Java ProPolymorphism: Shape base class with Circle, Square, and Triangle subclasses

// Shape.java
// Define an abstract class named Shape
abstract class Shape {

  // Declare an abstract method draw, to be implemented by subclasses
  public abstract void draw();

  // Declare an abstract method calculateArea, to be implemented by subclasses
  public abstract double calculateArea();
} 

// Circle.java
// Define a class named Circle that extends Shape
class Circle extends Shape {
  
  // Declare a private double variable radius
  private double radius;

  // Define a constructor that takes a double radius as a parameter
  public Circle(double radius) {
    // Assign the parameter radius to the instance variable radius
    this.radius = radius;
  }

  // Override the draw method from the Shape class
  @Override
  public void draw() {
    // Print "Drawing a circle" to the console
    System.out.println("Drawing a circle");
  }

  // Override the calculateArea method from the Shape class
  @Override
  public double calculateArea() {
    // Return the area of the circle using the formula π * radius^2
    return Math.PI * radius * radius;
  }
}

// Square.java
// Define a class named Square that extends Shape
class Square extends Shape {

  // Declare a private double variable side
  private double side;

  // Define a constructor that takes a double side as a parameter
  public Square(double side) {
    // Assign the parameter side to the instance variable side
    this.side = side;
  }

  // Override the draw method from the Shape class
  @Override
  public void draw() {
    // Print "Drawing a square" to the console
    System.out.println("Drawing a square");
  }

  // Override the calculateArea method from the Shape class
  @Override
  public double calculateArea() {
    // Return the area of the square using the formula side * side
    return side * side;
  }
}

// Triangle.java
// Define a class named Triangle that extends Shape
class Triangle extends Shape {
  
  // Declare private double variables base and height
  private double base;
  private double height;

  // Define a constructor that takes a double base and a double height as parameters
  public Triangle(double base, double height) {
    // Assign the parameter base to the instance variable base
    this.base = base;
    // Assign the parameter height to the instance variable height
    this.height = height;
  }

  // Override the draw method from the Shape class
  @Override
  public void draw() {
    // Print "Drawing a triangle" to the console
    System.out.println("Drawing a triangle");
  }

  // Override the calculateArea method from the Shape class
  @Override
  public double calculateArea() {
    // Return the area of the triangle using the formula 0.5 * base * height
    return 0.5 * base * height;
  }
}

// Main.java
// Define the Main class
public class Main {

  // Define the main method, the entry point of the program
  public static void main(String[] args) {
    // Create an instance of Circle with radius 7.0 and assign it to the variable circle of type Shape
    Shape circle = new Circle(7.0);
    // Create an instance of Square with side length 12.0 and assign it to the variable square of type Shape
    Shape square = new Square(12.0);
    // Create an instance of Triangle with base 5.0 and height 3.0 and assign it to the variable triangle of type Shape
    Shape triangle = new Triangle(5.0, 3.0);

    // Call the drawShapeAndCalculateArea method with circle as the argument
    drawShapeAndCalculateArea(circle);
    // Call the drawShapeAndCalculateArea method with square as the argument
    drawShapeAndCalculateArea(square);
    // Call the drawShapeAndCalculateArea method with triangle as the argument
    drawShapeAndCalculateArea(triangle);
  }

  // Define the drawShapeAndCalculateArea method that takes a Shape object as a parameter
  public static void drawShapeAndCalculateArea(Shape shape) {
    // Call the draw method on the shape object
    shape.draw();
    // Call the calculateArea method on the shape object and store the result in a variable area
    double area = shape.calculateArea();
    // Print the area of the shape to the console
    System.out.println("Area: " + area);
  }
}