1. The document describes 23 design patterns organized into three categories: creational patterns, structural patterns, and behavioral patterns. 2. The creational patterns provide mechanisms to create objects that implement given interfaces without specifying the exact class. The structural patterns focus on relationships and responsibilities between entities. The behavioral patterns are concerned with effective communication between entities. 3. Each pattern describes a recurring problem in object-oriented software design and presents a general reusable solution. The patterns promote loose coupling between classes and allow variation through inheritance.

1. C Abstract Factory S Facade S Proxy Memento
Memento
S Adapter C Factory Method B Observer Caretaker -state
Type: Behavioral
S Bridge S Flyweight C Singleton
What it is:
C Builder B Interpreter B State Without violating encapsulation, capture
and externalize an object's internal state
B Strategy so that the object can be restored to this
B Chain of Responsibility B Iterator Originator
state later.
B Command B Mediator B Template Method -state
B Visitor +setMemento(in m : Memento)
S Composite B Memento +createMemento()
S Decorator C Prototype
«interface»
notifies
«interface» successor Chain of Responsibility Observer Subject «interface»
Handler Observer
Client +attach(in o : Observer)
+handleRequest() Type: Behavioral Type: Behavioral +detach(in o : Observer) +update()
+notify()
What it is: What it is:
Avoid coupling the sender of a request to Define a one-to-many dependency between
its receiver by giving more than one object objects so that when one object changes
a chance to handle the request. Chain the state, all its dependents are notified and
receiving objects and pass the request updated automatically.
along the chain until an object handles it. ConcreteSubject observes ConcreteObserver
ConcreteHandler1 ConcreteHandler2
-subjectState -observerState
+handleRequest() +handleRequest()
+update()
Client Invoker Command State Context
+request()
Type: Behavioral Type: Behavioral «interface»
State
ConcreteCommand What it is: What it is: +handle()
+execute() Encapsulate a request as an object, Allow an object to alter its behavior when
thereby letting you parameterize clients its internal state changes. The object will
with different requests, queue or log appear to change its class.
requests, and support undoable operations.
Command
Receiver
+execute() ConcreteState1 ConcreteState2
+action()
+handle() +handle()
Client
Interpreter Strategy Context
Type: Behavioral «interface»
Type: Behavioral Strategy
«interface»
What it is:
Context AbstractExpression +execute()
What it is: Define a family of algorithms,
+interpret() Given a language, define a representation encapsulate each one, and make them
for its grammar along with an interpreter interchangeable. Lets the algorithm vary
that uses the representation to interpret independently from
sentences in the language. clients that use it.
ConcreteStrategyA ConcreteStrategyB
TerminalExpression NonterminalExpression +execute() +execute()
+interpret() : Context +interpret() : Context
Client
Iterator Template Method AbstractClass
Type: Behavioral Type: Behavioral +templateMethod()
«interface» «interface» #subMethod()
Aggregate Iterator What it is:
What it is:
+createIterator() +next() Provide a way to access the elements of Define the skeleton of an algorithm in an
an aggregate object sequentially without operation, deferring some steps to subclasses.
exposing its underlying representation. Lets subclasses redefine certain steps
of an algorithm without changing the
algorithm's structure. ConcreteClass
ConcreteAggregate ConcreteIterator +subMethod()
+createIterator() : Context +next() : Context
«interface»
informs
«interface» Mediator Visitor Visitor
Client
Mediator +visitElementA(in a : ConcreteElementA)
Colleague Type: Behavioral
Type: Behavioral +visitElementB(in b : ConcreteElementB)
What it is:
What it is: «interface»
Represent an operation to be
Define an object that encapsulates how a Element
performed on the elements of an ConcreteVisitor
set of objects interact. Promotes loose +accept(in v : Visitor)
object structure. Lets you define a
coupling by keeping objects from referring
new operation without changing +visitElementA(in a : ConcreteElementA)
updates to each other explicitly and it lets you vary
the classes of the elements on +visitElementB(in b : ConcreteElementB)
their interactions independently.
ConcreteMediator ConcreteColleague which it operates.
ConcreteElementA
ConcreteElementB
+accept(in v : Visitor)
Copyright © 2007 Jason S. McDonald Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1995). Design Patterns: Elements of +accept(in v : Visitor)
http://www.McDonaldLand.info Reusable Object-Oriented Software. Reading, Massachusetts: Addison Wesley Longman, Inc..

2. «interface»
Adapter
Adapter Proxy Client
Client
+operation() Type: Structural Type: Structural
«interface»
What it is: What it is: Subject
Convert the interface of a class into Provide a surrogate or placeholder for +request()
another interface clients expect. Lets another object to control access to it.
ConcreteAdapter classes work together that couldn't
Adaptee otherwise because of incompatible
-adaptee interfaces.
+adaptedOperation() represents
+operation() RealSubject Proxy
+request() +request()
Abstraction Client
Bridge Abstract Factory
+operation()
«interface»
«interface» Type: Structural
Type: Creational AbstractFactory
Implementor
+createProductA() «interface»
+operationImpl() What it is:
What it is: +createProductB() AbstractProduct
Decouple an abstraction from its
Provides an interface for creating
implementation so that the two can vary
families of related or dependent
independently.
objects without specifying their
concrete class.
ConcreteFactory
ConcreteImplementorA ConcreteImplementorB ConcreteProduct
+createProductA()
+operationImpl() +operationImpl() +createProductB()
«interface»
«interface»
Component Composite Builder Director
Builder
+operation() children
Type: Structural Type: Creational +construct() +buildPart()
+add(in c : Composite)
+remove(in c : Composite)
+getChild(in i : int) What it is: What it is:
Compose objects into tree structures to Separate the construction of a
represent part-whole hierarchies. Lets complex object from its representing
clients treat individual objects and so that the same construction
compositions of objects uniformly. process can create different ConcreteBuilder
Composite representations.
Leaf +buildPart()
+operation() +getResult()
+operation() +add(in c : Composite)
+remove(in c : Composite)
+getChild(in i : int)
«interface»
Component
ConcreteComponent Decorator Factory Method «interface» Creator
+operation() +operation() Product +factoryMethod()
Type: Structural Type: Creational
+anOperation()
Decorator What it is: What it is:
Attach additional responsibilities to an Define an interface for creating an
+operation() object dynamically. Provide a flexible object, but let subclasses decide which
alternative to sub-classing for extending class to instantiate. Lets a class defer
functionality. instantiation to subclasses.
ConcreteDecorator
ConcreteCreator
-addedState ConcreteProduct
+factoryMethod()
+operation()
+addedBehavior()
Client
Facade Facade Prototype
Complex system
Type: Structural Type: Creational «interface»
Prototype
What it is: What it is: +clone()
Provide a unified interface to a set of Specify the kinds of objects to create
interfaces in a subsystem. Defines a high- using a prototypical instance, and
level interface that makes the subsystem create new objects by copying this
easier to use. prototype.
ConcretePrototype1 ConcretePrototype2
+clone() +clone()
«interface»
FlyweightFactory
Flyweight
+getFlyweight(in key) +operation(in extrinsicState)
Flyweight Singleton
Singleton
Type: Structural Type: Creational
-static uniqueInstance
Client What it is:
What it is: -singletonData
Use sharing to support large numbers of Ensure a class only has one instance and +static instance()
fine grained objects efficiently. provide a global point of access to it. +SingletonOperation()
ConcreteFlyweight
-intrinsicState
UnsharedConcreteFlyweight
+operation(in extrinsicState)
-allState
+operation(in extrinsicState) Copyright © 2007 Jason S. McDonald Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1995). Design Patterns: Elements of
http://www.McDonaldLand.info Reusable Object-Oriented Software. Reading, Massachusetts: Addison Wesley Longman, Inc..