Summary - Polymorphism

Combine dynamic binding and pattern matching to write a shape renderer.

Summary - Polymorphism

In this group you learned polymorphism: a superclass reference holds a subclass instance; method calls dispatch dynamically. With override you replace a parent method; the @Override annotation checks this at compile time.

Abstract classes and interfaces provide contracts; instanceof and pattern matching let you test types. This summary brings them together in a small task.

Exercise A polymorphic bird list

Define abstract class Bird with an abstract String sound(); method. Define class Sparrow extends Bird that overrides sound with @Override returning "Chirp". Define class Crow extends Bird that overrides sound with @Override returning "Caw". In main, create a List<Bird> birds = new ArrayList<>();, add new Sparrow() and new Crow(), and print each sound on its own line with a for-each loop.

Show example solution

abstract class Bird {
    abstract String sound();
}

class Sparrow extends Bird {
    @Override
    String sound() { return "Chirp"; }
}

class Crow extends Bird {
    @Override
    String sound() { return "Caw"; }
}

public class Main {
    public static void main(String[] args) {
        List<Bird> birds = new ArrayList<>();
        birds.add(new Sparrow());
        birds.add(new Crow());
        for (Bird b : birds) {
            System.out.println(b.sound());
        }
    }
}

Test yourself

Which is the core polymorphism syntax?
- new Circle() = Shape s
- Shape s = new Circle();
- Circle extends Shape implements
- instanceof Circle s
Show answer

Shape s = new Circle(); — A supertype variable receives a subtype instance.

Complete the override annotation

class Sparrow extends Bird {
    ____
    String sound() { return "Chirp"; }
}

Show answer

class Sparrow extends Bird {
    @Override
    String sound() { return "Chirp"; }
}

Match the polymorphism summary

virtual

Default method behavior
abstract

Non-instantiable reference type
interface

Multiple inheritance contract
instanceof

Type check and pattern matching

Next lesson: Model design

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