Composition

What a session covers

The tutor works through these with you, in conversation and code. A session ends when you have shown them, not when a video does.

1. Explain how delegation differs from extending a class
2. Build a class from objects it holds and delegates to
3. Spot the class that extends HashMap to borrow one method

In practice

A few lines that show Composition doing its job.

public class OrderService {
    private final OrderRepository orders;
    private final PaymentGateway payments;
    private final Clock clock;

    public OrderService(OrderRepository orders, PaymentGateway payments, Clock clock) {
        this.orders = orders; this.payments = payments; this.clock = clock;
    }

    public Receipt place(Order order) {
        var charge = payments.charge(order.total());
        return orders.save(order.paidAt(clock.instant(), charge));
    }
}

The common mistake

What to know first

Your roadmap puts these before Composition, unless you already know them.

• Interfaces and abstract classes

How it stays learned

Once a session shows you understand Composition, it comes back for review. Each review takes a minute and tells the memory model how well you still know it.

1. Day 3First recall, while it is fresh
2. Day 7Second, a week on
3. Day 18Last, when it has settled

Where it leads

Topics that build on Composition.

• Dependency injection