Composition over inheritance

Definition

Composition means building a class by holding other objects and using them, instead of extending a class. It keeps classes loosely coupled and easy to change, so it is usually preferred over inheritance when you only want to reuse behaviour.

Example code

A notifier that can swap how it sends messages

interface Sender {
    void send(String message);
}

class EmailSender implements Sender {
    public void send(String message) { System.out.println("Email: " + message); }
}

class SmsSender implements Sender {
    public void send(String message) { System.out.println("SMS: " + message); }
}

class OrderNotifier {
    private final Sender sender;                 // "has a" sender, not "is a" sender

    OrderNotifier(Sender sender) { this.sender = sender; }

    void orderShipped(int orderId) { sender.send("Order " + orderId + " has shipped"); }
}

class Demo {
    public static void main(String[] args) {
        new OrderNotifier(new EmailSender()).orderShipped(101);
        new OrderNotifier(new SmsSender()).orderShipped(102);   // swapped without touching OrderNotifier
    }
}

// Output:
// Email: Order 101 has shipped
// SMS: Order 102 has shipped