Polymorphism

Definition

Polymorphism means one call can run different code depending on the real object. You call a method on a parent type, and Java picks the child's version while the program runs.

Example code

One loop, three different areas

abstract class Shape {
    abstract double area();
}

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

class Square extends Shape {
    private final double side;
    Square(double side) { this.side = side; }
    double area() { return side * side; }
}

class Triangle extends Shape {
    private final double base, height;
    Triangle(double base, double height) { this.base = base; this.height = height; }
    double area() { return 0.5 * base * height; }
}

class Demo {
    public static void main(String[] args) {
        Shape[] shapes = {new Circle(1), new Square(2), new Triangle(3, 4)};
        for (Shape shape : shapes) {
            // same call, the version that runs depends on the real object
            System.out.printf("%s -> %.2f%n", shape.getClass().getSimpleName(), shape.area());
        }
    }
}

// Output:
// Circle -> 3.14
// Square -> 4.00
// Triangle -> 6.00