Two Subclasses and Polymorphism

The full power of polymorphism appears when two or more subclasses share a parent and one piece of client code processes all of them uniformly. The array holds them as the parent type; each call dispatches to the right override.

This is the foundation of the AP CSA "hierarchy of shapes / vehicles / animals" FRQ pattern — and the cleanest demonstration of why polymorphism matters.

On the AP exam, this appears in MCQs about dispatch and in FRQs that ask you to write a generic processing loop over a mixed array.

Setup

public class Shape {
    public double area() { return 0; }
}

public class Circle extends Shape {
    private double r;
    public Circle(double r) { this.r = r; }
    @Override public double area() { return Math.PI * r * r; }
}

public class Square extends Shape {
    private double s;
    public Square(double s) { this.s = s; }
    @Override public double area() { return s * s; }
}

The polymorphic processor

public static double totalArea(Shape[] shapes) {
    double total = 0;
    for (Shape sh : shapes) {
        total += sh.area();
    }
    return total;
}

One loop. Each iteration calls area(), but the actual computation depends on the object's class.

Why no instanceof?

Polymorphism eliminates the need for explicit type checks. As long as every subclass overrides area(), the loop handles any new subclass added later without modification.