, such as circles, rectangles and triangles Polymorphism exists in three distinct forms in Java: • Method overloading • Method overriding through inheritance • Method overriding through the Java interface What is explicit casting? Explicit casting in the process in which the complier are specifically informed to about transforming the object. Example long i = 700.20; int j = (int) i; //Explicit casting What is the Java Virtual Machine (JVM)? The Java Virtual Machine is software that can be ported onto various hardware-based platforms What do you understand by downcasting? The process of Downcasting refers to the casting from a general to a more specific type, i.e. casting down the hierarchy What are Java Access Specifiers? Or What is the difference between public, private, protected and default Access Specifiers? Or What are different types of access modifiers? Access specifiers are keywords that determine the type of access to the member of a class. These keywords are for allowing privileges to parts of a program such as functions and variables. These are: • Public: accessible to all classes • Protected: accessible to the classes within the same package and any subclasses. • Private: accessible only to the class to which they belong • Default: accessible to the class to which they belong and to subclasses within the same package Which class is the superclass of every class? Object. Name primitive Java types. The 8 primitive types are byte, char, short, int, long, float, double, and boolean. What is the difference between static and non-static variables? Or What are class variables? Or What is static in java? Or What is a static method? A static variable is associated with the class as a whole rather than with specific instances of a class. Each object will share a common copy of the static variables i.e. there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class. These are declared outside a class and stored in static memory. Class variables are mostly used for constants. Static variables are always called by the class name. This variable is created when the program starts and gets destroyed when the programs stops. The scope of the class variable is same an instance variable. Its initial value is same as instance variable and gets a default value when its not initialized corresponding to the data type. Similarly, a static method is a method that belongs to the class rather than any object of the class and doesn't apply to an object or even require that any objects of the class have been instantiated. Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a non-static method. In other words, you can't change a static method into an instance method in a subclass. Non-static variables take on unique values with each object instance. What is the difference between the boolean & operator and the && operator? If an expression involving the boolean & operator is evaluated, both operands are evaluated, whereas the && operator is a short cut operator. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. If the first operand evaluates to false, the evaluation of the second operand is skipped. How does Java handle integer overflows and underflows? It uses those low order bytes of the result that can fit into the size of the type allowed by the operation. What if I write static public void instead of public static void? Program compiles and runs properly. What is the difference between declaring a variable and defining a variable? In declaration we only mention the type of the variable and its name without initializing it. Defining means declaration + initialization. E.g. String s; is just a declaration while String s = new String ("bob"); Or String s = "bob"; are both definitions. What type of parameter passing does Java support? In Java the arguments (primitives and objects) are always passed by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object. Explain the Encapsulation principle. Encapsulation is a process of binding or wrapping or hiding all the internal details of an object from the outside world and hence keeps the data safe from outside interface and misuse. Objects allow procedures to be encapsulated with their data to reduce potential interference. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper. What do you understand by a variable? Variable is a named memory location that can be easily referred in the program. The variable is used to hold the data and it can be changed during the course of the execution of the program. What do you understand by numeric promotion? The Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integral and floating-point operations may take place. In the numerical promotion process the byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required. What do you understand by casting in java language? What are the types of casting? The process of converting one data type to another is called Casting. There are two types of casting in Java; these are implicit casting and explicit casting. What is the first argument of the String array in main method? The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. If we do not provide any arguments on the command line, then the String array of main method will be empty but not null. How can one prove that the array is not null but empty? Print array.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print array.length. Can an application have multiple classes having main method? Yes. While starting the application we mention the class name to be run. The JVM will look for the main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
When is static
variable loaded? Is it at compile time or runtime? When exactly a
static block is loaded in Java?
Static variable are loaded when classloader brings the class
to the JVM. It is not necessary that an object has to be created. Static
variables will be allocated memory space when they have been loaded. The code
in a static block is loaded/executed only once i.e. when the class is first
initialized. A class can have any number of static blocks. Static block is
not member of a class, they do not have a return statement and they cannot be
called directly. Cannot contain this or super. They are primarily used to
initialize static fields.
Can I have multiple main methods in the same class?No the program fails to compile. The compiler says that the main method is already defined in the class.
Explain working
of Java Virtual Machine (JVM)?
JVM is an
abstract computing machine like any other real computing machine which first
converts .java file into .class file by using Compiler (.class is nothing but
byte code file.) and Interpreter reads byte codes.
How can I swap
two variables without using a third variable?
Add two variables
and assign the value into First variable. Subtract the Second value with the
result Value. and assign to Second variable. Subtract the Result of First
Variable With Result of Second Variable and Assign to First Variable.
Example:
int
a=5,b=10;a=a+b; b=a-b; a=a-b;
What is data
encapsulation?
Encapsulation may
be used by creating 'get' and 'set' methods in a class (JAVABEAN) which are
used to access the fields of the object. Typically the fields are made
private while the get and set methods are public. Encapsulation can be used
to validate the data that is to be stored, to do calculations on data that is
stored in a field or fields, or for use in introspection (often the case when
using javabeans in Struts, for instance). Wrapping of data and function into
a single unit is called as data encapsulation. Encapsulation is nothing but
wrapping up the data and associated methods into a single unit in such a way
that data can be accessed with the help of associated methods.
Encapsulation provides data security. It is nothing but data hiding.
What is reflection
API? How are they implemented?
Reflection is the
process of introspecting the features and state of a class at runtime and
dynamically manipulate at run time. This is supported using Reflection API
with built-in classes like Class, Method, Fields, Constructors etc. Example:
Using Java Reflection API we can get the class name, by using the getName
method.
Does JVM maintain
a cache by itself? Does the JVM allocate objects in heap? Is this the OS heap
or the heap maintained by the JVM? Why
Yes, the JVM maintains
a cache by itself. It creates the Objects on the HEAP, but references to
those objects are on the STACK.
What is phantom
memory?
Phantom memory is
false memory. Memory that does not exist in reality.
Can a method be
static and synchronized?
A static method
can be synchronized. If you do so, the JVM will obtain a lock on the
java.lang.Class instance associated with the object. It is similar to saying:
synchronized(XYZ.class) {
}
What is
difference between String and StringTokenizer?
A StringTokenizer
is utility class used to break up string.
Example:
StringTokenizer
st = new StringTokenizer("Hello World");
while (st.hasMoreTokens()) {
System.out.println(st.nextToken());
}
Output:
Hello
World
|
|
Java Package's
Interview Questions
|
Do I need to import java.lang package any time? Why?
Or
Which package is always imported by default?
No. It is by default loaded internally by the JVM. The java.lang package is always imported by default.
Can I import same package/class twice? Will the JVM load the package twice at runtime?
No. It is by default loaded internally by the JVM. The java.lang package is always imported by default.
Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple
times. Neither compiler nor JVM complains anything about it. And the JVM will
internally load the class only once no matter how many times you import the
same class.
Does importing a package imports the sub packages as
well? E.g. Does importing com.bob.* also import com.bob.code.*?
No you will have to import the sub packages explicitly. Importing com.bob.* will import classes in the package bob only. It will not import any class in any of its sub package’s.
No you will have to import the sub packages explicitly. Importing com.bob.* will import classes in the package bob only. It will not import any class in any of its sub package’s.
What is a Java package and how is it used?
Or
Explain the usage of Java packages.
A Java package is a naming context for classes and
interfaces. A package is used to create a separate name space for groups of
classes and interfaces. Packages are also used to organize related classes and
interfaces into a single API unit and to control accessibility to these classes
and interfaces.
For example: The Java API is grouped into libraries of related classes and interfaces; these libraries are known as package.
Are the imports checked for validity at compile time? e.g. will
the code containing an import such as java.lang.BOB compile?For example: The Java API is grouped into libraries of related classes and interfaces; these libraries are known as package.
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying, cannot resolve symbol.
What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
What is the difference between String and StringBuffer?
String objects are immutable whereas StringBuffer objects
are not. StringBuffer unlike Strings support growable and modifiable strings.
Can a private method of a superclass be declared within a
subclass?
Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features.
Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding (polymorphism) features.
What is the default value of an object reference declared
as an instance variable?
null unless we define it explicitly.
null unless we define it explicitly.
What is the difference between a constructor and a
method?
Or
How can a subclass call a method or a
constructor defined in a superclass?
A constructor is a member function of a class that is used to create objects of that class, invoked using the new operator. It has the same name as the class and has no return type. They are only called once, whereas member functions can be called many times. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator. Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
super.method(); is used to call a super class method from
a sub class. To call a constructor of the super class, we use the super();
statement as the first line of the subclass’s constructor.
Can a top-level class be private or protected?
No. A top-level class cannot be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access. If a top level class is declared as private/protected the compiler will complain that the "modifier private is not allowed here”.
Why Java does not support multiple inheritance?
Java does support multiple inheritance via interface implementation.
Where and how can you use a private constructor?
Private constructor can be used if you do not want any other class to instantiate the class. This concept is generally used in Singleton Design Pattern. The instantiation of such classes is done from a static public method.
How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
What is Method Overriding? What restrictions are placed on method overriding?
When a class defines a method using the same name, return type, and argument list as that of a method in its superclass, the method in the subclass is said to override the method present in the Superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass.
Restrictions placed on method overriding
• Overridden methods must have the same name, argument list, and return type.
• The overriding method may not limit the access of the method it overrides. Methods may be overridden to be more public, not more private.
• The overriding method may not throw any exceptions that may not be thrown by the overridden method.
No. A top-level class cannot be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access. If a top level class is declared as private/protected the compiler will complain that the "modifier private is not allowed here”.
Why Java does not support multiple inheritance?
Java does support multiple inheritance via interface implementation.
Where and how can you use a private constructor?
Private constructor can be used if you do not want any other class to instantiate the class. This concept is generally used in Singleton Design Pattern. The instantiation of such classes is done from a static public method.
How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
What is Method Overriding? What restrictions are placed on method overriding?
When a class defines a method using the same name, return type, and argument list as that of a method in its superclass, the method in the subclass is said to override the method present in the Superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass.
Restrictions placed on method overriding
• Overridden methods must have the same name, argument list, and return type.
• The overriding method may not limit the access of the method it overrides. Methods may be overridden to be more public, not more private.
• The overriding method may not throw any exceptions that may not be thrown by the overridden method.
What are the Object and Class classes used for? Which
class should you use to obtain design information about an object?
Differentiate between a Class and an Object?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program. The Class class is used to obtain information about an object's design. A Class is only a definition or prototype of real life object. Whereas an object is an instance or living representation of real life object. Every object belongs to a class and every class contains one or more related objects.
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program. The Class class is used to obtain information about an object's design. A Class is only a definition or prototype of real life object. Whereas an object is an instance or living representation of real life object. Every object belongs to a class and every class contains one or more related objects.
What is a singleton class?
Or
What is singleton pattern?
This design pattern is used by an application to ensure that at any time there is only one instance of a class created. You can achieve this by having the private constructor in the class and having a getter method which returns an object of the class and creates one for the first time if its null.
Or
What is singleton pattern?
This design pattern is used by an application to ensure that at any time there is only one instance of a class created. You can achieve this by having the private constructor in the class and having a getter method which returns an object of the class and creates one for the first time if its null.
What is method overloading and method overriding?
Or
What is difference between overloading and overriding?
Method overloading: When 2 or more methods in a class have the same method names with different arguments, it is said to be method overloading. Overloading does not block inheritance from the superclass. Overloaded methods must have different method signatures
Method overriding : When a method in a class has the same method name with same arguments as that of the superclass, it is said to be method overriding. Overriding blocks inheritance from the superclass. Overridden methods must have same signature.
Or
What is difference between overloading and overriding?
Method overloading: When 2 or more methods in a class have the same method names with different arguments, it is said to be method overloading. Overloading does not block inheritance from the superclass. Overloaded methods must have different method signatures
Method overriding : When a method in a class has the same method name with same arguments as that of the superclass, it is said to be method overriding. Overriding blocks inheritance from the superclass. Overridden methods must have same signature.
Basically overloading and overriding are different
aspects of polymorphism.
static/early binding polymorphism: overloading
dynamic/late binding polymorphism: overriding
static/early binding polymorphism: overloading
dynamic/late binding polymorphism: overriding
If a class is declared without any access modifiers,
where may the class be accessed?
A class that is declared without any access modifiers is
said to have package or default access. This means that the class can only be
accessed by other classes and interfaces that are defined within the same
package.
Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its super
classes.
Which java.util classes and interfaces support event
handling?
The EventObject class and the EventListener interface support event processing
Can an object's finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.
The EventObject class and the EventListener interface support event processing
Can an object's finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object's finalize() method may be invoked by other objects.
What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
It returns the runtime information like memory availability.
* Runtime.freeMemory() --> Returns JVM Free Memory
* Runtime.maxMemory() --> Returns the maximum amount of memory that the JVM will attempt to use. It also helps to run the garbage collector
* Runtime.gc()
The purpose of the Runtime class is to provide access to the Java runtime system.
It returns the runtime information like memory availability.
* Runtime.freeMemory() --> Returns JVM Free Memory
* Runtime.maxMemory() --> Returns the maximum amount of memory that the JVM will attempt to use. It also helps to run the garbage collector
* Runtime.gc()
What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
The purpose of the System class is to provide access to system resources.
Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable object.
What is a bean? Where can it be used?
A Bean is a reusable and self-contained software component. Beans created using java take advantage of all the security and
platform independent features of java. Bean can be plugged into any software application. Bean is a simple class which has set
and get methods. It could be used within a JSP using JSP tags to use them.
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable object.
What is a bean? Where can it be used?
A Bean is a reusable and self-contained software component. Beans created using java take advantage of all the security and
platform independent features of java. Bean can be plugged into any software application. Bean is a simple class which has set
and get methods. It could be used within a JSP using JSP tags to use them.
What is the functionality of instanceOf() ?
instanceOf opertaor is used to check whether an object
can be cast to a specific type without throwing ClassCastException.
What would
happen if you say this = null?
It will come up with Error Message
"The left-hand side of an
assignment must be a variable".
I want to
create two instances of a class ,But when trying for creating third instance it
should not allow me to create . What i have to do for making this?
One way of doing this would be:
public class test1
{
static int cntr=0;
test1()
{ cntr++;
if(cntr>2)
throw new
NullPointerException();//u can define a new exception // for this
}
public static void main(String
args[])
{
test1 t1= new test1();
System.out.println("hello
1");
test1 t2= new test1();
System.out.println("hello
2");
test1 t3= new test1();
}
}
What is the
difference between an object and an instance?
An Object May not have a class
definition. eg int a[] where a is an array.
An Instance should have a class
definition.
eg MyClass my=new MyClass();
my is an instance.
What is
heap in Java?
It is a memory area which stores all
the objects created by an executing program.
Why default
constructor of base class will be called first in java?
A subclass inherits all the methods
and fields (eligible one) from the base class, so base class is constructed in
the process of creation of subclass object (subclass is also an object of
superclass). Hence before initializing the default value of sub class the super
class should be initialized using the default constructor.
What are
the other ways to create an object other than creating as new object?
We can create object in different
ways;
1.new operator
2.class.forName: Classname obj =
Class.forName("Fully Qualified class Name").newInstance();
3.newInstance
4.object.clone
What is the
difference between instance, object, reference and a class?
Class: A class is a user defined data type
with set of data members & member functions
Object: An Object is an instance of a class
Reference: A reference is just like a pointer
pointing to an object
Instance: This represents the values of data
members of a class at a particular time
|
Java Garbage
Collection Interview Questions
|
Explain garbage collection?
Or How you can force the garbage collection?
Or What is the purpose of garbage collection in Java, and
when is it used?
Or What is Garbage Collection and how to call it explicitly?
Or Explain Garbage collection mechanism in Java?
Garbage collection is one of the most important features of Java. The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use.
In Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. Garbage collection is an automatic process and can't be forced. There is no guarantee that Garbage collection will start immediately upon request of System.gc().
Garbage collection is one of the most important features of Java. The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use.
In Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. Garbage collection is an automatic process and can't be forced. There is no guarantee that Garbage collection will start immediately upon request of System.gc().
What kind of thread is the Garbage collector thread?
It is a daemon thread.
Can an object’s finalize() method be invoked while it is
reachable?
An object’s finalize() method cannot be invoked by the
garbage collector while the object is still reachable. However, an object’s
finalize() method may be invoked by other objects.
Does garbage collection guarantee that a program will not
run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
What is the purpose of finalization?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
What is the purpose of finalization?
The purpose of finalization is to give an unreachable
object the opportunity to perform any cleanup, before the object gets garbage
collected. For example, closing an opened database Connection.
If an object is garbage collected, can it become
reachable again?
Once an object is garbage collected, It can no longer
become reachable again.
|
Java Object
Serialization Interview Questions
|
How many methods in the Serializable interface? Which
methods of Serializable interface should I implement?
There is no method in the Serializable interface. It’s an empty interface which does not contain any methods. The Serializable interface acts as a marker, telling the object serialization tools that the class is serializable. So we do not implement any methods.
There is no method in the Serializable interface. It’s an empty interface which does not contain any methods. The Serializable interface acts as a marker, telling the object serialization tools that the class is serializable. So we do not implement any methods.
What is the difference between
Serializalble and Externalizable interface? How can you control over the
serialization process i.e. how can you customize the seralization process?
When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject() two methods to control more complex object serailization process. When you use Externalizable interface, you have a complete control over your class's serialization process. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject() two methods to control more complex object serailization process. When you use Externalizable interface, you have a complete control over your class's serialization process. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
How to make a class or a bean serializable? How do I
serialize an object to a file?
Or What interface must an object implement before it can be
written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object. The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object. The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
What happens to the object references included in the
object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original object.
What is serialization?
The serialization is a kind of mechanism that makes a class or a bean persistent by having its properties or fields and state information saved and restored to and from storage. That is, it is a mechanism with which you can save the state of an object by converting it to a byte stream.
Common Usage of serialization.
Whenever an object is to be sent over the network or saved in a file, objects are serialized.
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original object.
What is serialization?
The serialization is a kind of mechanism that makes a class or a bean persistent by having its properties or fields and state information saved and restored to and from storage. That is, it is a mechanism with which you can save the state of an object by converting it to a byte stream.
Common Usage of serialization.
Whenever an object is to be sent over the network or saved in a file, objects are serialized.
What happens to the static fields of a class during
serialization?
There are three exceptions in which serialization doesn’t necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of any particular state.
2. Base class fields are only handled if the base class itself is serializable.
3. Transient fields.
There are three exceptions in which serialization doesn’t necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of any particular state.
2. Base class fields are only handled if the base class itself is serializable.
3. Transient fields.
What one should take care of while serializing the
object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
What is a transient variable?
Or Explain the usage of the keyword transient?
Or What are Transient and Volatile Modifiers
A transient variable is a variable that may not be serialized i.e. the value of the variable can’t be written to the stream in a Serializable class. If you don't want some field to be serialized, you can mark that field transient or static. In such a case when the class is retrieved from the ObjectStream the value of the variable is null.
A transient variable is a variable that may not be serialized i.e. the value of the variable can’t be written to the stream in a Serializable class. If you don't want some field to be serialized, you can mark that field transient or static. In such a case when the class is retrieved from the ObjectStream the value of the variable is null.
Volatile modifier applies
to variables only and it tells the compiler that the variable modified by
volatile can be changed unexpectedly by other parts of the program.
What is Serialization and deserialization?
Serialization is the process of writing the state of an
object to a byte stream. Deserialization is the process of restoring these
objects.
What is Externalizable?
Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
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Java Collections
Interview Questions
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What is HashMap and Map?
Map is Interface and Hashmap is class that implements this interface.
Map is Interface and Hashmap is class that implements this interface.
What is the significance of ListIterator?
Or What is the difference
b/w Iterator and ListIterator?
Iterator : Enables you to cycle through a collection in the
forward direction only, for obtaining or removing elements
ListIterator :It extends Iterator, allow bidirectional traversal of
list and the modification of elements
Difference between HashMap and
HashTable? Can we make hashmap synchronized?
1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).
2. HashMap does not guarantee that the order of the map will remain constant over time.
3. HashMap is non synchronized whereas Hashtable is synchronized.
4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
Note on Some Important Terms
1)Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.
2)Fail-safe is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke "set" method since it doesn’t modify the collection "structurally”. However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.
1. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).
2. HashMap does not guarantee that the order of the map will remain constant over time.
3. HashMap is non synchronized whereas Hashtable is synchronized.
4. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
Note on Some Important Terms
1)Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.
2)Fail-safe is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke "set" method since it doesn’t modify the collection "structurally”. However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.
HashMap can be synchronized by
Map m = Collections.synchronizeMap(hashMap);
What is the difference between set and list?
A Set stores elements in an unordered way and does not contain duplicate elements, whereas a list stores elements in an ordered way but may contain duplicate elements.
A Set stores elements in an unordered way and does not contain duplicate elements, whereas a list stores elements in an ordered way but may contain duplicate elements.
Difference between Vector and ArrayList? What is the
Vector class?
Vector is synchronized whereas ArrayList is not. The Vector class provides the capability to implement a growable array of objects. ArrayList and Vector class both implement the List interface. Both classes are implemented using dynamically resizable arrays, providing fast random access and fast traversal. In vector the data is retrieved using the elementAt() method while in ArrayList, it is done using the get() method. ArrayList has no default size while vector has a default size of 10. when you want programs to run in multithreading environment then use concept of vector because it is synchronized. But ArrayList is not synchronized so, avoid use of it in a multithreading environment.
What is an Iterator interface? Is Iterator a Class or Interface? What is its use?
The Iterator is an interface, used to traverse through the elements of a Collection. It is not advisable to modify the collection itself while traversing an Iterator.
What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.
What is the List interface?
The List interface provides support for ordered collections of objects.
How can we access elements of a collection?
We can access the elements of a collection using the following ways:
1.Every collection object has get(index) method to get the element of the object. This method will return Object.
2.Collection provide Enumeration or Iterator object so that we can get the objects of a collection one by one.
What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
Vector is synchronized whereas ArrayList is not. The Vector class provides the capability to implement a growable array of objects. ArrayList and Vector class both implement the List interface. Both classes are implemented using dynamically resizable arrays, providing fast random access and fast traversal. In vector the data is retrieved using the elementAt() method while in ArrayList, it is done using the get() method. ArrayList has no default size while vector has a default size of 10. when you want programs to run in multithreading environment then use concept of vector because it is synchronized. But ArrayList is not synchronized so, avoid use of it in a multithreading environment.
What is an Iterator interface? Is Iterator a Class or Interface? What is its use?
The Iterator is an interface, used to traverse through the elements of a Collection. It is not advisable to modify the collection itself while traversing an Iterator.
What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.
What is the List interface?
The List interface provides support for ordered collections of objects.
How can we access elements of a collection?
We can access the elements of a collection using the following ways:
1.Every collection object has get(index) method to get the element of the object. This method will return Object.
2.Collection provide Enumeration or Iterator object so that we can get the objects of a collection one by one.
What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
What’s the difference between a queue and a stack?
Stack is a data structure that is based on last-in-first-out rule (LIFO), while queues are based on First-in-first-out (FIFO) rule.
What is the Map interface?
The Map interface is used associate keys with values.
Stack is a data structure that is based on last-in-first-out rule (LIFO), while queues are based on First-in-first-out (FIFO) rule.
What is the Map interface?
The Map interface is used associate keys with values.
What is the Properties class?
The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.
The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.
Which implementation of the List interface provides for
the fastest insertion of a new element into the middle of the list?
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a.
|
Vector
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b.
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ArrayList
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c.
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LinkedList
|
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d.
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None of the above
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ArrayList and Vector both use an array to store the
elements of the list. When an element is inserted into the middle of the list
the elements that follow the insertion point must be shifted to make room for
the new element. The LinkedList is implemented using a doubly linked list; an
insertion requires only the updating of the links at the point of insertion.
Therefore, the LinkedList allows for fast insertions and deletions.
How can we use hashset in
collection interface?
This class implements the set interface, backed by a hash
table (actually a HashMap instance). It makes no guarantees as to the iteration
order of the set; in particular, it does not guarantee that the order will
remain constant over time. This class permits the Null element.
This class offers constant time performance for the basic
operations (add, remove, contains and size), assuming the hash function
disperses the elements properly among the buckets.
What are differences between
Enumeration, ArrayList, Hashtable and Collections and Collection?
Enumeration: It is
series of elements. It can be use to enumerate through the elements of a
vector, keys or values of a hashtable. You can not remove elements from
Enumeration.
ArrayList: It is
re-sizable array implementation. Belongs to 'List' group in collection. It
permits all elements, including null. It is not thread -safe.
Hashtable: It maps
key to value. You can use non-null value for key or value. It is part of group
Map in collection.
Collections: It
implements Polymorphic algorithms which operate on collections.
Collection: It is
the root interface in the collection hierarchy.
What is difference between
array & arraylist?
An ArrayList is resizable, where as, an array is not.
ArrayList is a part of the Collection Framework. We can store any type of
objects, and we can deal with only objects. It is growable. Array is collection
of similar data items. We can have array of primitives or objects. It is of
fixed size. We can have multi dimensional arrays.
Array: can store primitive ArrayList: Stores object only
Array: fix
size
ArrayList: resizable
Array: can have multi dimensional
Array:
lang
ArrayList: Collection framework
Can you limit the initial
capacity of vector in java?
Yes you can limit the initial capacity. We can construct
an empty vector with specified initial capacity
public vector(int initialcapacity)
What method should the key
class of Hashmap override?
The methods to override are equals() and hashCode().
What is the difference between
Enumeration and Iterator?
The functionality of Enumeration interface is duplicated
by the Iterator interface. Iterator has a remove() method while Enumeration
doesn't. Enumeration acts as Read-only interface, because it has the methods
only to traverse and fetch the objects, where as using Iterator we can
manipulate the objects also like adding and removing the objects.
So Enumeration is used when ever we want to make
Collection objects as Read-only.
Java Abstract Class and Interface Interview Questions |
Or When should you use an abstract class, when an interface, when both?
Or What is similarities/difference between an Abstract class and Interface?
Or What is the difference between interface and an abstract class?
1. Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. An abstract class can contain no abstract methods also i.e. abstract class may contain concrete methods. A Java Interface can contain only method declarations and public static final constants and doesn't contain their implementation. The classes which implement the Interface must provide the method definition for all the methods present.
2. Abstract class definition begins with the keyword "abstract" keyword followed by Class definition. An Interface definition begins with the keyword "interface".
3. Abstract classes are useful in a situation when some general methods should be implemented and specialization behavior should be implemented by subclasses. Interfaces are useful in a situation when all its properties need to be implemented by subclasses
4. All variables in an Interface are by default - public static final while an abstract class can have instance variables.
5. An interface is also used in situations when a class needs to extend an other class apart from the abstract class. In such situations its not possible to have multiple inheritance of classes. An interface on the other hand can be used when it is required to implement one or more interfaces. Abstract class does not support Multiple Inheritance whereas an Interface supports multiple Inheritance.
6. An Interface can only have public members whereas an abstract class can contain private as well as protected members.
7. A class implementing an interface must implement all of the methods defined in the interface, while a class extending an abstract class need not implement any of the methods defined in the abstract class.
8. The problem with an interface is, if you want to add a new feature (method) in its contract, then you MUST implement those method in all of the classes which implement that interface. However, in the case of an abstract class, the method can be simply implemented in the abstract class and the same can be called by its subclass
9. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast
10.Interfaces are often used to describe the peripheral abilities of a class, and not its central identity, E.g. an Automobile class might implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.
Note: There is no difference between a fully abstract class (all methods declared as abstract and all fields are public static final) and an interface.
Note: If the various objects are all of-a-kind, and share a common state and behavior, then tend towards a common base class. If all they share is a set of method signatures, then tend towards an interface.
Similarities:
Neither Abstract classes nor Interface can be instantiated.
What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Only its subclasses can be instantiated. A class that has one or more abstract methods must be declared abstract. A subclass that does not provide an implementation for its inherited abstract methods must also be declared abstract. You indicate that a class is abstract with the abstract keyword like this:
public abstract class AbstractClass
Abstract classes may contain abstract methods. A method declared abstract is not actually implemented in the class. It exists only to be overridden in subclasses. Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or itself be declared abstract. Only the method’s prototype is provided in the class definition. Also, a final method can not be abstract and vice versa. Methods specified in an interface are implicitly abstract.
. It has no body. For example,
public abstract float getInfo()
What must a class do to implement an interface?
The class must provide all of the methods in the interface and identify the interface in its implements clause.
What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
What is interface? How to support multiple inhertance in Java?
Or What is a cloneable interface and how many methods does it contain?
An Interface are implicitly abstract and public. Interfaces with empty bodies are called marker interfaces having certain property or behavior. Examples: java.lang.Cloneable,java.io.Serializable,java.util.EventListener. An interface body can contain constant declarations, method prototype declarations, nested class declarations, and nested interface declarations.
Interfaces provide support for multiple inheritance in Java. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Example of Interface:
public interface sampleInterface {
public void functionOne();
public long CONSTANT_ONE = 1000;
}
What is an abstract class?
Or Can you make an instance of an abstract class?
Abstract classes can contain abstract and concrete methods. Abstract classes cannot be instantiated directly i.e. we cannot call the constructor of an abstract class directly nor we can create an instance of an abstract class by using
“Class.forName().newInstance()”
(Here we get java.lang.InstantiationException).
However, if we create an instance of a class that extends an Abstract class, compiler will initialize both the classes. Here compiler will implicitly call the constructor of the Abstract class. Any class that contain an abstract method must be declared “abstract” and abstract methods can have definitions only in child classes. By overriding and customizing the abstract methods in more than one subclass makes “Polymorphism” and through Inheritance we define body to the abstract methods.
Basically an abstract class serves as a template. Abstract class must be extended/subclassed for it to be implemented. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated. Abstract class is a class that provides some general functionality but leaves specific implementation to its inheriting classes.
Example of Abstract class:
abstract class AbstractClassExample{
protected String name;
public String getname() {
return name;
}
public abstract void function();
}
Example: Vehicle is an abstract class and Bus Truck, car etc are specific implementations
No! You cannot make an instance of an abstract class. An abstract class has to be sub-classed. If you have an abstract class and you want to use a method which has been implemented, you may need to subclass that abstract class, instantiate your subclass and then call that method.
What is meant by "Abstract Interface"?
Firstly, an interface is abstract. That means you cannot have any implementation in an interface. All the methods declared in an interface are abstract methods or signatures of the methods.
How to define an Interface?
In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Example of Interface:
public interface SampleInterface {
public void functionOne();
public long CONSTANT_ONE = 1000;
}
Can Abstract Class have constructors? Can interfaces have constructors?
Abstract class's can have a constructor, but you cannot access it through the object, since you cannot instantiate abstract
class. To access the constructor create a sub class and extend the abstract class which is having the constructor.
Example
public abstract class AbstractExample {
public AbstractExample(){
System.out.println("In AbstractExample()");
}
}
public class Test extends AbstractExample{
public static void main(String args[]){
Test obj=new Test();
}
}
If interface &
abstract class have same methods and those methods contain no implementation,
which one would you prefer?
Obviously one
should ideally go for an interface, as we can only extend one class.
Implementing an interface for a class is very much effective rather than
extending an abstract class because we can extend some other useful class for
this subclass.
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Java Exceptions
Questions
|
Explain the user defined Exceptions?
User defined Exceptions are custom Exception classes defined by the user for specific purpose. A user defined exception can be created by simply sub-classing an Exception class or a subclass of an Exception class. This allows custom exceptions to be generated (using throw clause) and caught in the same way as normal exceptions.
Example:
class CustomException extends Exception {
}
User defined Exceptions are custom Exception classes defined by the user for specific purpose. A user defined exception can be created by simply sub-classing an Exception class or a subclass of an Exception class. This allows custom exceptions to be generated (using throw clause) and caught in the same way as normal exceptions.
Example:
class CustomException extends Exception {
}
What classes of exceptions may be caught by a catch
clause?
A catch clause can catch any exception that may be
assigned to the Throwable type. This includes the Error and Exception types.
Errors are generally irrecoverable conditions
What is the difference between exception and error?
Error's are irrecoverable exceptions. Usually a program
terminates when an error is encountered.
What is the difference between throw and throws keywords?
The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as an argument. The exception will be caught by an enclosing try-catch block or propagated further up the calling hierarchy. The throws keyword is a modifier of a method that denotes that an exception may be thrown by the method. An exception can be rethrown.
What is the difference between throw and throws keywords?
The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as an argument. The exception will be caught by an enclosing try-catch block or propagated further up the calling hierarchy. The throws keyword is a modifier of a method that denotes that an exception may be thrown by the method. An exception can be rethrown.
What class of exceptions are generated by the Java
run-time system?
The Java runtime system generates Runtime Exceptions and
Errors.
What is the base class for Error and Exception?
What is the base class for Error and Exception?
Throwable
What are Checked and Unchecked Exceptions?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the exception may be thrown. Checked exceptions must be caught at compile time. Example: IOException.
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. Example: ArrayIndexOutOfBoundsException. Errors are often irrecoverable conditions.
What are Checked and Unchecked Exceptions?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the exception may be thrown. Checked exceptions must be caught at compile time. Example: IOException.
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. Example: ArrayIndexOutOfBoundsException. Errors are often irrecoverable conditions.
Does the code in finally block get executed if there is
an exception and a return statement in a catch block?
Or
What is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught. If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(0) statement is executed earlier or on system shut down earlier or the memory is used up earlier before the thread goes to finally block.
Or
What is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught. If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(0) statement is executed earlier or on system shut down earlier or the memory is used up earlier before the thread goes to finally block.
try{
//some statements
}
catch{
//statements when exception is caught
}
finally{
//statements executed whether exception occurs or not
}
Does the order of placing catch statements matter in the
catch block?
Yes, it does. The FileNoFoundException is inherited from
the IOException. So FileNoFoundException is caught before IOException.
Exception’s subclasses have to be caught first before the General Exception
|
Java Swing
Interview Questions
|
What is the difference between Swing and AWT components?
AWT components are heavy-weight, whereas Swing components are lightweight. Hence Swing works faster than AWT. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component. Pluggable look and feel possible using java Swing. Also, we can switch from one look and feel to another at runtime in swing which is not possible in AWT.
Name the containers which use Border Layout as their default layout?
window, Frame and Dialog classes.
AWT components are heavy-weight, whereas Swing components are lightweight. Hence Swing works faster than AWT. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component. Pluggable look and feel possible using java Swing. Also, we can switch from one look and feel to another at runtime in swing which is not possible in AWT.
Name the containers which use Border Layout as their default layout?
window, Frame and Dialog classes.
Name Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.
What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.
Which package has light weight components?
javax.Swing package contains light weight components. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.
What are peerless components?
The peerless components are called light weight components.
What is a Container in a GUI?
A Container contains and arranges other components (including other containers) through the use of layout managers, which use specific layout policies to determine where components should go as a function of the size of the container.
How are the elements of a GridBagLayout organized?
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.
What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.
Which package has light weight components?
javax.Swing package contains light weight components. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.
What are peerless components?
The peerless components are called light weight components.
What is a Container in a GUI?
A Container contains and arranges other components (including other containers) through the use of layout managers, which use specific layout policies to determine where components should go as a function of the size of the container.
How are the elements of a GridBagLayout organized?
Or What is a layout manager and what are different types of
layout managers available in java Swing?
Or How are the elements of different layouts organized?
A layout manager is an object that is used to organize components in a container. The different layouts available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion.
BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container.
CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards.
GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid.
GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements may be different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
What advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.
What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout. For example, setLayout(new FlowLayout()); will be set the layout as FlowLayout.
Which Container method is used to cause a container to be laid out and redisplayed?
validate()
Name Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular component. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.
What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.
What do heavy weight components mean?
Heavy weight components like Abstract Window Toolkit (AWT) depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component.
What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is just a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.
Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
A layout manager is an object that is used to organize components in a container. The different layouts available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion.
BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container.
CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards.
GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid.
GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements may be different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
What advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.
What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout. For example, setLayout(new FlowLayout()); will be set the layout as FlowLayout.
Which Container method is used to cause a container to be laid out and redisplayed?
validate()
Name Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular component. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.
What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.
What do heavy weight components mean?
Heavy weight components like Abstract Window Toolkit (AWT) depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component.
What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is just a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display normally.
Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
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Java Threads
Interview Questions
|
What are three ways in which a thread can enter the
waiting state?
Or What are different ways in which a thread can enter the
waiting state?
A thread can enter the waiting state by the following ways:
1. Invoking its sleep() method,
2. By blocking on I/O
3. By unsuccessfully attempting to acquire an object's lock
4. By invoking an object's wait() method.
5. It can also enter the waiting state by invoking its (deprecated) suspend() method.
A thread can enter the waiting state by the following ways:
1. Invoking its sleep() method,
2. By blocking on I/O
3. By unsuccessfully attempting to acquire an object's lock
4. By invoking an object's wait() method.
5. It can also enter the waiting state by invoking its (deprecated) suspend() method.
What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the
ready state, either from waiting, running or after its creation. When a task
invokes its sleep() method, it returns to the waiting state from a running
state.
How to create multithreaded program? Explain different ways of using thread? When a thread is created and started, what is its initial state?
Or Extending Thread class or implementing Runnable Interface. Which is better?
You have two ways to do so. First, making your class "extends" Thread class. The other way is making your class implement "Runnable" interface. The latter is more advantageous, cause when you are going for multiple inheritance, then only interface can help. . If you are already inheriting a different class, then you have to go for Runnable Interface. Otherwise you can extend Thread class. Also, if you are implementing interface, it means you have to implement all methods in the interface. Both Thread class and Runnable interface are provided for convenience and use them as per the requirement. But if you are not extending any class, better extend Thread class as it will save few lines of coding. Otherwise performance wise, there is no distinguishable difference.
A thread is in the ready state after it has been created and started.
How to create multithreaded program? Explain different ways of using thread? When a thread is created and started, what is its initial state?
Or Extending Thread class or implementing Runnable Interface. Which is better?
You have two ways to do so. First, making your class "extends" Thread class. The other way is making your class implement "Runnable" interface. The latter is more advantageous, cause when you are going for multiple inheritance, then only interface can help. . If you are already inheriting a different class, then you have to go for Runnable Interface. Otherwise you can extend Thread class. Also, if you are implementing interface, it means you have to implement all methods in the interface. Both Thread class and Runnable interface are provided for convenience and use them as per the requirement. But if you are not extending any class, better extend Thread class as it will save few lines of coding. Otherwise performance wise, there is no distinguishable difference.
A thread is in the ready state after it has been created and started.
What is mutual exclusion? How can you take care of mutual
exclusion using Java threads?
Mutual exclusion is a phenomenon where no two processes can access critical regions of memory at the same time. Using Java multithreading we can arrive at mutual exclusion. For mutual exclusion, you can simply use the synchronized keyword and explicitly or implicitly provide an Object, any Object, to synchronize on. The synchronized keyword can be applied to a class, to a method, or to a block of code. There are several methods in Java used for communicating mutually exclusive threads such as wait( ), notify( ), or notifyAll( ). For example, the notifyAll( ) method wakes up all threads that are in the wait list of an object.
Mutual exclusion is a phenomenon where no two processes can access critical regions of memory at the same time. Using Java multithreading we can arrive at mutual exclusion. For mutual exclusion, you can simply use the synchronized keyword and explicitly or implicitly provide an Object, any Object, to synchronize on. The synchronized keyword can be applied to a class, to a method, or to a block of code. There are several methods in Java used for communicating mutually exclusive threads such as wait( ), notify( ), or notifyAll( ). For example, the notifyAll( ) method wakes up all threads that are in the wait list of an object.
What is the difference between preemptive scheduling and
time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then re-enters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
What invokes a thread's run() method?
After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.
What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(), notify() and notifyAll() methods are used to provide an efficient way for thread inter-communication. When a thread executes an object’s wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object’s notify() or notifyAll() methods.
What is thread? What are the high-level thread states?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then re-enters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
What invokes a thread's run() method?
After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.
What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(), notify() and notifyAll() methods are used to provide an efficient way for thread inter-communication. When a thread executes an object’s wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object’s notify() or notifyAll() methods.
What is thread? What are the high-level thread states?
Or What are the states associated in the thread?
A thread is an independent path of execution in a system. The high-level thread states are ready, running, waiting and dead.
A thread is an independent path of execution in a system. The high-level thread states are ready, running, waiting and dead.
What is deadlock?
When two threads are waiting for each other and can’t proceed until the first thread obtains a lock on the other thread or vice versa, the program is said to be in a deadlock.
How does multithreading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.
When two threads are waiting for each other and can’t proceed until the first thread obtains a lock on the other thread or vice versa, the program is said to be in a deadlock.
How does multithreading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.
What are synchronized methods and synchronized
statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Can Java object be locked down for exclusive use by a given thread?
Or What happens when a thread cannot acquire a lock on an object?
Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Can Java object be locked down for exclusive use by a given thread?
Or What happens when a thread cannot acquire a lock on an object?
Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.
If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.
What’s the difference between the methods sleep() and
wait()?
The sleep method is used when the thread has to be put aside for a fixed amount of time. Ex: sleep(1000), puts the thread aside for exactly one second. The wait method is used to put the thread aside for up to the specified time. It could wait for much lesser time if it receives a notify() or notifyAll() call. Ex: wait(1000), causes a wait of up to one second. The method wait() is defined in the Object and the method sleep() is defined in the class Thread.
The sleep method is used when the thread has to be put aside for a fixed amount of time. Ex: sleep(1000), puts the thread aside for exactly one second. The wait method is used to put the thread aside for up to the specified time. It could wait for much lesser time if it receives a notify() or notifyAll() call. Ex: wait(1000), causes a wait of up to one second. The method wait() is defined in the Object and the method sleep() is defined in the class Thread.
What is the difference between process and thread?
A thread is a separate path of execution in a program. A
Process is a program in execution.
What is daemon thread and which method is used to create
the daemon thread?
Daemon threads are threads with low priority and runs in the back ground doing the garbage collection operation for the java runtime system. The setDaemon() method is used to create a daemon thread. These threads run without the intervention of the user. To determine if a thread is a daemon thread, use the accessor method isDaemon()
Daemon threads are threads with low priority and runs in the back ground doing the garbage collection operation for the java runtime system. The setDaemon() method is used to create a daemon thread. These threads run without the intervention of the user. To determine if a thread is a daemon thread, use the accessor method isDaemon()
When a standalone application is run then as long as any
user threads are active the JVM cannot terminate, otherwise the JVM terminates
along with any daemon threads which might be active. Thus a daemon thread is at
the mercy of the runtime system. Daemon threads exist only to serve user
threads.
What do you understand by Synchronization?
What do you understand by Synchronization?
Or What is synchronization and why is it important?
Or Describe
synchronization in respect to multithreading?
Or What is synchronization?
With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption which may otherwise lead to dirty reads and significant errors.
E.g. synchronizing a function:
public synchronized void Method1 () {
// method code.
}
E.g. synchronizing a block of code inside a function:
public Method2 (){
synchronized (this) {
// synchronized code here.
}
}
With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption which may otherwise lead to dirty reads and significant errors.
E.g. synchronizing a function:
public synchronized void Method1 () {
// method code.
}
E.g. synchronizing a block of code inside a function:
public Method2 (){
synchronized (this) {
// synchronized code here.
}
}
When you will synchronize a piece of your code?
When you expect that your shared code will be accessed by
different threads and these threads may change a particular data causing data
corruption, then they are placed in a synchronized construct or a synchronized
method.
Why would you use a synchronized block vs. synchronized
method?
Synchronized blocks place locks for shorter periods than synchronized methods.
What is an object's lock and which objects have locks?
Answer: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
Synchronized blocks place locks for shorter periods than synchronized methods.
What is an object's lock and which objects have locks?
Answer: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.
What state does a thread enter when it terminates its
processing?
When a thread terminates its processing, it enters the
dead state.
How would you implement a thread pool?
public class ThreadPool implements ThreadPoolInt
This class is an generic implementation of a thread pool,
which takes the following input
a) Size of the pool to be constructed
b) Name of the class which implements Runnable and
constructs a thread pool with active threads that are waiting for activation.
Once the threads have finished processing they come back and wait once again in
the pool.
This thread pool engine can be locked i.e. if some
internal operation is performed on the pool then it is preferable that the
thread engine be locked. Locking ensures that no new threads are issued by the
engine. However, the currently executing threads are allowed to continue till
they come back to the passivePool.
Is there a separate stack for each thread in Java?
Yes. Every thread maintains its own separate stack,
called Runtime Stack but they share the same memory. Elements of the stack are
the method invocations, called activation records or stack frame. The
activation record contains pertinent information about a method like local
variables.
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Java Wrapper
Classes Interview Questions
|
What are Wrapper Classes? Describe the wrapper classes in
Java.
Wrapper classes are classes that allow primitive types to be accessed as objects. Wrapper class is wrapper around a primitive data type.
Following table lists the primitive types and the corresponding wrapper classes:
Wrapper classes are classes that allow primitive types to be accessed as objects. Wrapper class is wrapper around a primitive data type.
Following table lists the primitive types and the corresponding wrapper classes:
|
Primitive
|
Wrapper
|
|
Boolean
|
java.lang.Boolean
|
|
Byte
|
java.lang.Byte
|
|
Char
|
java.lang.Character
|
|
double
|
java.lang.Double
|
|
Float
|
java.lang.Float
|
|
Int
|
java.lang.Integer
|
|
Long
|
java.lang.Long
|
|
Short
|
java.lang.Short
|
|
Void
|
java.lang.Void
|
30 Java Interview Questions
* Q1. How could Java classes
direct program messages to the system console, but error messages, say to a
file?
A. The
class System has a variable out that represents the standard output, and
the variable err that represents the standard error device. By default,
they both point at the system console. This how the standard output could be
re-directed:
Stream
st = new Stream(new FileOutputStream("output.txt"));
System.setErr(st); System.setOut(st);
* Q2. What's the difference between an interface and an abstract class?
A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.
* Q3. Why would you use a synchronized block vs. synchronized method?
A. Synchronized blocks place locks for shorter periods than synchronized methods.
* Q4. Explain the usage of the keyword transient?
A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).
* Q5. How can you force garbage collection?
A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.
* Q6. How do you know if an explicit object casting is needed?
A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:
Object a; Customer b; b =
(Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.
* Q7. What's the difference between the methods sleep() and wait()
A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.
* Q8. Can you write a Java class that could be used both as an applet as well as an application?
A. Yes. Add a main() method to the applet.
* Q9. What's the difference between constructors and other methods?
A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.
* Q10. Can you call one constructor from another if a class has multiple constructors
A. Yes. Use this() syntax.
* Q11. Explain the usage of Java packages.
A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.
* Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?
A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java
com.xyz.hr.Employee
* Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?
A.There's no difference, Sun Microsystems just re-branded this version.
* Q14. What would you use to compare two String variables - the operator == or the method equals()?
A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.
* Q15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.
* Q16. Can an inner class declared inside of a method access local variables of this method?
A. It's possible if these variables are final.
* Q17. What can go wrong if you replace && with & in the following code:
String a=null; if (a!=null &&
a.length()>10) {...}
A. A single ampersand here
would lead to a NullPointerException.
* Q18. What's the main difference between a Vector and an ArrayList
A. Java Vector class is internally synchronized and ArrayList is not.
* Q19. When should the method invokeLater()be used?
A. This method is used to ensure that Swing components are updated through the event-dispatching thread.
* Q20. How can a subclass
call a method or a constructor defined in a superclass?
A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.
For senior-level developers:
** Q21. What's the difference
between a queue and a stack?
A. Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule
** Q22. You can create an
abstract class that contains only abstract methods. On the other hand, you can
create an interface that declares the same methods. So can you use abstract
classes instead of interfaces?
A. Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.
** Q23. What comes to mind
when you hear about a young generation in Java?
A. Garbage collection.
** Q24. What comes to mind
when someone mentions a shallow copy in Java?
A. Object cloning.
** Q25. If you're overriding
the method equals() of an object, which other method you might also consider?
A. hashCode()
** Q26. You are planning to
do an indexed search in a list of objects. Which of the two Java collections
should you use:
ArrayList or LinkedList?
ArrayList or LinkedList?
A. ArrayList
** Q27. How would you make a
copy of an entire Java object with its state?
A. Have this class implement Cloneable interface and call its method clone().
** Q28. How can you minimize
the need of garbage collection and make the memory use more effective?
A. Use object pooling and weak object references.
** Q29. There are
two classes: A and B. The class B need to inform a class A when some important
event has happened. What Java technique would you use to implement it?
A. If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.
** Q30. What access level do
you need to specify in the class declaration to ensure that only classes from
the same directory can access it?
A. You do not need to specify any access level, and Java will use a default package access level.
Good questions asked during Java interview
- Is “abc” a primitive value? - The String literal “abc” is not a primitive value. It is a String object.
- What restrictions are placed on the values of each case of a switch statement? - During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
- What modifiers may be used with an interface declaration? - An interface may be declared as public or abstract.
- Is a class a subclass of itself? - A class is a subclass of itself.
- What is the difference between a while statement and a do statement? - A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
- What modifiers can be used with a local inner class? - A local inner class may be final or abstract.
- What is the purpose of the File class? - The File class is used to create objects that provide access to the files and directories of a local file system.
- Can an exception be rethrown? - Yes, an exception can be rethrown.
- When does the compiler supply a default constructor for a class? - The compiler supplies a default constructor for a class if no other constructors are provided.
- If a method is declared as protected, where may the method be accessed? - A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
- Which non-Unicode letter characters may be used as the first character of an identifier? - The non-Unicode letter characters $ and _ may appear as the first character of an identifier
- What restrictions are placed on method overloading? - Two methods may not have the same name and argument list but different return types.
- What is casting? - There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
- What is the return type of a program’s main() method? - A program’s main() method has a void return type.
- What class of exceptions are generated by the Java run-time system? - The Java runtime system generates RuntimeException and Error exceptions.
- What class allows you to read objects directly from a stream? - The ObjectInputStream class supports the reading of objects from input streams.
- What is the difference between a field variable and a local variable? - A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.
- How are this() and super() used with constructors? - this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
- What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution? - A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.
- Why are the methods of the Math class static? - So they can be invoked as if they are a mathematical code library.
- What are the legal operands of the instanceof operator? - The left operand is an object reference or null value and the right operand is a class, interface, or array type.
- What an I/O filter? - An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
- If an object is garbage collected, can it become reachable again? - Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.
- What are E and PI? - E is the base of the natural logarithm and PI is mathematical value pi.
- Are true and false keywords? - The values true and false are not keywords.
- What is the difference between the File and RandomAccessFile classes? - The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.
- What happens when you add a double value to a String? - The result is a String object.
- What is your platform’s default character encoding? - If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1.
- Which package is always imported by default? - The java.lang package is always imported by default.
- What interface must an object implement before it can be written to a stream as an object? - An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
- How can my application get to know when a HttpSession is removed? - Define a Class HttpSessionNotifier which implements HttpSessionBindingListener and implement the functionality what you need in valueUnbound() method. Create an instance of that class and put that instance in HttpSession.
- Whats the difference between notify() and notifyAll()? - notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a “writer” lock on a file might permit all “readers” to resume).
- Why can’t I say just abs()
or sin() instead of Math.abs() and Math.sin()? - The import statement
does not bring methods into your local name space. It lets you abbreviate
class names, but not get rid of them altogether. That’s just the way it
works, you’ll get used to it. It’s really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn’t need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can’t use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can’t use it on java.lang.Math at all, because Math is a “final” class which means it can’t be extended. - Why are there no global variables in Java? - Global variables are considered bad form for a variety of reasons: Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables), State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state, When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once. For these reasons, Java decided to ban global variables.
- What does it mean that a class or member is final? - A final class can no longer be subclassed. Mostly this is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. Methods may be declared final as well. This means they may not be overridden in a subclass. Fields can be declared final, too. However, this has a completely different meaning. A final field cannot be changed after it’s initialized, and it must include an initializer statement where it’s declared. For example, public final double c = 2.998; It’s also possible to make a static field final to get the effect of C++’s const statement or some uses of C’s #define, e.g. public static final double c = 2.998;
- What does it mean that a method or class is abstract? - An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate that a class is abstract with the abstract keyword like this:
37. public abstract class Container extends Component {
Abstract classes may contain abstract methods. A
method declared abstract is not actually implemented in the current class. It
exists only to be overridden in subclasses. It has no body. For example,
public abstract float price();
Abstract methods may only be included in abstract
classes. However, an abstract class is not required to have any abstract
methods, though most of them do. Each subclass of an abstract class must
override the abstract methods of its superclasses or itself be declared
abstract.
- What is a transient variable? - transient variable is a variable that may not be serialized.
- How are Observer and Observable used? - Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
- Can a lock be acquired on a class? - Yes, a lock can be acquired on a class. This lock is acquired on the class’s Class object.
- What state does a thread enter when it terminates its processing? - When a thread terminates its processing, it enters the dead state.
- How does Java handle integer overflows and underflows? - It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
- What is the difference between the >> and >>> operators? - The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
- Is sizeof a keyword? - The sizeof operator is not a keyword.
- Does garbage collection guarantee that a program will not run out of memory? - Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection
- Can an object’s finalize() method be invoked while it is reachable? - An object’s finalize() method cannot be invoked by the garbage collector while the object is still reachable. However, an object’s finalize() method may be invoked by other objects.
- What value does readLine() return when it has reached the end of a file? - The readLine() method returns null when it has reached the end of a file.
- Can a for statement loop indefinitely? - Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;
- To what value is a variable of the String type automatically initialized? - The default value of an String type is null.
- What is a task’s priority and how is it used in scheduling? - A task’s priority is an integer value that identifies the relative order in which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.
- What is the range of the short type? - The range of the short type is -(2^15) to 2^15 - 1.
- What is the purpose of garbage collection? - The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.
- What do you understand by private, protected and public? - These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.
- What is Downcasting ? - Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy
- Can a method be overloaded based on different return type but same argument type ? - No, because the methods can be called without using their return type in which case there is ambiquity for the compiler
- What happens to a static var that is defined within a method of a class ? - Can’t do it. You’ll get a compilation error
- How many static init can you have ? - As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.
- What is the difference amongst JVM Spec, JVM Implementation, JVM Runtime ? - The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM implementation is the actual implementation of the spec by a vendor and the JVM runtime is the actual running instance of a JVM implementation
- Describe what happens when an object is created in Java? - Several things happen in a particular order to ensure the object is constructed properly: Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data. The instance variables of the objects are initialized to their default values. The constructor for the most derived class is invoked. The first thing a constructor does is call the consctructor for its superclasses. This process continues until the constrcutor for java.lang.Object is called, as java.lang.Object is the base class for all objects in java. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.
- What does the “final” keyword mean in front of a variable? A method? A class? - FINAL for a variable: value is constant. FINAL for a method: cannot be overridden. FINAL for a class: cannot be derived
- What is the difference between instanceof and isInstance? - instanceof is used to check to see if an object can be cast into a specified type without throwing a cast class exception. isInstance() Determines if the specified Object is assignment-compatible with the object represented by this Class. This method is the dynamic equivalent of the Java language instanceof operator. The method returns true if the specified Object argument is non-null and can be cast to the reference type represented by this Class object without raising a ClassCastException. It returns false otherwise.
- Why does it take so much time to access an Applet having Swing Components the first time? - Because behind every swing component are many Java objects and resources. This takes time to create them in memory. JDK 1.3 from Sun has some improvements which may lead to faster execution of Swing applications.
Master list of Java interview questions - 115 questions
115 questions total, not for the weak. Covers everything from basics to JDBC connectivity, AWT and JSP.- What is the difference between procedural and object-oriented programs?- a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOP program, unit of program is object, which is nothing but combination of data and code. b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible with in the object and which in turn assures the security of the code.
- What are Encapsulation, Inheritance and Polymorphism?- Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse. Inheritance is the process by which one object acquires the properties of another object. Polymorphism is the feature that allows one interface to be used for general class actions.
- What is the difference between Assignment and Initialization?- Assignment can be done as many times as desired whereas initialization can be done only once.
- What is OOPs?- Object oriented programming organizes a program around its data, i. e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.
- What are Class, Constructor and Primitive data types?- Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform. Constructor is a special kind of method that determines how an object is initialized when created. Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.
- What is an Object and how do you allocate memory to it?- Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
- What is the difference between constructor and method?- Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.
- What are methods and how are they defined?- Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. A method’s signature is a combination of the first three parts mentioned above.
- What is the use of bin and lib in JDK?- Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.
- What is casting?- Casting is used to convert the value of one type to another.
- How many ways can an argument be passed to a subroutine and explain them?- An argument can be passed in two ways. They are passing by value and passing by reference. Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
- What is the difference between an argument and a parameter?- While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
- What are different types of access modifiers?- public: Any thing declared as public can be accessed from anywhere. private: Any thing declared as private can’t be seen outside of its class. protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages. default modifier : Can be accessed only to classes in the same package.
- What is final, finalize() and finally?- final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can’t be overridden. A final variable can’t change from its initialized value. finalize() : finalize() method is used just before an object is destroyed and can be called just prior to garbage collection. finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.
- What is UNICODE?- Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.
- What is Garbage Collection and how to call it explicitly?- When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection. System. gc() method may be used to call it explicitly.
- What is finalize() method?- finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.
- What are Transient and Volatile Modifiers?- Transient: The transient modifier applies to variables only and it is not stored as part of its object’s Persistent state. Transient variables are not serialized. Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.
- What is method overloading and method overriding?- Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
- What is difference between overloading and overriding?- a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method. b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass. c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass. d) Overloading must have different method signatures whereas overriding must have same signature.
- What is meant by Inheritance and what are its advantages?- Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.
- What is the difference between this() and super()?- this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.
- What is the difference between superclass and subclass?- A super class is a class that is inherited whereas sub class is a class that does the inheriting.
- What modifiers may be used with top-level class?- public, abstract and final can be used for top-level class.
- What are inner class and anonymous class?- Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private. Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.
- What is a package?- A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.
- What is a reflection package?- java. lang. reflect package has the ability to analyze itself in runtime.
- What is interface and its use?- Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object’s programming interface without revealing the actual body of the class.
- What is an abstract class?- An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.
- What is the difference between Integer and int?- a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.
- What is a cloneable interface and how many methods does it contain?- It is not having any method because it is a TAGGED or MARKER interface.
- What is the difference between abstract class and interface?- a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can’t have subclasses.
- Can you have an inner class inside a method and what variables can you access?- Yes, we can have an inner class inside a method and final variables can be accessed.
- What is the difference between String and String Buffer?- a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.
- What is the difference between Array and vector?- Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.
- What is the difference between exception and error?- The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying to open the file, which does not exist, the network connection is disrupted, operands being manipulated are out of prescribed ranges, the class file you are interested in loading is missing. The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.
- What is the difference between process and thread?- Process is a program in execution whereas thread is a separate path of execution in a program.
- What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?- Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait() : When a thread executes a call to wait() method, it surrenders the object lock and enters into a waiting state. notify() or notifyAll() : To remove a thread from the waiting state, some other thread must make a call to notify() or notifyAll() method on the same object.
- What is the class and interface in java to create thread and which is the most advantageous method?- Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.
- What are the states associated in the thread?- Thread contains ready, running, waiting and dead states.
- What is synchronization?- Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.
- When you will synchronize a piece of your code?- When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.
- What is deadlock?- When two threads are waiting each other and can’t precede the program is said to be deadlock.
- What is daemon thread and which method is used to create the daemon thread?- Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
- Are there any global variables in Java, which can be accessed by other part of your program?- No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.
- What is an applet?- Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.
- What is the difference between applications and applets?- a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.
- How does applet recognize the height and width?- Using getParameters() method.
- When do you use codebase in applet?- When the applet class file is not in the same directory, codebase is used.
- What is the lifecycle of an applet?- init() method - Can be called when an applet is first loaded start() method - Can be called each time an applet is started. paint() method - Can be called when the applet is minimized or maximized. stop() method - Can be used when the browser moves off the applet’s page. destroy() method - Can be called when the browser is finished with the applet.
- How do you set security in applets?- using setSecurityManager() method
- What is an event and what are the models available for event handling?- An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model
- What are the advantages of the model over the event-inheritance model?- The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.
- What is source and listener?- source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.
- What is adapter class?- An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged()and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged() .
- What is meant by controls and what are different types of controls in AWT?- Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component.
- What is the difference between choice and list?- A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.
- What is the difference between scrollbar and scrollpane?- A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling.
- What is a layout manager and what are different types of layout managers available in java AWT?- A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.
- How are the elements of different layouts organized?- FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
- Which containers use a Border layout as their default layout?- Window, Frame and Dialog classes use a BorderLayout as their layout.
- Which containers use a Flow layout as their default layout?- Panel and Applet classes use the FlowLayout as their default layout.
- What are wrapper classes?- Wrapper classes are classes that allow primitive types to be accessed as objects.
- What are Vector, Hashtable, LinkedList and Enumeration?- Vector : The Vector class provides the capability to implement a growable array of objects. Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object’s keys. Hash codes are integer values that identify objects. LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive locations. Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements() and nextElement(). HasMoreElemnts() tests if this enumeration has more elements and nextElement method returns successive elements of the series.
- What is the difference between set and list?- Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.
- What is a stream and what are the types of Streams and classes of the Streams?- A Stream is an abstraction that either produces or consumes information. There are two types of Streams and they are: Byte Streams: Provide a convenient means for handling input and output of bytes. Character Streams: Provide a convenient means for handling input & output of characters. Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.
- What is the difference between Reader/Writer and InputStream/Output Stream?- The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.
- What is an I/O filter?- An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
- What is serialization and deserialization?- Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
- What is JDBC?- JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.
- What are drivers available?- a) JDBC-ODBC Bridge driver b) Native API Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver
- What is the difference between JDBC and ODBC?- a) OBDC is for Microsoft and JDBC is for Java applications. b) ODBC can’t be directly used with Java because it uses a C interface. c) ODBC makes use of pointers which have been removed totally from Java. d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required. e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.
- What are the types of JDBC Driver Models and explain them?- There are two types of JDBC Driver Models and they are: a) Two tier model and b) Three tier model Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server. Three tier model: A middle tier is introduced in this model. The functions of this model are: a) Collection of SQL statements from the client and handing it over to the database, b) Receiving results from database to the client and c) Maintaining control over accessing and updating of the above.
- What are the steps involved for making a connection with a database or how do you connect to a database?a) Loading the driver : To load the driver, Class. forName() method is used. Class. forName(”sun. jdbc. odbc. JdbcOdbcDriver”); When the driver is loaded, it registers itself with the java. sql. DriverManager class as an available database driver. b) Making a connection with database: To open a connection to a given database, DriverManager. getConnection() method is used. Connection con = DriverManager. getConnection (”jdbc:odbc:somedb”, “user”, “password”); c) Executing SQL statements : To execute a SQL query, java. sql. statements class is used. createStatement() method of Connection to obtain a new Statement object. Statement stmt = con. createStatement(); A query that returns data can be executed using the executeQuery() method of Statement. This method executes the statement and returns a java. sql. ResultSet that encapsulates the retrieved data: ResultSet rs = stmt. executeQuery(”SELECT * FROM some table”); d) Process the results : ResultSet returns one row at a time. Next() method of ResultSet object can be called to move to the next row. The getString() and getObject() methods are used for retrieving column values: while(rs. next()) { String event = rs. getString(”event”); Object count = (Integer) rs. getObject(”count”);
- What type of driver did you use in project?- JDBC-ODBC Bridge driver (is a driver that uses native(C language) libraries and makes calls to an existing ODBC driver to access a database engine).
- What are the types of statements in JDBC?- Statement: to be used createStatement() method for executing single SQL statement PreparedStatement — To be used preparedStatement() method for executing same SQL statement over and over. CallableStatement — To be used prepareCall() method for multiple SQL statements over and over.
- What is stored procedure?- Stored procedure is a group of SQL statements that forms a logical unit and performs a particular task. Stored Procedures are used to encapsulate a set of operations or queries to execute on database. Stored procedures can be compiled and executed with different parameters and results and may have any combination of input/output parameters.
- How to create and call stored procedures?- To create stored procedures: Create procedure procedurename (specify in, out and in out parameters) BEGIN Any multiple SQL statement; END; To call stored procedures: CallableStatement csmt = con. prepareCall(”{call procedure name(?,?)}”); csmt. registerOutParameter(column no. , data type); csmt. setInt(column no. , column name) csmt. execute();
- What is servlet?- Servlets are modules that extend request/response-oriented servers, such as java-enabled web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company’s order database.
- What are the classes and interfaces for servlets?- There are two packages in servlets and they are javax. servlet and
- What is the difference between an applet and a servlet?- a) Servlets are to servers what applets are to browsers. b) Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
- What is the difference between doPost and doGet methods?- a) doGet() method is used to get information, while doPost() method is used for posting information. b) doGet() requests can’t send large amount of information and is limited to 240-255 characters. However, doPost()requests passes all of its data, of unlimited length. c) A doGet() request is appended to the request URL in a query string and this allows the exchange is visible to the client, whereas a doPost() request passes directly over the socket connection as part of its HTTP request body and the exchange are invisible to the client.
- What is the life cycle of a servlet?- Each Servlet has the same life cycle: a) A server loads and initializes the servlet by init () method. b) The servlet handles zero or more client’s requests through service() method. c) The server removes the servlet through destroy() method.
- Who is loading the init() method of servlet?- Web server
- What are the different servers available for developing and deploying Servlets?- a) Java Web Server b) JRun g) Apache Server h) Netscape Information Server i) Web Logic
- How many ways can we track client and what are they?- The servlet API provides two ways to track client state and they are: a) Using Session tracking and b) Using Cookies.
- What is session tracking and how do you track a user session in servlets?- Session tracking is a mechanism that servlets use to maintain state about a series requests from the same user across some period of time. The methods used for session tracking are: a) User Authentication - occurs when a web server restricts access to some of its resources to only those clients that log in using a recognized username and password. b) Hidden form fields - fields are added to an HTML form that are not displayed in the client’s browser. When the form containing the fields is submitted, the fields are sent back to the server. c) URL rewriting - every URL that the user clicks on is dynamically modified or rewritten to include extra information. The extra information can be in the form of extra path information, added parameters or some custom, server-specific URL change. d) Cookies - a bit of information that is sent by a web server to a browser and which can later be read back from that browser. e) HttpSession- places a limit on the number of sessions that can exist in memory. This limit is set in the session. maxresidents property.
- What is Server-Side Includes (SSI)?- Server-Side Includes allows embedding servlets within HTML pages using a special servlet tag. In many servlets that support servlets, a page can be processed by the server to include output from servlets at certain points inside the HTML page. This is accomplished using a special internal SSINCLUDE, which processes the servlet tags. SSINCLUDE servlet will be invoked whenever a file with an. shtml extension is requested. So HTML files that include server-side includes must be stored with an . shtml extension.
- What are cookies and how will you use them?- Cookies are a mechanism that a servlet uses to have a client hold a small amount of state-information associated with the user. a) Create a cookie with the Cookie constructor: public Cookie(String name, String value) b) A servlet can send a cookie to the client by passing a Cookie object to the addCookie() method of HttpServletResponse: public void HttpServletResponse. addCookie(Cookie cookie) c) A servlet retrieves cookies by calling the getCookies() method of HttpServletRequest: public Cookie[ ] HttpServletRequest. getCookie().
- Is it possible to communicate from an applet to servlet and how many ways and how?- Yes, there are three ways to communicate from an applet to servlet and they are: a) HTTP Communication(Text-based and object-based) b) Socket Communication c) RMI Communication
- What is connection pooling?- With servlets, opening a database connection is a major bottleneck because we are creating and tearing down a new connection for every page request and the time taken to create connection will be more. Creating a connection pool is an ideal approach for a complicated servlet. With a connection pool, we can duplicate only the resources we need to duplicate rather than the entire servlet. A connection pool can also intelligently manage the size of the pool and make sure each connection remains valid. A number of connection pool packages are currently available. Some like DbConnectionBroker are freely available from Java Exchange Works by creating an object that dispenses connections and connection Ids on request. The ConnectionPool class maintains a Hastable, using Connection objects as keys and Boolean values as stored values. The Boolean value indicates whether a connection is in use or not. A program calls getConnection() method of the ConnectionPool for getting Connection object it can use; it calls returnConnection() to give the connection back to the pool.
- Why should we go for interservlet communication?- Servlets running together in the same server communicate with each other in several ways. The three major reasons to use interservlet communication are: a) Direct servlet manipulation - allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse - allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration - requires to communicate with each other by sharing specific information (through method invocation)
- Is it possible to call servlet with parameters in the URL?- Yes. You can call a servlet with parameters in the syntax as (?Param1 = xxx || m2 = yyy).
- What is Servlet chaining?- Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet’s output is piped to the next servlet’s input. This process continues until the last servlet is reached. Its output is then sent back to the client.
- How do servlets handle multiple simultaneous requests?- The server has multiple threads that are available to handle requests. When a request comes in, it is assigned to a thread, which calls a service method (for example: doGet(), doPost() and service()) of the servlet. For this reason, a single servlet object can have its service methods called by many threads at once.
- What is the difference between TCP/IP and UDP?- TCP/IP is a two-way communication between the client and the server and it is a reliable and there is a confirmation regarding reaching the message to the destination. It is like a phone call. UDP is a one-way communication only between the client and the server and it is not a reliable and there is no confirmation regarding reaching the message to the destination. It is like a postal mail.
- What is Inet address?- Every computer connected to a network has an IP address. An IP address is a number that uniquely identifies each computer on the Net. An IP address is a 32-bit number.
- What is Domain Naming Service(DNS)?- It is very difficult to remember a set of numbers(IP address) to connect to the Internet. The Domain Naming Service(DNS) is used to overcome this problem. It maps one particular IP address to a string of characters. For example, www. mascom. com implies com is the domain name reserved for US commercial sites, moscom is the name of the company and www is the name of the specific computer, which is mascom’s server.
- What is URL?- URL stands for Uniform Resource Locator and it points to resource files on the Internet. URL has four components: http://www. address. com:80/index.html, where http - protocol name, address - IP address or host name, 80 - port number and index.html - file path.
- What is RMI and steps involved in developing an RMI object?- Remote Method Invocation (RMI) allows java object that executes on one machine and to invoke the method of a Java object to execute on another machine. The steps involved in developing an RMI object are: a) Define the interfaces b) Implementing these interfaces c) Compile the interfaces and their implementations with the java compiler d) Compile the server implementation with RMI compiler e) Run the RMI registry f) Run the application
- What is RMI architecture?- RMI architecture consists of four layers and each layer performs specific functions: a) Application layer - contains the actual object definition. b) Proxy layer - consists of stub and skeleton. c) Remote Reference layer - gets the stream of bytes from the transport layer and sends it to the proxy layer. d) Transportation layer - responsible for handling the actual machine-to-machine communication.
- what is UnicastRemoteObject?- All remote objects must extend UnicastRemoteObject, which provides functionality that is needed to make objects available from remote machines.
- Explain the methods, rebind() and lookup() in Naming class?- rebind() of the Naming class(found in java. rmi) is used to update the RMI registry on the server machine. Naming. rebind(”AddSever”, AddServerImpl); lookup() of the Naming class accepts one argument, the rmi URL and returns a reference to an object of type AddServerImpl.
- What is a Java Bean?- A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
- What is a Jar file?- Jar file allows to efficiently deploying a set of classes and their associated resources. The elements in a jar file are compressed, which makes downloading a Jar file much faster than separately downloading several uncompressed files. The package java. util. zip contains classes that read and write jar files.
- What is BDK?- BDK, Bean Development Kit is a tool that enables to create, configure and connect a set of set of Beans and it can be used to test Beans without writing a code.
- What is JSP?- JSP is a dynamic scripting capability for web pages that allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix traditionally ends with .jsp to indicate to the web server that the file is a JSP files. JSP is a server side technology - you can’t do any client side validation with it. The advantages are: a) The JSP assists in making the HTML more functional. Servlets on the other hand allow outputting of HTML but it is a tedious process. b) It is easy to make a change and then let the JSP capability of the web server you are using deal with compiling it into a servlet and running it.
- What are JSP scripting elements?- JSP scripting elements lets to insert Java code into the servlet that will be generated from the current JSP page. There are three forms: a) Expressions of the form <%= expression %> that are evaluated and inserted into the output, b) Scriptlets of the formthat are inserted into the servlet’s service method, and c) Declarations of the form <%! Code %>that are inserted into the body of the servlet class, outside of any existing methods.
- What are JSP Directives?- A JSP directive affects the overall structure of the servlet class. It usually has the following form:<%@ directive attribute=”value” %> However, you can also combine multiple attribute settings for a single directive, as follows:<%@ directive attribute1=”value1″ attribute 2=”value2″ . . . attributeN =”valueN” %> There are two main types of directive: page, which lets to do things like import classes, customize the servlet superclass, and the like; and include, which lets to insert a file into the servlet class at the time the JSP file is translated into a servlet
- What are Predefined variables or implicit objects?- To simplify code in JSP expressions and scriptlets, we can use eight automatically defined variables, sometimes called implicit objects. They are request, response, out, session, application, config, pageContext, and page.
- What are JSP ACTIONS?- JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include: jsp:include - Include a file at the time the page is requested. jsp:useBean - Find or instantiate a JavaBean. jsp:setProperty - Set the property of a JavaBean. jsp:getProperty - Insert the property of a JavaBean into the output. jsp:forward - Forward the requester to a newpage. Jsp: plugin - Generate browser-specific code that makes an OBJECT or EMBED
- How do you pass data (including JavaBeans) to a JSP from a servlet?- (1) Request Lifetime: Using this technique to pass beans, a request dispatcher (using either “include” or forward”) can be called. This bean will disappear after processing this request has been completed. Servlet: request. setAttribute(”theBean”, myBean); RequestDispatcher rd = getServletContext(). getRequestDispatcher(”thepage. jsp”); rd. forward(request, response); JSP PAGE:<jsp: useBean id=”theBean” scope=”request” class=”. . . . . ” />(2) Session Lifetime: Using this technique to pass beans that are relevant to a particular session (such as in individual user login) over a number of requests. This bean will disappear when the session is invalidated or it times out, or when you remove it. Servlet: HttpSession session = request. getSession(true); session. putValue(”theBean”, myBean); /* You can do a request dispatcher here, or just let the bean be visible on the next request */ JSP Page:<jsp:useBean id=”theBean” scope=”session” class=”. . . ” /> 3) Application Lifetime: Using this technique to pass beans that are relevant to all servlets and JSP pages in a particular app, for all users. For example, I use this to make a JDBC connection pool object available to the various servlets and JSP pages in my apps. This bean will disappear when the servlet engine is shut down, or when you remove it. Servlet: GetServletContext(). setAttribute(”theBean”, myBean); JSP PAGE:<jsp:useBean id=”theBean” scope=”application” class=”. . . ” />
- How can I set a cookie in JSP?- response. setHeader(”Set-Cookie”, “cookie string”); To give the response-object to a bean, write a method setResponse (HttpServletResponse response) - to the bean, and in jsp-file:<% bean. setResponse (response); %>
- How can I delete a cookie with JSP?- Say that I have a cookie called “foo, ” that I set a while ago & I want it to go away. I simply: <% Cookie killCookie = new Cookie(”foo”, null); KillCookie. setPath(”/”); killCookie. setMaxAge(0); response. addCookie(killCookie); %>
- How are Servlets and JSP Pages related?- JSP pages are focused around HTML (or XML) with Java codes and JSP tags inside them. When a web server that has JSP support is asked for a JSP page, it checks to see if it has already compiled the page into a servlet. Thus, JSP pages become servlets and are transformed into pure Java and then compiled, loaded into the server and executed.
Core Java Interview Questions
Question: What is transient variable?
Answer: Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
Answer: Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
Question: Name the containers which uses Border Layout as their default layout?
Answer: Containers which uses Border Layout as their default are: window, Frame and Dialog classes.
Question: What do you understand by Synchronization?
Answer: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}
Question: What is Collection API?
Answer: The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes:
HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.Example of interfaces:
Collection, Set,
List and Map.Question: Is Iterator a Class or Interface? What is its use?
Answer: Iterator is an interface which is used to step through the elements of a Collection.
Question: What is similarities/difference between an Abstract class and Interface?
Answer: Differences are as follows:
- Interfaces provide a form of multiple inheritance. A class can extend only one other class.
- Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
- A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
- Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.
- Neither Abstract classes
or Interface can be instantiated.
Question: How to define an Abstract class?
Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
protected String myString;
public String getMyString() {
return myString;
}
public abstract string anyAbstractFunction();
}
Question:
How to define an Interface?Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
protected String myString;
public String getMyString() {
return myString;
}
public abstract string anyAbstractFunction();
}
Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Emaple of Interface:
public interface sampleInterface {
public void functionOne();
public long CONSTANT_ONE = 1000;
}
Question: Explain the user defined Exceptions?
Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
// The class simply has to exist to be an exception
}
Question: Explain the new Features of JDBC 2.0 Core API?
Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities.
New Features in JDBC 2.0 Core API:
- Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position
- JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.
- Java applications can now use the ResultSet.updateXXX methods.
- New data types - interfaces mapping the SQL3 data types
- Custom mapping of user-defined types (UTDs)
- Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values.
Question: Explain garbage collection?
Answer: Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.
Question: How you can force the garbage collection?
Answer: Garbage collection automatic process and can't be forced.
Question: What is OOPS?
Answer: OOP is the common abbreviation for Object-Oriented Programming.
Question: Describe the principles of OOPS.
Answer: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.
Question: Explain the Encapsulation principle.
Answer: Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
Question: Explain the Inheritance principle.
Answer: Inheritance is the process by which one object acquires the properties of another object.
Question: Explain the Polymorphism principle.
Answer: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".
Question: Explain the different forms of Polymorphism.
Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:
- Method overloading
- Method overriding through inheritance
- Method overriding through the Java interface
Answer: Access specifiers are keywords that determines the type of access to the member of a class. These are:
- Public
- Protected
- Private
- Defaults
Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.
Following table lists the primitive types and the corresponding wrapper classes:
|
Primitive
|
Wrapper
|
|
boolean
|
java.lang.Boolean
|
|
byte
|
java.lang.Byte
|
|
char
|
java.lang.Character
|
|
double
|
java.lang.Double
|
|
float
|
java.lang.Float
|
|
int
|
java.lang.Integer
|
|
long
|
java.lang.Long
|
|
short
|
java.lang.Short
|
|
void
|
java.lang.Void
|
public class test {
public static void main(String [] args) {
int x = 3;
int y = 1;
if (x = y)
System.out.println("Not equal");
else
System.out.println("Equal");
}
}
What is the result?
A. The output is “Equal”
B. The output in “Not Equal”
C. An error at " if (x = y)" causes compilation to fall.
D. The program executes but no output is show on console.
Answer: C
Question: what is the class variables ?
Answer: When we create a number of objects of the same class, then each object will share a common copy of variables. That means that there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class, but mind it that it should be declared outside outside a class. These variables are stored in static memory. Class variables are mostly used for constants, variable that never change its initial value. Static variables are always called by the class name. This variable is created when the program starts i.e. it is created before the instance is created of class by using new operator and gets destroyed when the programs stops. The scope of the class variable is same a instance variable. The class variable can be defined anywhere at class level with the keyword static. It initial value is same as instance variable. When the class variable is defined as int then it's initial value is by default zero, when declared boolean its default value is false and null for object references. Class variables are associated with the class, rather than with any object.
Question: What is the difference between the instanceof and getclass, these two are same or not ?
Answer: instanceof is a operator, not a function while getClass is a method of java.lang.Object class. Consider a condition where we use
if(o.getClass().getName().equals("java.lang.Math")){ }
This method only checks if the classname we have passed is equal to java.lang.Math. The class java.lang.Math is loaded by the bootstrap ClassLoader. This class is an abstract class.This class loader is responsible for loading classes. Every Class object contains a reference to the ClassLoader that defines. getClass() method returns the runtime class of an object. It fetches the java instance of the given fully qualified type name. The code we have written is not necessary, because we should not compare getClass.getName(). The reason behind it is that if the two different class loaders load the same class but for the JVM, it will consider both classes as different classes so, we can't compare their names. It can only gives the implementing class but can't compare a interface, but instanceof operator can.
The instanceof operator compares an object to a specified type. We can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface. We should try to use instanceof operator in place of getClass() method. Remember instanceof opeator and getClass are not same. Try this example, it will help you to better understand the difference between the two.
Interface one{
}
Class Two implements one {
}
Class Three implements one {
}
public class Test {
public static void main(String args[]) {
one test1 = new Two();
one test2 = new Three();
System.out.println(test1 instanceof one); //true
System.out.println(test2 instanceof one); //true
System.out.println(Test.getClass().equals(test2.getClass())); //false
}
}
