What are the three categories of GoF patterns?

Creational patterns deal with object creation (5 patterns like Factory Method and Singleton). Structural patterns compose objects into larger structures (7, like Adapter and Decorator). Behavioural patterns manage communication and responsibility (11, like Observer and Strategy).

What is the difference between Strategy and State?

Both swap behaviour behind an interface. Strategy lets a client choose an interchangeable algorithm, like a sorting or pricing method. State changes behaviour as an object moves through its own lifecycle, and the transitions are usually internal rather than client-selected.

When should I avoid the Singleton pattern?

Singletons introduce hidden global state that makes testing and concurrency harder, so use them sparingly. Prefer passing dependencies explicitly (dependency injection). Reserve a true singleton for genuinely single resources like a configuration registry or connection pool.

Are these patterns still relevant in modern languages?

Yes, though some are built into the language. Iterator and Observer are often native, and first-class functions can replace Strategy or Command with a closure. The patterns remain a shared vocabulary for describing structure and intent across teams.

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Gang of Four Design Pattern Reference

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A shared vocabulary for object-oriented design

The 1994 book Design Patterns by Gamma, Helm, Johnson and Vlissides — the Gang of Four — catalogued 23 reusable solutions to recurring design problems. This reference lists all 23, grouped by purpose, with each pattern’s intent and a concrete use case so you can recognise where one fits. Search by keyword or filter by group.

How it works

The patterns split into three families by what they manage:

Creational (5)   → how objects are created
  Abstract Factory, Builder, Factory Method, Prototype, Singleton
Structural (7)   → how objects are composed
  Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy
Behavioural (11) → how objects communicate and share responsibility
  Chain of Responsibility, Command, Interpreter, Iterator, Mediator,
  Memento, Observer, State, Strategy, Template Method, Visitor

Each pattern names a problem and a tested solution, so “use an Adapter here” or “this needs an Observer” conveys a whole design in two words. The value is the shared vocabulary as much as the code structure.

Tips and notes

Reach for a pattern when it genuinely simplifies a design — over-applying them adds indirection without benefit.
Several patterns are now language features: iteration, events and first-class functions can replace Iterator, Observer and Strategy boilerplate.
Decorator, Proxy and Adapter look similar but differ in intent: Decorator adds behaviour, Proxy controls access, and Adapter changes an interface.
Favour composition (Strategy, Decorator) over deep inheritance hierarchies.