Challenge 1: A Rectangle Class with a Constructor and area() — Possible Solution
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Rectangle.java:

public class Rectangle {
    private double width;
    private double height;

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

    public double area() {
        return width * height;
    }

    public static void main(String[] args) {
        Rectangle r = new Rectangle(4.0, 5.0);
        System.out.println("Area: " + r.area());
    }
}

Output:

    Area: 20.0

Explanation:
width and height are private, so nothing outside Rectangle can read
or write them directly. The constructor parameters share the fields'
names on purpose, so this.width/this.height are required inside the
constructor to distinguish the field from the parameter -- without
this, `width = width;` would just assign the parameter to itself.

WHY THIS WORKS AS AN ANSWER
------------------------------
Both fields are private per the chapter's own "default to private"
guidance, the constructor uses this to disambiguate exactly as
demonstrated in the chapter's own Account example, and area() is a
public instance method computing a real value from that private state.