Question 24 of 28: Sample Code for Polymorphism, Dynamic Binding, Overloading, and Overriding

Question 24: Sample Code for Polymorphism, Dynamic Binding, Overloading, and Overriding (Part V)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Polymorphism is the ability of a single name (here, a Shape reference) to refer to objects of more than one type. Dynamic binding is the runtime mechanism that resolves shape.area() to the actual overriding method based on the object's real type. Overriding is Circle/Square each supplying their own area() with the identical signature declared in Shape. Overloading is describe() appearing twice with different parameter lists, resolved at compile time.

abstract class Shape {
    abstract double area();                 // overridden below
}
class Circle extends Shape {
    double radius;
    Circle(double r) { radius = r; }
    double area() { return Math.PI * radius * radius; }   // OVERRIDING
}
class Square extends Shape {
    double side;
    Square(double s) { side = s; }
    double area() { return side * side; }                 // OVERRIDING
}
class Printer {
    void describe(Shape s) {                               // OVERLOADED #1
        System.out.println("Shape area: " + s.area());
    }
    void describe(String label, Shape s) {                 // OVERLOADED #2
        System.out.println(label + " area: " + s.area());
    }
}
// POLYMORPHISM + DYNAMIC BINDING:
Shape[] shapes = { new Circle(2.0), new Square(3.0) };
Printer p = new Printer();
for (Shape shape : shapes) {
    p.describe(shape);   // shape.area() dispatches at RUNTIME to Circle.area() or Square.area()
}

The shapes array holds references of the single declared type Shape (polymorphism), yet each call to shape.area() inside the loop resolves at runtime to whichever concrete subclass's overriding method the object actually is (dynamic binding) — whereas the choice between describe(Shape) and describe(String, Shape) is settled by the compiler from the number/type of arguments at the call site (overloading), with no runtime dispatch involved at all.