Dynamic Method Dispatch

When an overridden method is called through a superclass reference, Java determines which version of that method to execute.

Here is an example that illustrates dynamic method dispatch:

 
class Base {
  void callme() {
    System.out.println("Inside A's callme method");
  }
}
class SubClass extends Base {
  void callme() {
    System.out.println("Inside B's callme method");
  }
}
class SubClass2 extends Base {
  void callme() {
    System.out.println("Inside C's callme method");
  }
}
public class Main {
  public static void main(String args[]) {
    Base a = new Base(); 
    SubClass b = new SubClass(); 
    SubClass2 c = new SubClass2();
    Base r; 
    r = a; 
    r.callme(); 
    r = b;
    r.callme(); 
    r = c; 
    r.callme();
  }
}

The output from the program is shown here:


Inside A's callme method
Inside B's callme method
Inside C's callme method

polymorphism

The following example use the run-time polymorphism.

 
class Shape {
  double height;
  double width;
  Shape(double a, double b) {
    height = a;
    width = b;
  }
  double area() {
    System.out.println("Area for Figure is undefined.");
    return 0;
  }
}
class Rectangle extends Shape {
  Rectangle(double a, double b) {
    super(a, b);
  }
  // override area for rectangle
  double area() {
    System.out.println("Inside Area for Rectangle.");
    return height * width;
  }
}
class Triangle extends Shape {
  Triangle(double a, double b) {
    super(a, b);
  }
  // override area for right triangle
  double area() {
    System.out.println("Inside Area for Triangle.");
    return height * width / 2;
  }
}
public class Main {
  public static void main(String args[]) {
    Shape f = new Shape(10, 10);
    Rectangle r = new Rectangle(9, 5);
    Triangle t = new Triangle(10, 8);

    Shape 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());
  }
}

The output from the program is shown here:


Inside Area for Rectangle.
Area is 45.0
Inside Area for Triangle.
Area is 40.0
Area for Figure is undefined.
Area is 0.0

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