Design Patterns

In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations. Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved. Many patterns imply object-orientation or more generally mutable state, and so may not be as applicable in functional programming languages, in which data is immutable or treated as such.

Design patterns reside in the domain of modules and interconnections. At a higher level there are architectural patterns that are larger in scope, usually describing an overall pattern followed by an entire system.

There are many types of design patterns, like

  • Algorithm strategy patterns addressing concerns related to high-level strategies describing how to exploit application characteristic on a computing platform.
  • Computational design patterns addressing concerns related to key computation identification.
  • Execution patterns that address concerns related to supporting application execution, including strategies in executing streams of tasks and building blocks to support task synchronization.
  • Implementation strategy patterns addressing concerns related to implementing source code to support
  1. program organization, and
  2. the common data structures specific to parallel programming.
  • Structural design patterns addressing concerns related to high-level structures of applications being developed.

Design Patterns by Jason McDonald

Creational patterns

Abstract factory

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


Separate the construction of a complex object from its representation allowing the same construction process to create various 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 (dependency injection).

Lazy initialization

Tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed.


Ensure a class has only named instances, and provide global point of access to them.

Object pool

Avoid expensive acquisition and release of resources by recycling objects that are no longer in use. Can be considered a generalisation of connection pool and thread pool patterns.


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

Registry (Multiton)

Resource acquisition is initialization

Ensure that resources are properly released by tying them to the lifespan of suitable objects.


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

Structural patterns

Adapter or Wrapper

Convert the interface of a class into another interface clients expect. An adapter lets classes work together that could not otherwise because of incompatible interfaces.


Decouple an abstraction from its implementation allowing the two to vary independently.


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


Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality.


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.

Front Controller

The pattern relates to the design of web applications. It provides a centralized entry point for handling requests.


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


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

Behavioral patterns


Generalized observer, which allows multiple readers and writers. Communicates information system-wide.

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.


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


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


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


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.


Without violating encapsulation, capture and externalize an object's internal state allowing the object to be restored to this state later.

Null object

Avoid null references by providing a default object.

Observer or Publish/subscribe

Define a one-to-many dependency between objects where a state change in one object results with all its dependents being notified and updated automatically.


Define common functionality for a group of classes


Recombinable business logic in a Boolean fashion


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


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 method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.


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.

Concurrency patterns

Active Object

Decouples method execution from method invocation that reside in their own thread of control. The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests.


Only execute an action on an object when the object is in a particular state.

Binding properties

Combining multiple observers to force properties in different objects to be synchronized or coordinated in some way.

Messaging design pattern (MDP)

Allows the interchange of information (i.e. messages) between components and applications.

Double-checked locking

Reduce the overhead of acquiring a lock by first testing the locking criterion (the 'lock hint') in an unsafe manner; only if that succeeds does the actual lock proceed.
Can be unsafe when implemented in some language/hardware combinations. It can therefore sometimes be considered an anti-pattern.

Event-based asynchronous

Addresses problems with the asynchronous pattern that occur in multithreaded programs.

Guarded suspension

Manages operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed.


One thread puts a "lock" on a resource, preventing other threads from accessing or modifying it.

Monitor object

An object whose methods are subject to mutual exclusion, thus preventing multiple objects from erroneously trying to use it at the same time.


A reactor object provides an asynchronous interface to resources that must be handled synchronously.

Read-write lock

Allows concurrent read access to an object, but requires exclusive access for write operations.


Explicitly control when threads may execute single-threaded code.

Thread pool

A number of threads are created to perform a number of tasks, which are usually organized in a queue. Typically, there are many more tasks than threads. Can be considered a special case of the object pool pattern.

Thread-specific storage

Static or "global" memory local to a thread.

GOF Design Patterns (Quick References) :



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