AP Computer Science Exam Question: Inheritance and Polymorphism

Question:

Consider the following hierarchy of classes representing different shapes:

class Shape {
    protected String color;
    protected double area;
    
    public double calculateArea() {
        return area;
    }
}

class Circle extends Shape {
    private double radius;

    public Circle(String color, double radius) {
        this.color = color;
        this.radius = radius;
    }

    public double calculateArea() {
        area = Math.PI * Math.pow(radius, 2);
        return area;
    }
}

class Rectangle extends Shape {
    private double length;
    private double width;

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

    public double calculateArea() {
        area = length * width;
        return area;
    }
}

class Triangle extends Shape {
    private double base;
    private double height;

    public Triangle(String color, double base, double height) {
        this.color = color;
        this.base = base;
        this.height = height;
    }

    public double calculateArea() {
        area = 0.5 * base * height;
        return area;
    }
}

You are given a class TestShapes which has the following method:

public class TestShapes {
    public static void main(String[] args) {
        Shape[] shapes = {
                new Circle("Red", 5.0),
                new Rectangle("Blue", 6.0, 9.0),
                new Triangle("Green", 4.5, 7.0)
        };

        for (Shape shape : shapes) {
            System.out.print("The " + shape.getColor() + " " + shape.getClass().getSimpleName() + " has an area of ");
            System.out.println(shape.calculateArea());
        }
    }
}

1. Explain why the calculateArea methods in the subclasses Circle, Rectangle, and Triangle are declared with the @Override annotation. (4 points)
2. In the TestShapes class, modify the loop to calculate the total area of all the shapes and print it. (6 points)

Answer:

1. The @Override annotation is used to indicate that a method in a subclass is intended to override a method in the superclass. It helps to ensure that the method being overridden is spelled correctly and has the correct signature. In this case, the calculateArea methods in the subclasses Circle, Rectangle, and Triangle are meant to override the calculateArea method in the Shape class. By declaring them with @Override, it provides a clearer indication of the intention and helps the compiler to catch any mistakes or inconsistencies.

2. To modify the loop in the TestShapes class to calculate the total area of all the shapes, we can introduce a variable totalArea initialized to 0 before the loop. Inside the loop, we can add the area of each shape to the totalArea variable. Finally, we print the totalArea at the end of the loop. Below is the modified code:

public class TestShapes {
    public static void main(String[] args) {
        Shape[] shapes = {
                new Circle("Red", 5.0),
                new Rectangle("Blue", 6.0, 9.0),
                new Triangle("Green", 4.5, 7.0)
        };

        double totalArea = 0.0;

        for (Shape shape : shapes) {
            System.out.print("The " + shape.getColor() + " " + shape.getClass().getSimpleName() + " has an area of ");
            double area = shape.calculateArea();
            System.out.println(area);
            totalArea += area;
        }

        System.out.println("\nThe total area of all shapes is: " + totalArea);
    }
}

By introducing the totalArea variable and accumulating the area of each shape, we can calculate the total area of all the shapes. Finally, we print the total area after the loop.

The program will produce output similar to:

The Red Circle has an area of 78.53981633974483
The Blue Rectangle has an area of 54.0
The Green Triangle has an area of 15.75

The total area of all shapes is: 148.28981633974482

The total area of all shapes is calculated by adding up the individual areas of each shape. In this case, the total area is 148.2898 (rounded to 4 decimal places).