Examples of GoF Design Patterns in Java s core libraries

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I am learning GoF Java Design Patterns and I want to see some real life examples of them  What are some good examples of these Design Patterns in Java s core libraries

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Factory method   java util Collection Iterator is a good example of a Factory Method  Depending on the concrete subclass of Collection you use  it will create an Iterator implementation  Because both the Factory superclass  Collection  and the Iterator created are interfaces  it is sometimes confused with AbstractFactory  Most of the examples for AbstractFactory in the the accepted answer  BalusC  are examples of Factory  a simplified version of Factory Method  which is not part of the original GoF patterns  In Facory the Factory class hierarchy is collapsed and the factory uses other means to choose the product to be returned    Abstract Factory   An abstract factory has multiple factory methods  each creating a different product  The products produced by one factory are intended to be used together  your printer and cartridges better be from the same  abstract  factory   As mentioned in answers above the families of AWT GUI components  differing from platform to platform  are an example of this  although its implementation differs from the structure described in Gof

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Flyweight is used with some values of Byte  Short  Integer  Long and String  Facade is used in many place but the most obvious is Scripting interfaces  Singleton - java lang Runtime comes to mind  Abstract Factory - Also Scripting and JDBC API  Command - TextComponent s Undo Redo  Interpreter - RegEx  java util regex   and SQL  java sql   API  Prototype - Not 100  sure if this count  but I thinkg clone   method can be used for this purpose

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RMI is based on Proxy   Should be possible to cite one for most of the 23 patterns in GoF    Abstract Factory  java sql interfaces all get their concrete implementations from JDBC JAR when driver is registered  Builder  java lang StringBuilder  Factory Method  XML factories  among others  Prototype  Maybe clone    but I m not sure I m buying that  Singleton  java lang System Adapter  Adapter classes in java awt event  e g   WindowAdapter  Bridge  Collection classes in java util  List implemented by ArrayList  Composite  java awt  java awt Component   java awt Container Decorator  All over the java io package  Facade  ExternalContext behaves as a facade for performing cookie  session scope and similar operations  Flyweight  Integer  Character  etc  Proxy  java rmi package Chain of Responsibility  Servlet filters Command  Swing menu items Interpreter  No directly in JDK  but JavaCC certainly uses this  Iterator  java util Iterator interface  can t be clearer than that  Mediator  JMS  Memento   Observer  java util Observer Observable  badly done  though  State   Strategy   Template   Visitor     I can t think of examples in Java for 10 out of the 23  but I ll see if I can do better tomorrow   That s what edit is for

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You can find an overview of a lot of design patterns in Wikipedia  It also mentions which patterns are mentioned by GoF  I ll sum them up here and try to assign as many pattern implementations as possible  found in both the Java SE and Java EE APIs   Creational patterns Abstract factory  recognizeable by creational methods returning the factory itself which in turn can be used to create another abstract interface type   javax xml parsers DocumentBuilderFactory newInstance   javax xml transform TransformerFactory newInstance   javax xml xpath XPathFactory newInstance    Builder  recognizeable by creational methods returning the instance itself   java lang StringBuilder append    unsynchronized  java lang StringBuffer append    synchronized  java nio ByteBuffer put    also on CharBuffer  ShortBuffer  IntBuffer  LongBuffer  FloatBuffer and DoubleBuffer  javax swing GroupLayout Group addComponent   All implementations of java lang Appendable java util stream Stream Builder  Factory method  recognizeable by creational methods returning an implementation of an abstract interface type   java util Calendar getInstance   java util ResourceBundle getBundle   java text NumberFormat getInstance   java nio charset Charset forName   java net URLStreamHandlerFactory createURLStreamHandler String   Returns singleton object per protocol  java util EnumSet of   javax xml bind JAXBContext createMarshaller   and other similar methods  Prototype  recognizeable by creational methods returning a different instance of itself with the same properties   java lang Object clone    the class has to implement java lang Cloneable   Singleton  recognizeable by creational methods returning the same instance  usually of itself  everytime   java lang Runtime getRuntime   java awt Desktop getDesktop   java lang System getSecurityManager     Structural patterns Adapter  recognizeable by creational methods taking an instance of different abstract interface type and returning an implementation of own another abstract interface type which decorates overrides the given instance   java util Arrays asList   java util Collections list   java util Collections enumeration   java io InputStreamReader InputStream   returns a Reader  java io OutputStreamWriter OutputStream   returns a Writer  javax xml bind annotation adapters XmlAdapter marshal   and  unmarshal    Bridge  recognizeable by creational methods taking an instance of different abstract interface type and returning an implementation of own abstract interface type which delegates uses the given instance   None comes to mind yet  A fictive example would be new LinkedHashMap LinkedHashSet lt K gt   List lt V gt   which returns an unmodifiable linked map which doesn t clone the items  but uses them  The java util Collections newSetFromMap   and singletonXXX   methods however comes close   Composite  recognizeable by behavioral methods taking an instance of same abstract interface type into a tree structure   java awt Container add Component   practically all over Swing thus  javax faces component UIComponent getChildren    practically all over JSF UI thus   Decorator  recognizeable by creational methods taking an instance of same abstract interface type which adds additional behaviour   All subclasses of java io InputStream  OutputStream  Reader and Writer have a constructor taking an instance of same type  java util Collections  the checkedXXX    synchronizedXXX   and unmodifiableXXX   methods  javax servlet http HttpServletRequestWrapper and HttpServletResponseWrapper javax swing JScrollPane  Facade  recognizeable by behavioral methods which internally uses instances of different independent abstract interface types   javax faces context FacesContext  it internally uses among others the abstract interface types LifeCycle  ViewHandler  NavigationHandler and many more without that the enduser has to worry about it  which are however overrideable by injection   javax faces context ExternalContext  which internally uses ServletContext  HttpSession  HttpServletRequest  HttpServletResponse  etc   Flyweight  recognizeable by creational methods returning a cached instance  a bit the  quot multiton quot  idea   java lang Integer valueOf int   also on Boolean  Byte  Character  Short  Long and BigDecimal   Proxy  recognizeable by creational methods which returns an implementation of given abstract interface type which in turn delegates uses a different implementation of given abstract interface type   java lang reflect Proxy java rmi   javax ejb EJB  explanation here  javax inject Inject  explanation here  javax persistence PersistenceContext   Behavioral patterns Chain of responsibility  recognizeable by behavioral methods which  indirectly  invokes the same method in another implementation of same abstract interface type in a queue   java util logging Logger log   javax servlet Filter doFilter    Command  recognizeable by behavioral methods in an abstract interface type which invokes a method in an implementation of a different abstract interface type which has been encapsulated by the command implementation during its creation   All implementations of java lang Runnable All implementations of javax swing Action  Interpreter  recognizeable by behavioral methods returning a structurally different instance type of the given instance type  note that parsing formatting is not part of the pattern  determining the pattern and how to apply it is   java util Pattern java text Normalizer All subclasses of java text Format All subclasses of javax el ELResolver  Iterator  recognizeable by behavioral methods sequentially returning instances of a different type from a queue   All implementations of java util Iterator  thus among others also java util Scanner    All implementations of java util Enumeration  Mediator  recognizeable by behavioral methods taking an instance of different abstract interface type  usually using the command pattern  which delegates uses the given instance   java util Timer  all scheduleXXX   methods  java util concurrent Executor execute   java util concurrent ExecutorService  the invokeXXX   and submit   methods  java util concurrent ScheduledExecutorService  all scheduleXXX   methods  java lang reflect Method invoke    Memento  recognizeable by behavioral methods which internally changes the state of the whole instance   java util Date  the setter methods do that  Date is internally represented by a long value  All implementations of java io Serializable All implementations of javax faces component StateHolder  Observer  or Publish Subscribe   recognizeable by behavioral methods which invokes a method on an instance of another abstract interface type  depending on own state   java util Observer java util Observable  rarely used in real world though  All implementations of java util EventListener  practically all over Swing thus  javax servlet http HttpSessionBindingListener javax servlet http HttpSessionAttributeListener javax faces event PhaseListener  State  recognizeable by behavioral methods which changes its behaviour depending on the instance s state which can be controlled externally   javax faces lifecycle LifeCycle execute    controlled by FacesServlet  the behaviour is dependent on current phase  state  of JSF lifecycle   Strategy  recognizeable by behavioral methods in an abstract interface type which invokes a method in an implementation of a different abstract interface type which has been passed-in as method argument into the strategy implementation   java util Comparator compare    executed by among others Collections sort    javax servlet http HttpServlet  the service   and all doXXX   methods take HttpServletRequest and HttpServletResponse and the implementor has to process them  and not to get hold of them as instance variables    javax servlet Filter doFilter    Template method  recognizeable by behavioral methods which already have a  quot default quot  behaviour defined by an abstract type   All non-abstract methods of java io InputStream  java io OutputStream  java io Reader and java io Writer  All non-abstract methods of java util AbstractList  java util AbstractSet and java util AbstractMap  javax servlet http HttpServlet  all the doXXX   methods by default sends a HTTP 405  quot Method Not Allowed quot  error to the response  You re free to implement none or any of them   Visitor  recognizeable by two different abstract interface types which has methods defined which takes each the other abstract interface type  the one actually calls the method of the other and the other executes the desired strategy on it   javax lang model element AnnotationValue and AnnotationValueVisitor javax lang model element Element and ElementVisitor javax lang model type TypeMirror and TypeVisitor java nio file FileVisitor and SimpleFileVisitor javax faces component visit VisitContext and VisitCallback

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The Abstract Factory pattern is used in various places   E g   DatagramSocketImplFactory  PreferencesFactory  There are many more---search the Javadoc for interfaces which have the word  Factory  in their name   Also there are quite a few instances of the Factory pattern  too

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Even though I m sort of a broken clock with this one  Java XML API uses Factory a lot  I mean just look at this   Document doc   DocumentBuilderFactory newInstance   newDocumentBuilder   parse source   String title   XPathFactory newInstance   newXPath   evaluate    title   doc        and so on and so forth   Additionally various Buffers  StringBuffer  ByteBuffer  StringBuilder  use Builder

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Observer pattern throughout whole swing  Observable  Observer  MVC also in swing Adapter pattern  InputStreamReader and OutputStreamWriter  NOTE  ContainerAdapter  ComponentAdapter  FocusAdapter  KeyAdapter  MouseAdapter are not adapters  they are actually Null Objects  Poor naming choice by Sun  Decorator pattern  BufferedInputStream can decorate other streams such as FilterInputStream  AbstractFactory Pattern for the AWT Toolkit and the Swing pluggable look-and-feel classes java lang Runtime getRuntime   is Singleton ButtonGroup for Mediator pattern Action  AbstractAction may be used for different visual representations to execute same code -  Command pattern Interned Strings or CellRender in JTable for Flyweight Pattern  Also think about various pools - Thread pools  connection pools  EJB object pools - Flyweight is really about management of shared resources  The Java 1 0 event model is an example of Chain of Responsibility  as are Servlet Filters  Iterator pattern in Collections Framework Nested containers in AWT Swing use the Composite pattern Layout Managers in AWT Swing are an example of Strategy   and many more I guess

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