Inheritance in Java

 To create a child class in Java we must first start with a parent class (or superclass), like this one...

public class Rectangle {

    private int length;
    private int width;

    // Constructor that accepts length and width
    public Rectangle(int length, int width) {
        this.length = length;
        this.width = width;
    }

    // Default constructor
    public Rectangle() {
        this(3, 5); // Calls the constructor with length 3 and width 5
    }

    // Getter for length
    public int getLength() {
        return length;
    }

    // Getter for width
    public int getWidth() {
        return width;
    }

    // Getter for area
    public int getArea() {
        return length * width;
    }

    // Setter for length
    public void setLength(int length) {
        this.length = length;
    }

    // Setter for width
    public void setWidth(int width) {
        this.width = width;
    }

} // Rectangle

Now we want to create a child class (or subclass) that will extend the parent...

public class Box extends Rectangle {

    private int height;

    // Constructor
    public Box(int length, int width, int height) {
        super(length, width); //calls parent class constructor
        this.height = height;
    }

    // Default constructor
    public Box() {
        super();
        this.height = 5;
    }

    // Getters & Setters
    public int getHeight() {
        return height;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public int getVolume() {
        return super.getArea() * height;
    }

}//Box

Then when we can use the new Box class in our program...

public class MainClass {

  public static void main(String[] args) { 


  Box myBox = new Box();

  System.out.println(myBox.getLength());
  System.out.println(myBox.getWidth());
  System.out.println(myBox.getHeight());
  System.out.println(myBox.getVolume());
 
  } //main
} //MainClass

The output is...

3
5
5
75

But here is a problem... Should our Box class still have a getArea() method? Let us say for a moment that I did not want Box to have a getArea() method. Well, that is just too bad because Java does not offer a way to limit what methods the child class will inherit from the parent.

public class MainClass {

  public static void main(String[] args) { 


  Box myBox = new Box();

  System.out.println(myBox.getLength()); 
  System.out.println(myBox.getWidth());
  System.out.println(myBox.getHeight());
  System.out.println(myBox.getVolume());
  System.out.println(myBox.getArea());
 
  } //main
} //MainClass

The best we can hope for is to override the getArea() method with a runtime exception throw.

public class Box extends Rectangle {

    private int height;

    // Constructor
    public Box(int length, int width, int height) {
        super(length, width);
        this.height = height;
    }

    // Default constructor
    public Box() {
        super();
        this.height = 5;
    }

    // Getters & Setters
    public int getHeight() {
        return height;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public int getVolume() {
        return super.getArea() * height; // Calls Rectangle's protected getArea()
    }

    @Override
    public int getArea() {
      throw new UnsupportedOperationException("getArea() is not supported for Box objects.");
    }

}//Box

It is an annoying work around, but one we can live with.