Has-A Relationship - Week 5 Visual

When one class contains instances of another, that's a has-a relationship. Hand has-a Card. Deck has-a Card. But Hand doesn't care that Deck exists.

Two has-a relationships, one shared class. Both Hand and Deck have an ArrayList<Card> field. Neither has any reference to the other — main is the one that tells them to talk. That's loose coupling at work: two classes can collaborate without knowing about each other.

Coupling — the partner concept to cohesion

Loose vs Tight Coupling

Cohesion is about within a class — how related the parts are. Coupling is about between classes — how dependent they are on each other. Aim for loose coupling. The card-game setup above already shows it — here's what would happen if it slipped.

✓ Loose (this is what we built)

Hand and Deck stay independent

Hand and Deck both depend on Card — that's fine, Card is a building block. But neither holds a reference to the other. Change Deck's shuffle algorithm, and Hand doesn't notice.

✕ Tight (avoid this)

Hand reaches into Deck directly

Now Hand stores a Deck field and calls deck.deal() itself. Any change to Deck can break Hand. Tests get harder — you can't test Hand without also building a Deck.