by Michael Thomas
Java Design Patterns Home Page
This is a survival guide for Java Design Patterns.
The 23 Gof (Gang of Four) design patterns comes from the book: Design Patterns: Elements of
Reusable Object-Oriented Software, Addison-Wesley, by Erich Gamma, Richard Helm,
Ralph Johnson, John Vissides. All 23 patterns are listed when you first open the
book.
Grouping of Patterns (by Wikipedia)
| Design Pattern | Description |
Creational Patterns |
|
| Abstract Factory (Group: Creational Patterns) |
Wikipedia: Abstract Factory groups object factories that have a common theme. Gof: Provide an interface for creating families of related or dependent objects without specifying their concrete classes. (pg 87) |
| Builder (Group: Creational Patterns) |
Wikipedia:
Builder constructs complex objects by separating construction and
representation. Gof: Separate the construction of a complex object from its representation so that the same construction processes can create different representations. (pg 97) |
| Factory Method (Group: Creational Patterns) |
Wikipedia:
Factory Method creates objects without specifying the exact class to create. Gof: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. (pg 107) |
| Prototype (Group: Creational Patterns) |
Wikipedia:
Prototype creates objects by cloning an existing object. Gof: Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. (pg 117) |
|
Singleton (Group: Creational Patterns) |
Wikipedia's
Singleton:
Singleton restricts object creation for a class to only one instance. Gof: Ensure a class only has one instance, and provide a global point of access to it. (pg 127) |
Structural Patterns |
|
| Adapter (Group: Structural patterns) |
Wikipedia: Adapter allows classes with incompatible interfaces to work together by
wrapping its own interface around that of an already existing class. Gof: Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces. (pg 139) |
| Bridge (Group: Structural patterns) |
Wikipedia:
Bridge decouples an abstraction from its implementation so that the two can vary
independently. Gof: Decouple an abstraction from its implementation so that the two can vary independently. (pg 151) |
| Composite (Group: Structural patterns) |
Wikipedia:
Composite composes zero-or-more similar objects so that they can be manipulated
as one object. Gof: Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. (pg 163) |
| Decorator (Group: Structural patterns) |
Wikipedia:
Decorator dynamically adds/overrides behavior in an existing method of an
object. Gof: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. (pg 175) |
| Facade (Group: Structural patterns) |
Wikipedia:
Facade provides a simplified interface to a large body of code. Gof: Provide a unified interface to a set of interfaces in a system. Facade defines a higher-level interface that makes the subsystem easier to use. (pg 175) |
| Flyweight (Group: Structural patterns) |
Wikipedia:
Flyweight reduces the cost of creating and manipulating a large number of
similar objects. Gof: Use sharing to support large numbers of fine-grained objects efficiently. A flyweight is a shared object that can be used in multiple contexts simultaneously. The flyweight acts as an independent object in each contect - it's indistinguishable from an instance of the object that's not shared. (pg 185) |
| Proxy (Group: Structural patterns) |
Wikipedia:
Proxy provides a placeholder for another object to control access, reduce cost,
and reduce complexity. Gof: Provide a surrogate or placeholder for another object to control access to it. (pg 207) |
Behavioral Patterns |
|
| Chain of Responsibility (Group: Behavioral patterns) |
Wikipedia: Chain of responsibility delegates commands to a chain of processing objects. Gof: 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. (pg 223) |
| Command (Group: Behavioral patterns) |
Wikipedia:
Command creates objects which encapsulate actions and parameters. Gof: Encapsulate a request as an object, thereby letting you parameterize clients with different request, queue or log request, and support undoable operations. (pg 223) |
| Interpreter (Group: Behavioral patterns) |
Wikipedia:
Interpreter implements a specialized language. Gof: Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. (pg 243) |
| Iterator (Group: Behavioral patterns) |
Wikipedia:
Iterator accesses the elements of an object sequentially without exposing its
underlying representation. Gof: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representations. (pg 257) |
| Mediator (Group: Behavioral patterns) |
Wikipedia:
Mediator allows loose coupling between classes by being the only class that has
detailed knowledge of their methods. Gof: 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. (pg 273) |
| Memento (Group: Behavioral patterns) |
Wikipedia:
Memento provides the ability to restore an object to its previous state (undo). Gof: Without violating encapsulation, capture and enternalize an object's internal state so that the object can be restored to this state later. (pg 283) |
| Observer (Group: Behavioral patterns) |
Wikipedia:
Observer is a publish/subscribe pattern which allows a number of observer
objects to see an event. Gof: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. (pg 293) |
| State (Group: Behavioral patterns) |
Wikipedia:
State allows an object to alter its behavior when its internal state changes. Gof: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. (pg 305) |
| Strategy (Group: Behavioral patterns) |
Wikipedia:
Strategy allows one of a family of algorithms to be selected on-the-fly at
runtime. Gof: Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. (pg 315) |
| Template Method (Group: Behavioral patterns) |
Wikipedia:
Template method defines the skeleton of an algorithm as an abstract class,
allowing its subclasses to provide concrete behavior. (pg 325) Gof: Define the skeleton of an algorithm without changing the algorithm's structure. |
| Visitor (Group: Behavioral patterns) |
Wikipedia:
Visitor separates an algorithm from an object structure by moving the hierarchy
of methods into one object. Gof: 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. (pg 331) |