Design Pattern Quick Reference (Applied)

The OOP module covers each Gang of Four pattern in depth — intent, structure, participants, full Java implementations. This topic is the applied companion: not what each pattern is, but the trigger phrase in an LLD prompt that should make you reach for it. In an interview you don't have time to recall a UML diagram; you need a fast mapping from a symptom in the requirements to the pattern that resolves it.

Pattern as a response to an axis of change

Every pattern exists to absorb one kind of variation without forcing edits to existing code (Open/Closed). The skill is recognizing that axis in the prompt:

• "Pricing can be hourly, flat, or weekend, and we'll add more." → Strategy
• "Different displays should update when a spot frees up." → Observer
• "An order moves through Placed → Paid → Shipped, with different rules per stage." → State
• "We need to create cars, bikes, or trucks by type." → Factory

If you can't name the axis of variation, you don't need the pattern — see the anti-patterns question in the framework topic.

The three families

CREATIONAL   "How do I build this object?"     Factory, Abstract Factory,
                                             Builder, Singleton
STRUCTURAL   "How do I compose / wrap it?"     Decorator, Adapter, Proxy,
                                             Facade, Composite
BEHAVIORAL   "How do objects interact?"        Strategy, Observer, State,
                                             Template Method, Command,
                                             Chain of Responsibility

How to use this in the round

When you hit step 5 of the framework (apply patterns), scan the requirements for these trigger phrases. Name the pattern and the phrase that justified it: "because pricing varies along an axis we expect to grow, I'll put it behind a PricingStrategy interface." That single sentence demonstrates you reach for patterns deliberately, not decoratively.

The three questions below are organized by family, each giving the trigger, a one-line "reach for it when," and a tiny Java sketch you can reproduce under time pressure. For the deeper theory and trade-offs, cross-reference the OOP module's creational, structural, and behavioral pattern topics.