Gang of Four (GoF) Overview

The Complete Gang of Four Pattern Catalog

The Gang of Four identified 23 patterns organized into three categories.

Creational Patterns (5)

Pattern	Purpose	Use When
Abstract Factory	Create families of related objects	Need to create product families without specifying concrete classes
Builder	Construct complex objects step by step	Object has many optional parameters or complex construction
Factory Method	Define interface for creating objects	Don't know exact types of objects needed until runtime
Prototype	Create objects by cloning	Creating new objects is expensive; better to clone existing ones
Singleton	Ensure only one instance exists	Need exactly one instance (config, connection pool, logger)

Structural Patterns (7)

Pattern	Purpose	Use When
Adapter	Make incompatible interfaces compatible	Integrating with legacy code or third-party libraries
Bridge	Separate abstraction from implementation	Want to avoid permanent binding between abstraction and implementation
Composite	Treat individual and composite objects uniformly	Working with tree structures or hierarchies
Decorator	Add responsibilities dynamically	Need to add functionality without subclassing
Facade	Simplify complex subsystems	Need simple interface to complex system
Flyweight	Share objects to save memory	Need many similar objects; memory is a concern
Proxy	Control access to an object	Need lazy initialization, access control, or logging

Behavioral Patterns (11)

Pattern	Purpose	Use When
Chain of Responsibility	Pass request along chain of handlers	Multiple objects can handle a request
Command	Encapsulate request as object	Need to parameterize objects with operations, queue requests, or support undo
Interpreter	Define grammar and interpret sentences	Need to interpret a language or expressions
Iterator	Access elements sequentially	Need to traverse collection without exposing structure
Mediator	Centralize complex communications	Objects communicate in complex ways; want to reduce coupling
Memento	Capture and restore object state	Need to implement undo/redo or snapshots
Observer	Notify dependents of state changes	Change to one object requires changes to others
State	Alter behavior when state changes	Object behavior depends on its state
Strategy	Encapsulate interchangeable algorithms	Need different variants of an algorithm
Template Method	Define algorithm skeleton	Have algorithm with steps that can vary
Visitor	Add operations to object structure	Need to perform operations on elements of complex object structure