Abstract Class and Abstract Method

abstract class Figure        // Abstract class
{
 double dim1;
 double dim2;
 
 Figure(double a, double b)
 {
  dim1=a;
  dim2=b;
 }
 
 abstract double area();  // Abstract Method
}
//-----------
class Rectangle extends Figure
{
 Rectangle(double a,double b)
 {
  super(a,b);
 }
 
 double area()
 {
  System.out.println("\nInside Area for Rectangle");
  return dim1 * dim2;
 }
}
//-----------
class Triangle extends Figure
{
 Triangle(double a,double b)
 {
  super(a,b);
 }
 
 double area()
 {
  System.out.println("Inside Area for Triangle");
  return dim1 * dim2/2;
 }
}
//----------
class AbstractAreas
{
 public static void main(String args[])
 {
  //Figure f=new Figure(10,10);        //illegal now
  Rectangle r=new Rectangle(9,5);
  Triangle t=new Triangle(10,8);
  
  Figure figref;
  
  figref=r;
  System.out.println("Area is "+figref.area());
  
  figref=t;
  System.out.println("Area is "+figref.area());
 }
}

Output:

Overriding: Find the Area of Figures

class Figure
{
 double dim1;
 double dim2;
 
 Figure(double a, double b)
 {
  dim1=a;
  dim2=b;
 }
 
 double area()
 {
  System.out.println("Area for figure is undefined");
  return 0;
 }
}

class Rectangle extends Figure
{
 Rectangle(double a,double b)
 {
  super(a,b);
 }
 
 double area()
 {
  System.out.println("\nInside Area for Rectangle");
  return dim1 * dim2;
 }
}

class Triangle extends Figure
{
 Triangle(double a,double b)
 {
  super(a,b);
 }
 
 double area()
 {
  System.out.println("Inside Area for Triangle");
  return dim1 * dim2/2;
 }
}

class FindAreas
{
 public static void main(String args[])
 {
  Figure f=new Figure(10,10);
  Rectangle r=new Rectangle(9,5);
  Triangle t=new Triangle(10,8);
  
  Figure figref;
  
  figref=r;
  System.out.println("Area is "+figref.area());
  
  figref=t;
  System.out.println("Area is "+figref.area());

  figref=f;
  System.out.println("Area is "+figref.area());
 }
}

----- output: