Top 10 java interview questions

Java Interview Questions and Answers

1. What is Collection  API ? 
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.

2.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.

3.What is similarities/difference  between an  Abstract  class and Interface? 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.
Similarities: Neither  Abstract classes or Interface can  be instantiated.

4.How to define  an  Abstract  class?
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();

5.How to define  an Interface in  Java ? 
In Java Interface  defines the  methods but  does  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;

 6.If a  class is located in a  package,  what  do you  need to  change  in the OS environment to be  able to  use  it? 
Security  Issue Get  Norton  Security  Scan  and Spyware  Doctor free  for your  Computer  from  Google. The  Pack contains nearly  14 plus software  . Pick the  one which is suited  for you  Make your PC more  useful.  Get the  free Google Pack. 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; and  is located in the file  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 .

7.How many  methods in the  Serializable interface? There is no method  in the  Serializable interface.  The Serializable  interface acts as a marker, telling  the object  serialization tools that your class is serializable.

8.How many  methods in the  Externalizable  interface? There are  two methods in the Externalizable  interface.  You have to  implement  these two methods in order to  make  your  class externalizable.  These  two methods are readExternal() and  writeExternal(). 

9.What is the  difference  between  Serializalble and Externalizable interface? 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.
10.What is a transient variable  in Java? A transient  variable  is a variable  that  may  not be  serialized. If you  don’t want  some  field to  be serialized,  you  can mark  that  field transient  or static.

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