Java-Inheritance

—Rectangle.class—

public class Rectangle {
    private int length = 0;
    private int breadth = 0;
    public Rectangle() {}
    public Rectangle(int value) {
        length = value;
        breadth = value;
    }
    public Rectangle(int length, int breadth) {
        this.length = length;
        this.breadth = breadth;
    }
    public int getLength() {
        return length;
    }
    public int getBreadth() {
        return breadth;
    }
    public void setLength(int length) {
        this.length = length;
    }
    public void setBreadth(int breadth) {
        this.breadth = breadth;
    }
    public int calcArea() {
        return length * breadth;
    }
}

—TestRectangle.class—

class TestRectangle {
    public static void main(String[] args) {
        Rectangle r1 = new Rectangle();
        System.out.println("r1 Length: " + r1.getLength()); // 0
        System.out.println("r1 Breadth: " + r1.getBreadth()); // 0
        System.out.println("r1 Area: " + r1.calcArea()); // 0
        System.out.println("-----------------------------------------");

        Rectangle r2 = new Rectangle(10);
        System.out.println("r2 Length: " + r2.getLength()); // 10
        System.out.println("r2 Breadth: " + r2.getBreadth()); // 10
        System.out.println("r2 Area: " + r2.calcArea()); // 100
        System.out.println("-----------------------------------------");

        Rectangle r3 = new Rectangle(10, 20);
        System.out.println("r3 Length: " + r3.getLength()); // 10
        System.out.println("r3 Breadth: " + r3.getBreadth()); // 20
        System.out.println("r3 Area: " + r3.calcArea()); // 200
        System.out.println("-----------------------------------------");

        r1.setLength(100);
        r1.setBreadth(200);
        System.out.println("r1 Length: " + r1.getLength()); // 100
        System.out.println("r1 Breadth: " + r1.getBreadth()); // 200
        System.out.println("r1 Area: " + r1.calcArea()); // 20000
    }
}

—Container.class—

public class Container extends Rectangle {
    private int height = 0;

    public Container() {
        super(0);
    }
    public Container(int value) {
        super(value);
        height = value;
    }
    public Container(int length, int breadth, int height) {
        super(length, breadth);
        this.height = height;
    }
    public void setHeight(int height) {
        this.height = height;
    }
    public int getHeight() {
        return height;
    }
    public int calcVolume() {
        return calcArea() * height;
    }
}

—TestContainer.class—

class TestContainer {
    public static void main(String[] args) {
        Container c1 = new Container();
        System.out.println("c1.getLength(): " + c1.getLength()); //0
        System.out.println("c1.getBreadth(): " + c1.getBreadth()); //0
        System.out.println("c1.getHeight(): " + c1.getHeight()); //0
        System.out.println("c1.calcArea(): " + c1.calcArea()); //0
        System.out.println("c1.calcVolume(): " + c1.calcVolume()); //0
        System.out.println("----------------------------------------");

        Container c2 = new Container(10);
        System.out.println("c2.getLength(): " + c2.getLength()); //10
        System.out.println("c2.getBreadth(): " + c2.getBreadth()); //10
        System.out.println("c2.getHeight(): " + c2.getHeight()); //10
        System.out.println("c2.calcArea(): " + c2.calcArea()); //100
        System.out.println("c2.calcVolume(): " + c2.calcVolume()); //1000
        System.out.println("----------------------------------------");

        Container c3 = new Container(10, 20, 30);
        System.out.println("c3.getLength(): " + c3.getLength()); //10
        System.out.println("c3.getBreadth(): " + c3.getBreadth()); //20
        System.out.println("c3.getHeight(): " + c3.getHeight()); //30
        System.out.println("c3.calcArea(): " + c3.calcArea()); //200
        System.out.println("c3.calcVolume(): " + c3.calcVolume()); //6000
    }
}

Rectangle r = new Container(10, 20, 30); /*valid because where ever a class (instance) is expected, we can pass its instance or the instance of its direct or indirect subclass*/

Container c = new Rectangle(10, 20); /*invalid because of the reason above; Here class Rectangle is the super class of class Container*/