Lesson 10: Polymorphism & Method Overriding

Polymorphism lets code work with the parent type while the actual object's overridden method runs — 'the right behavior, chosen at runtime'.

Real-time example

A shipping system calls calculateCost() on any Shipment, whether it's AirShipment or SeaShipment — the caller doesn't care which.

abstract class Shipment {
    abstract double calculateCost(double kg);
}
class AirShipment extends Shipment {
    double calculateCost(double kg) { return kg * 12.5; }
}
class SeaShipment extends Shipment {
    double calculateCost(double kg) { return kg * 3.2; }
}
Shipment s = new AirShipment();
System.out.println(s.calculateCost(20)); // 250.0

What's happening

The variable's declared type is Shipment, but calculateCost runs the actual object's version — dynamic dispatch.

This is why you can add a new shipment type later without touching any code that already calls calculateCost().