abstract and interfaces

[hemapriya rao]

could any one tell me the exact difference between abstract and interface class

[Rob Spoor]

1) A class can extend only one abstract class but implement multiple interfaces.

2) An interface can only declare constants (any field) and public abstract methods (any method).
An abstract class can have anything other classes can have: private fields, instance fields, protected members, constructors, you name it. Unlike other classes, it can also have abstract methods. Unlike interfaces, these abstract methods can also be protected or have no access modifier.
Please note that they cannot be private - private methods cannot be overridden, and therefore private abstract methods would never get an implementation.

[hemapriya rao]

1)so,should a immediate subclass of abstract class implement(define) all methods of abstract class?

2)when the class implements interface ,should that class override all methods of the interface?

[Rob Spoor]

For both situations, a class must either implement all abstract methods (so also methods defined inside an interface the class is implementing), or be abstract itself.

If a class is not abstract, it must make sure that all remaining abstract methods must be implemented.

Example:

interface Interface
{
    public void doSomething();
 
    public void doSomethingElse();
}
 
abstract class AbstractClass implements Interface
{
    public void doSomething()
    {
        System.out.println("Hello world!");
    }
 
    // don't implement doSomethingElse - keep it abstract!
 
    public abstract void myAbstractMethod();
}
 
class ConcreteClass extends AbstractClass
// because it extends AbstractClass, it indirectly implements Interface
{
    // inherit doSomething() from AbstractClass
 
    // the following two methods MUST be implemented
    public void doSomethingElse()
    {
        System.out.println("Bye Bye!");
    }
 
    public void myAbstractMethod()
    {
        System.out.println("Actually not abstract anymore");
    }
}

[Jesper de Jong]

This is a frequently asked question.
See the answer in the JavaRanch FAQ: InterfaceVsAbstractClass

[Saurabh Gupta]

Like to add one point in term of use of Abstract class and Interface:-

It's generally considered good OO practice to specify the "contract" which you want to adhere to via Java interfaces.

Use abstract Java classes when you want to provide some standard base code but want/need to force the user's of your class to complete the implementation.