Project 2 - Revised created by Ayysir - https://repl.it/@Ayysir/Project-2-Revised

Circle.java

public class Circle extends Shape {
    private final double radius;
    final double pi = Math.PI;

    public Circle() {
        this(1);
    }   
    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double calculateArea() {
        return pi * Math.pow(radius, 2);
    }
}

Rectangle.java

public class Rectangle extends Shape {
    private final double width, length; //sides

    public Rectangle() {
        this(1,1);
    }
    public Rectangle(double width, double length) {
        this.width = width;
        this.length = length;
    }

    @Override
    public double calculateArea() {
        return width * length;
    }
}

Shape.java

public interface Shape {
    public double calculateArea();
}

TestShape.java

public class TestShape {
    public static void main(String[] args) {

        // Rectangle test
        double width = 5, length = 7;
        Shape rectangle = new Rectangle(width, length);
        System.out.println("Rectangle width: " + width + " and length: " + length
                + "\nArea: " + rectangle.area() + "\n");

        // Circle test
        double radius = 5;
        Shape circle = new Circle(radius);
        System.out.println("Circle radius: " + radius
            + "\nArea: " + circle.area() + "\n");

        // Triangle test
        double a = 5, b = 3, c = 4;
        Shape triangle = new Triangle(a,b,c);
        System.out.println("Triangle sides lengths: " + a + ", " + b + ", " + c
                + "\nArea: " + triangle.area() + "\n");
    }
}

Triangle.java

public class Triangle extends Shape {
    private final double a, b, c;

    public Triangle() {
        this(1,1,1);
    }
    public Triangle(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    @Override
    public double calculateArea() {
        double s = (a + b + c) / 2;
        return Math.sqrt(s * (s - a) * (s - b) * (s - c));
    }
}