GOF Design Patterns Implementations

Creational patterns - concern the process of object creation.

1. 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. Note: Returns various instances, Multiple constructors, Interface driven, Adaptable to environment more easily Concepts:

   • Doesn't expose instantiation logic
   • Defer to subclasses
   • Common interface
   • Specified by architecture, implemented by user Examples: java.util.Calendar, java.util.ResourceBundle, java.text.NumberFormat

2. Abstract Factory - Provide an interface for creating families of related or dependent objects without specifying their concrete classes. Note: Implemented with a Factory, Hides the Factory, Abstracts Environment, Build through Composition Short definition: Factory of factories. Concepts:

   • Factory of Factories
   • Factory of related objects
   • Common Interface
   • Defer to Subclasses Examples: javax.xml.parsers.DocumentBuilderFactory

3. Builder - Separate the construction of a complex object from its representation so that the same construction process can create different representations. Note: Handles complex constructors, large number of parameters, immutability Short definition: Pattern for handling the construction of objects that might contain a lot of parameters, and we want to make the object immutable when we done the construction. Concepts:

   • Handles complex constructors
   • Large number of parameters
   • Immutability Example: java.lang.StringBuilder

4. Prototype - Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Prototype is often an alternative to Abstract Factory. Short definition: Used when the type of object to create is determined by a prototypical instance which is cloned to produce an new instance. Ofter prototype is used to get a unique instance of the same object. Concepts:

   • Avoids costly creation
   • Avoids subclassing
   • Typically doesn't use "new"
   • Often utilizes an Interface
   • Usually implemented with a Registry Examples: java.lang.Object#clone()

5. Singleton - Ensure a class only has one instance, and provide a global point of access to it. Note: getInstance method shouldn't contains any arguments, otherwise it should me Factory patterns. Concepts:

   • Only one instance creted
   • Guarantees control of a resource
   • Usually lazily loaded Example: java.lang.Runtime, Spring beans with Singleton scope

Structural patterns deal with the composition of classes or objects.

6. 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. Note: Works after code is designed, Legacy Short definition: Pattern for connecting new code to legacy code, without having to change the working contract that was produced from the legacy code originally. Concepts:

   • Plug adaptor
   • Convert interface into another interface
   • Legacy
   • Translate requests
   • Client, Adapter, Adaptee Example: Arrays -> Lists (not exactly, just functionality...), Streams.

7. Bridge - Decouple an abstraction from its implementation so that the two can vary independently. Short definition: The Bridge pattern is very similar to the Adapter with a main difference: bridge works with new code whereas an Adapter works with legacy code. Concepts:

   • Decouple Abstraction and implementation
   • Encapsulation, Composition, Inheritence
   • Changes in Abstraction won't affect client
   • Details won't be right Examples: JDBC Driver (DriverManager.registerDriver())...

8. Composite - Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat Short definition: Composite is a hierarchical type pattern that deals with tree structures of information. individual objects and compositions of objects uniformly. Composite is often used with Iterator or Visitor. Concepts:

   • Components represent part of whole structure
   • Compose objects into tree structures
   • Individual object treated as a Composite
   • Some operations applied on individual and composites Examples: java.awt.Component, RESTful service GETs

9. Decorator - Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. Structure diagrams of Composite and Decorator are similar Concepts:

   • Also called wrapper
   • Add behavior without affecting others
   • More then just inheritance
   • Single Responsibility Principle
   • Compose behavior dynamically Examples: java.io.InputStream, java.io.BufferedReader

10. 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. Concepts:

    • Make an API easier to use
    • Reduce dependencies on outside code
    • Simplify the interface or client usage
    • Usually a refactoring pattern Examples: java.net.URL Design:
    • Class that utilizes composition
    • Shouldn't have a need for inheritance
    • Typically encompasses full lifecycle

11. Flyweight - Use sharing to support large numbers of fine-grained objects efficiently. Concepts:

    • More efficient use of memory
    • Large number of similar objects
    • Immutable
    • Most of the object states can be extrinsic Examples: java.lang.String, Integer.valueOf - pools

12. Proxy - Provide a surrogate or placeholder for another object to control access to it. Short Definition: Proxy - is a pattern that act as an interface to something else. Concepts:

    • Interface by wrapping
    • Can add functionality
    • Security, Simplicity, Remote, Cost
    • Proxy called to access real object Examples: java.lang.reflect.Proxy, java.rmi.*,

Behavioral patterns characterize the ways in which classes or objects interact and distribute responsibility.

13. Interpreter - Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language. Short Definition: It's behavioral pattern that you use to represent the grammar of the language. Concepts:

    • Represent grammar
    • Interpret a sentence
    • Map a domain
    • AST Examples: java.util.Pattern, java.text.Format

14. 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. Short Definition: Used to define an algorithm that allows subclasses to redefine parts of the algorithm without changing it's structure. Concepts:

    • Code reuse
    • Common in libraries/frameworks
    • IoC
    • Algorithm emphasis Examples: java.util.Collections#sort(), java.util.AbstractList#indexOf() Components: AbstractBase, ConcreteClass

15. 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. Short Definition: It's behavioral pattern that decouples request from a handling object in a chain of handlers util it is finally recognized. Concepts:

    • Decoupling of sender and receiver
    • Receiver contains reference to next receiver
    • Promotes loose coupling
    • No Handler - OK Examples: java.util.logging.Logger#log(), javax.servlet.Filter#doFilter(), Spring security filters

16. Command - Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Short Definition: It's a design pattern that lets you encapsulate each request as an object. Concepts:

    • Encapsulate request as an Object
    • Object-oriented callback
    • Decouple sender from processor
    • Often used for "Undo" functionality Examples: java.lang.Runnable

17. Iterator - Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. Short Definition: It's a design pattern for providing navigation without exposing of a structure of an object. Concepts:

    • Traverse a container
    • Doesn't expose underlying structure
    • Decouples algorithms
    • Sequential Examples: java.util.Iterator, java.util.Enumeration

18. 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. Short Definition: Pattern that used to define how objects interacts with one another without have them refer to each other explicitly. (Similar to Observer) Concepts:

    • Loose coupling
    • Well-defined, but complex
    • Reusable components
    • Hub / Router Examples: java.util.Timer, java.lang.reflect.Method#invoke()

19. Memento - Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later. Short Definition: Pattern that used to externalize an object state usually to provide rollback functionality. (components: Originator, Caretaker, Memento) Concepts:

    • Restore Object to previous state
    • Externalize internal state
    • Shield complex internals Examples: java.util.Date, java.io.Serializable

20. Observer - Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Short Definition: Observer is a decoupling pattern, when we have a subject that need to be observed by one or more observers. (Similar to Mediator) Concepts:

    • One to Many
    • Decoupled
    • Event Handling
    • Pub / Sub
    • Most used in MVC Examples: java.util.Observer, java.util.EventListener, javax.jms.Topic Components: Subject, Concrete Subject, Observer, Concrete Observer

21. State - Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. Short Definition: Used when we need to represent state of an application. Concepts:

    • Localize state behaviour
    • State Object
    • Separates What from Where
    • OCP (Open-Close Principle) Examples: JSF Components: Context, State, ConcreteState

22. Strategy - Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. Short Definition: Used when you want to enable the strategy or algorithm to be selected at runtime. Concepts:

    • Eliminate conditional statements
    • Behaviour encapsulated in classes
    • Difficult to add new strategies
    • Client aware of strategies
    • Client chooses strategy Examples: java.util.Comparator Components: Context, Strategy, ConcreteStrategy

23. 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. Short Definition: Way to separate an algorithm from an object structure. Concepts:

    • Separate Algorithm from Object
    • Adding new features
    • Maintain Open/Closed principle
    • Visitor changes Examples: java.lang.model.elementElementVisitor Components: Visitor, ConcreteVisitor, Element, ConcreteElement

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Design aspects that design patterns let you vary:

Creational patterns:

1. Factory Method - subclass of object that is instantiated
2. Abstract Factory - families of product objects
3. Builder - how to composite object gets created
4. Prototype - class of object that is instantiated
5. Singleton - the sole instance of a class

Structural patterns:

6. Adapter - interface to an object
7. Bridge - implementation of an object
8. Composite - structure and composition of an object
9. Decorator - responsibilities of an object without subclassing
10. Facade - interface to a subsystem
11. Flyweight - storage costs of objects
12. Proxy - how to object is accessed; its lication

Behavioral patterns:

13. Interpreter - grammar and interpretation of a language
14. Template Method - steps of an algorithm
15. Chain of Responsibility - object that can fulfill a request
16. Command - when and how a request is fullfilled
17. Iterator - how an aggregate's elements are accessed, traversed
18. Mediator - how and which objects interact with each other
19. Memento - what private information is stored outside an object, and when
20. Observer - number of objects that depend on another object; how the dependent objects stay up to date
21. State - states of an object
22. Strategy - an algorithm
23. Visitor - operations that can be applied to object(s) without changing their class(es)