Java Language Polymorphism Adding behaviour by adding classes without touching existing code


Example

import java.util.ArrayList;
import java.util.List;

import static java.lang.System.out;

public class PolymorphismDemo {

    public static void main(String[] args) {
        List<FlyingMachine> machines = new ArrayList<FlyingMachine>();
        machines.add(new FlyingMachine());
        machines.add(new Jet());
        machines.add(new Helicopter());
        machines.add(new Jet());

        new MakeThingsFly().letTheMachinesFly(machines);
    }
}

class MakeThingsFly {
    public void letTheMachinesFly(List<FlyingMachine> flyingMachines) {
        for (FlyingMachine flyingMachine : flyingMachines) {
            flyingMachine.fly();
        }
    }
}

class FlyingMachine {
    public void fly() {
        out.println("No implementation");
    }
}

class Jet extends FlyingMachine {
    @Override
    public void fly() {
        out.println("Start, taxi, fly");
    }

    public void bombardment() {
        out.println("Fire missile");
    }
}

class Helicopter extends FlyingMachine {
    @Override
    public void fly() {
        out.println("Start vertically, hover, fly");
    }
}

Explanation

a) The MakeThingsFly class can work with everything that is of type FlyingMachine.

b) The method letTheMachinesFly also works without any change (!) when you add a new class, for example PropellerPlane:

public void letTheMachinesFly(List<FlyingMachine> flyingMachines) {
        for (FlyingMachine flyingMachine : flyingMachines) {
            flyingMachine.fly();
        }
    }
}

That's the power of polymorphism. You can implement the open-closed-principle with it.