When to use a pattern

The smell is a switch. The pattern is a seam.

Use a pattern when you can name the smell in plain language. “Every new vehicle type edits three switches” is Factory Method. “Hourly, airport, and event fees keep landing in ParkingLot” is Strategy. “A payment SDK does not look like Payable” is Adapter. If you cannot name the pain in plain language, you do not need the pattern yet.

// Smell: every new fee rule edits ParkingLot.
class ParkingLot {
  Money fee(Ticket t, Instant exit) {
    if (t.kind() == Kind.HOURLY)  return hourly(t, exit);
    if (t.kind() == Kind.AIRPORT) return airport(t, exit);
    throw new IllegalArgumentException();
  }
}

// Seam: PricingStrategy is the named variation.
// That is Strategy — a behavioral pattern — not a creational one.
class ParkingLot {
  private final PricingStrategy pricing;
  ParkingLot(PricingStrategy pricing) { this.pricing = pricing; }
  Money fee(Ticket t, Instant exit) { return pricing.fee(t, exit); }
}

Notice the family. The switch was about how to compute a fee, not about which class to construct. That is behavioral. A factory would be the wrong first name even though both hide a switch.

All 23 Gang of Four patterns

Five creational, seven structural, eleven behavioral. Each card is a full page. Open the one that matches the smell — do not memorize the list as a checklist.

Smell to family to pattern

Cousins that get mixed up