Abstract Class in Java Programming

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Java Abstract classes are used to declare common characteristics of subclasses.
 An abstract class cannot be instantiated.

It can only be used as a superclass for other classes that extend the abstract class. 
Abstract classes are declared with the abstract keyword. 
Abstract classes are used to provide a template or design for concrete subclasses down the inheritance tree.

 As mentioned, it is not possible to instantiate an abstract class.Therefore no objects of class A are declared in the program.

    abstract class A 
		{
    abstract void callme();
    // concrete methods are still allowed in abstract classes
    void callmetoo()
		{
    System.out.println("This is a concrete method.");
    }
    }

    class B extends A
{
    void callme()
		{
    System.out.println("B is implementation of callme.");
    }
    }

    class AbstractDemo 
		{
    public static void main(String args[]) {
    B b = new B();
    b.callme();
    b.callmetoo();
    }
    }