SOLID - Liskov Substitution Principle

/* Liskov Substitution Principle */

// Before
class Rectangle {

    protected int width;
    protected int height;

    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public int getWidth() {
        return width;
    }

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

    public int getHeight() {
        return height;
    }

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

    public int area() {
        return width * height;
    }

}

class Square extends Rectangle {

    public Square(int side) {
        super(side, side);
    }

}

class App {

    public static void main(String[] args) {
        Rectangle rectangle = new Rectangle(20, 40);
        Square square = new Square(20);

        rectangle.area(); // 800
        square.area(); // 400

        modifyRectangle(rectangle);
        modifyRectangle(square);

        rectangle.area(); // 1000
        square.area(); // 1000 instead of 2500
    }

    static void modifyRectangle(Rectangle rectangle) {
        rectangle.setHeight(50);
    }

}

// After
class Rectangle {

    private int width;
    private int height;

    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public int getWidth() {
        return width;
    }

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

    public int getHeight() {
        return height;
    }

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

    public int area() {
        return width * height;
    }

}

class Square {

    private int side;

    public Square(int side) {
        this.side = side;
    }

    public int getSide() {
        return side;
    }

    public void setSide(int side) {
        this.side = side;
    }

    public int area() {
        return side * side;
    }

}

class App {

    public static void main(String[] args) {
        Rectangle rectangle = new Rectangle(20, 40);
        Square square = new Square(20);

        rectangle.area(); // 800
        square.area(); // 400

        modifyRectangle(rectangle);
        modifySquare(square);

        rectangle.area(); // 1000
        square.area(); // 2500
    }

    static void modifyRectangle(Rectangle rectangle) {
        rectangle.setHeight(50);
    }

    static void modifySquare(Square square) {
        square.setSide(50);
    }

}