public abstract class Shape {
    protected double area;

    public double getArea() {
        return area;
    }

    protected abstract void onAreaChange();
}

public class Triangle extends Shape {
    protected double base;
    protected double height;

    public Triangle setBase(double base) {
        this.base = base;
        onAreaChange();
        return this;
    }

    public Triangle setHeight(double height) {
        this.height = height;
        onAreaChange();
        return this;
    }

    @Override
    public void onAreaChange() {
        this.area = 0.5 * this.base * this.height;
    }
}

public class Rectangle extends Shape {
    protected double height;
    protected double width;

    public Rectangle setHeight(double height) {
        this.height = height;
        onAreaChange();
        return this;
    }

    public Rectangle setWidth(double width) {
        this.width = width;
        onAreaChange();
        return this;
    }

    @Override
    public void onAreaChange() {
        this.area = this.height * this.width;
    }
}

public class Circle extends Shape {
    protected double radius;
    protected final double PI = 3.14159265;

    public Circle setRadius(double radius) {
        this.radius = radius;
        onAreaChange();
        return this;
    }

    @Override
    public void onAreaChange() {
        this.area = this.radius * this.radius * PI;
    }
}

import java.util.Scanner;

public class Main {
    public static Scanner scan = new Scanner(System.in);
    public static void main(String[] args) {
        while (true) {
            System.out.println("Enter the number which you want to compute the area");
            System.out.print("(1) Triangle (2) Rectangle (3) Circle ? ");
            switch (scan.nextInt()) {
            case 1:
                Triangle triangle = new Triangle();
                System.out.print("Base: ");
                triangle.setBase(scan.nextDouble());
                System.out.print("Height: ");
                triangle.setHeight(scan.nextDouble());
                System.out.println("Area of triangle: " + triangle.getArea());
                break;
            case 2:
                Rectangle rectangle = new Rectangle();
                System.out.print("Width: ");
                rectangle.setWidth(scan.nextDouble());
                System.out.print("Height: ");
                rectangle.setHeight(scan.nextDouble());
                System.out.println("Area of rectangle: " + rectangle.getArea());
                break;
            case 3:
                Circle circle = new Circle();
                System.out.print("Radius: ");
                circle.setRadius(scan.nextDouble());
                System.out.println("Area of circle: " + circle.getArea());
                break;
            default:
                System.out.println("What do u mean?");
            }
        }
    }
}