Adding behaviour by adding classes without touching existing code
suggest changeimport 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
- The
MakeThingsFly
class can work with everything that is of typeFlyingMachine
. - The method
letTheMachinesFly
also works without any change (!) when you add a new class, for examplePropellerPlane
:
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.
Found a mistake? Have a question or improvement idea?
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