• A class that is marked with abstract keyword, is known as abstract class in java.

• Abstract can have abstract and non-abstract methods (method with body).

• A method that doesn't have its definition is called abstract method, and to declare a method as abstract use keyword abstract.

  Syntax :

  abstract return-type method-name(arg-list);

• If Any class contains direct or indirect ( through parent ), one or more abstract method, that class must be declared as abstract.

• An abstract class cannot have objects because it is not complete, but it can have references.

• Static methods and constructors cannot be declared as abstract.

  abstract static void show();

• A concrete method can be overridden to become abstract.

• It is illegal to declare a class and method both final and abstract.

  abstract final void show();
  abstract final class Test {}

 //abstract class
abstract class Shape
{
 //abstract method
 abstract double area();

//concrete method
 void displayDetail()
 {
  System.out.println("Details of Shape");
 }
}
class Square extends Shape
{
 double side;
//Overriding the abstract method.
 double area() 
 { 
  return side*side;
 }
}

USE : A super class that only defines a generalized form that will be shared by all its sub classes, leaving the implementation details to its subclasses is said to an abstract class.

Example :

abstract class Animal
{
 abstract void talk();
}
class Dog extends Animal
{
  void talk()
   {
    System.out.println("bark");
   }
}
class Cat extends Animal
{
  void talk()
   {
    System.out.println("mew");
   }
}
class MainClass
{
  public static void main(String[] args)
  {
//ERROR, can not create object
   Animal ob = new Animal();

//can have references
    Animal ob1,ob2;

//Dynamic Polymorphism
    ob1 = new Dog();
    ob2 = new Cat();
    ob1.talk();
    ob2.talk();
  }
}           

bark
mew