In software engineering, a software design pattern is a general, reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into source or machine code. Rather, it is a description or template for how to solve a problem that can be used in many different situations.

Creational Patterns

  • Abstract Factory
    Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

  • Builder
    Separete the construction of a complex object from its representation so that the same construction process can create different representations.

  • Factory Method
    Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory method lets a class defer instantiation to subclasses.

  • Prototype
    Specify the kind of objects to create using a prototypical instance, and create new objects by copying this prototype.

  • Singleton
    Ensure a class only has one instance, and provide a global point of access to it.

Structural Patterns

  • Adapter
    Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.

  • Bridge
    Decouple an abstraction from its implementation so that the two can vary independently.

  • Composite
    Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of the objects uniformly.

  • Decorator
    Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

  • Facade
    Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

  • Flyweight
    Use sharing to support large numbers of fine-grained objects efficiently.

  • Proxy
    Provide a surrogate or placeholder for another object to control access to it.

Behavioral Patterns

  • Chain of Responsibility
    Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

  • Command
    Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

  • Interpreter
    Given a language, define a representation for its grammer along with an interpreter that uses the representation to interpret sentences in the language.

  • Iterator
    Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

  • Mediator
    Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

  • Memento
    Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

  • Observer
    Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

  • State
    Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

  • Strategy
    Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

  • Template Method
    Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template methods lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.

  • Visitor
    Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.


  1. Design Patterns: Elements of Reusable Object-Oriented Software