9.3 Overriding Superclass Methods

In a class hierarchy, when a method in a subclass has the same name and type signature as a method in its superclass, then the method in the subclass is said to override the method in the superclass. When an overridden method is called from within a subclass, it will always refer to the version of that method defined by the subclass. The version of the method defined by the superclass will be hidden.

A subclass may call an overridden superclass method by prefixing its name with the super key word and a dot (.). Consider the following:

Program (Rectangle.java)

public class Rectangle
{
   private double length;
   private double width;

   /**
    *  Sets length and width of rectangle.
    */
   public void setRectangle(double length, double width)
   {
      this.length = length;
      this.width  = width;
   }

   /**
    * Returns length and width as string
    */
   public String toString()
   {
      return "Length : " + length + "\nWidth : " + width;
   }
}

Program (Box.java)

public class Box extends Rectangle
{
   private double height;

   /**
    *  Sets length, width and height of box.
    */
   public void setBox(double length, double width, double height)
   {
      setRectangle(length, width);
      this.height  = height;
   }

   /**
    * The toString method overrides the Rectangle's toString method
    * and returns length, width and height of box as string
    */
   public String toString()
   {
      return super.toString() +    //Calling Rectangle's toString method
           "\nHeight : " + height;
   }
}

Program (BoxDemo.java)

import java.util.Scanner;

/**
 * This program demonstrates the Box class.
 */

public class BoxDemo
{
   public static void main(String[] args)
   {
      double length, width, height;
           
      // Create a Scanner object for keybaord input.
      Scanner console = new Scanner(System.in);

      // Get the length of box.
      System.out.print("Enter the length of box : ");
      length = console.nextDouble();

      // Get the width of box.
      System.out.print("Enter the width of box : ");
      width = console.nextDouble();

      // Get the height of box.
      System.out.print("Enter the height of box : ");
      height = console.nextDouble();

      // Create a box object.
      Box myBox = new Box();

      // Set the length, width and height of box.
      myBox.setBox(length, width, height);

      // Display the box detalis.
      System.out.println(myBox);
   }
}