Composition over inheritance

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 the fragile base class problem in your own words
2. Rewrite a small inheritance hierarchy as composed objects and functions
3. Spot a subclass created to reuse a single method

In practice

A few lines that show Composition over inheritance doing its job.

type Logger = { log(message: string): void };
type Clock = { now(): Date };

class OrderService {
  constructor(private readonly logger: Logger, private readonly clock: Clock) {}

  place(order: Order) {
    this.logger.log("placed at " + this.clock.now().toISOString());
  }
}

new OrderService(console, { now: () => new Date() });

The common mistake

What to know first

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

• Implementing interfaces

How it stays learned

Once a session shows you understand Composition over inheritance, 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