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What is an Abstract class?

By Sai Rohith Karanam
    
crux points:
  1. An abstract class is a class which is declared with the keyword "abstract".
  2. Abstract classes - No instantiation (Objects cannot be created).
  3. Abstract classes can be extended by other classes (Inheritance is possible).
  4. Abstract classes may or may not contain abstract methods.

What is an abstract method?
An abstract method is a method which has the only declaration but no implementation.
eg: abstract void thedevelopingminds();
                
The implementation of this abstract method is provided by the class which gets inherited from the abstract class.
                
 Note:
 The major difference between abstract classes and interfaces is-
 Abstract class   -  may or may not have abstract methods
 interface           -  All the methods are abstract strictly.
                           From Java 8, we can include default methods too.
       
An example demonstrating abstract class:
                  
                  abstract class Geometricobject
                 {
                                  void display() //non-abstract method
                                  {
                                           System.out.println("I am Geometric object");
                                  }
                                   abstract void area(); //abstract method
                 }
                  class Circle extends Geometricobject
                 {
                           double radius=2.8;
                          //abstract method must be implemented by the inherited class
                           void area()
                           {
                                System.out.println("I am circle with area:"+""+3.14*radius*radius);
                            }
                  }
                  class Rectangle extends Geometricobject
                  {
                           int length=3;
                           int breadth=5;
                           //abstract method must be implemented by the inherited class
                            void area()
                            {
                                System.out.println("I am rectangle with area:"+""+length*breadth);
                            }
                  }
                  class TestArea
                  {
                       public static void main(String... args)
                        {
                               Circle c=new Circle();
                               c.area();
                               Rectangle r=new Rectangle();
                               r.area();
                        }
                   }


 Output:
 I am circle with area:24.6176
 I am rectangle with area:15
                 
                                   
            
                         
                                          
                                   

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