Challenge 1: Shape and a Circle Subclass — Possible Solution
====================================================================

Shape.java:

public class Shape {
    protected String name;

    public Shape(String name) {
        this.name = name;
    }

    public double area() {
        return 0.0;
    }
}

Circle.java:

public class Circle extends Shape {
    private double radius;

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

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }

    public static void main(String[] args) {
        Circle c = new Circle(3.0);
        System.out.println(c.name + " area: " + c.area());
    }
}

Output:

    Circle area: 28.274333882308138

Explanation:
Circle extends Shape (exactly one superclass), and its constructor's
first statement is super("Circle"), setting the inherited protected
name field. @Override marks area() as intentionally replacing Shape's
own 0.0-returning version, and the compiler confirms the signature
genuinely matches an existing superclass method.

WHY THIS WORKS AS AN ANSWER
------------------------------
This follows the chapter's own Animal/Dog shape exactly -- a single
extends, an explicit super() call as the constructor's first
statement, and @Override used correctly on a method that truly
overrides its superclass's version.