Relationships Between Objects

A change to Order might force a change to ReceiptPrinter. The printer only touches an order while print runs. It keeps no reference.

class ReceiptPrinter {
  print(order: Order) {          // Order appears only as a parameter
    return order.lines().map(formatLine).join("\n");
  }
}

A courier always has access to its vehicle through a field, so the link lasts beyond one method call. Association is a dependency that is always reachable.

class Courier {
  constructor(private vehicle: Vehicle) {}  // a lasting link

  deliver(parcel: Parcel) {
    this.vehicle.driveTo(parcel.address);
  }
}

A playlist holds many tracks, but a track exists on its own and can sit in many playlists. Deleting the playlist does not delete the tracks.

class Playlist {
  private tracks: Track[] = [];

  add(track: Track) {             // the track was created elsewhere
    this.tracks.push(track);
  }
}

An order is made of line items, and a line item has no meaning outside its order. The order creates its items and they disappear with it.

class Order {
  private items: LineItem[] = [];

  addProduct(product: Product, qty: number) {
    this.items.push(new LineItem(product, qty)); // the order owns the item's life
  }
}

Song provides the methods that Playable declares. Any code that expects a Playable can receive a Song.

interface Playable {
  play(): void;
}

class Song implements Playable {
  play() { /* stream the audio */ }
}

Podcast inherits both the interface and the implementation of MediaItem, and can extend them. A podcast can be used anywhere a media item is expected.

abstract class MediaItem {
  constructor(protected title: string) {}
  describe() { return this.title; }
}

class Podcast extends MediaItem {
  constructor(title: string, private episode: number) { super(title); }
  describe() { return super.describe() + " #" + this.episode; }
}