I am learning GoF Java Design Patterns and I want to see some real life examples of them. Can you guys point to some good usage of these Design Patterns.(preferably in Java's core libraries).
Thank you
+12
A:
- Observer pattern throughout whole swing (
Observable,Observer) - MVC also in swing
- Adapter pattern: InputStreamReader and OutputStreamWriter
NOTE:
ContainerAdapter,ComponentAdapter,FocusAdapter,KeyAdapter,MouseAdapterare not adapters; they are actually Null Objects. Poor naming choice by Sun. - Decorator pattern (
BufferedInputStreamcan decorate other streams such asFilterInputStream) - AbstractFactory Pattern for the AWT Toolkit and the Swing pluggable look-and-feel classes
java.lang.Mathcould be considered a kind of Singleton (final + all methods static)ButtonGroupfor Mediator patternAction,AbstractActionmay be used for different visual represntations 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
jitter
2009-11-04 13:51:11
java.lang.Math (6th one) is not singleton, you don't have an instance to start with, everything is static. That's not singleton
Ion Todirel
2010-04-25 05:11:29
+4
A:
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.
uckelman
2009-11-04 13:59:12
+1
A:
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.
Esko
2009-11-04 14:07:13
+3
A:
- 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.
NawaMan
2009-11-04 14:09:28
+2
A:
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:
- Factory Method: XML factories, among others.
- Prototype: Maybe clone(), but I'm not sure I'm buying that.
- Singleton: java.lang.System
- Adapter:
- Bridge:
- Composite:
- Decorator: All over the java.io package.
- Facade:
- 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.
duffymo
2009-11-05 02:26:58
+126
A:
You can find an overview of a lot design patterns in Wikipedia. It also mentions which patterns are mentioned by GoF. I'll sum them up here and try to assign as much as possible pattern implementations found in both the Java SE and Java EE API's.
Creational patterns
Abstract factory (recognizeable by creational methods returning an abstract/interface type)
java.util.Calendar#getInstance()java.util.Arrays#asList()java.util.ResourceBundle#getBundle()java.net.URL#openConnection()java.sql.DriverManager#getConnection()java.sql.Connection#createStatement()java.sql.Statement#executeQuery()java.text.NumberFormat#getInstance()java.lang.management.ManagementFactory(allgetXXX()methods)java.nio.charset.Charset#forName()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 onCharBuffer,ShortBuffer,IntBuffer,LongBuffer,FloatBufferandDoubleBuffer)javax.swing.GroupLayout.Group#addComponent()- All implementations of
java.lang.Appendable
Factory method (recognizeable by creational methods returning a concrete type)
java.lang.Object#toString()(overrideable in all subclasses)java.lang.Class#newInstance()java.lang.Integer#valueOf(String)(also onBoolean,Byte,Character,Short,Long,FloatandDouble)java.lang.Class#forName()java.lang.reflect.Array#newInstance()java.lang.reflect.Constructor#newInstance()
Prototype (recognizeable by creational methods returning a different instance of itself with the same properties)
java.lang.Object#clone()(the class has to implementjava.lang.Cloneable)
Singleton (recognizeable by creational methods returning the same instance (usually of itself) everytime)
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.io.InputStreamReader(InputStream)(returns aReader)java.io.OutputStreamWriter(OutputStream)(returns aWriter)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<K>, List<V>)which returns an unmodifiable linked map which doesn't clone the items, but uses them. Thejava.util.Collections#newSetFromMap()andsingletonXXX()methods however comes close.
Composite (recognizeable by behavioral methods taking an instance of same abstract/interface type)
java.util.Map#putAll(Map)java.util.List#addAll(Collection)java.util.Set#addAll(Collection)java.nio.ByteBuffer#put(ByteBuffer)(also onCharBuffer,ShortBuffer,IntBuffer,LongBuffer,FloatBufferandDoubleBuffer)java.awt.Container#add(Component)(practically all over Swing thus)
Decorator (recognizeable by creational methods taking an instance of same abstract/interface type)
- All subclasses of
java.io.InputStream,OutputStream,ReaderandWriterhave a constructor taking an instance of same type. - Almost all implementations of
java.util.List,SetandMaphave a constructor taking an instance of same type. java.util.Collections, thecheckedXXX(),synchronizedXXX()andunmodifiableXXX()methods.javax.servlet.http.HttpServletRequestWrapperandHttpServletResponseWrapper
Facade (recognizeable by behavioral methods which internally uses instances of different independent abstract/interface types)
javax.faces.webapp.FacesServlet, it internally uses under each the abstract/interface typesServletContext,LifeCycle,ViewHandler,NavigationHandlerand many more without that the enduser has to worry about it (which are however overrideable by injection).
Flyweight (recognizeable by creational methods returning a cached instance, a bit the "multiton" idea)
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.Proxyjava.rmi.*, the whole API actually.
The Wikipedia example is IMHO a bit poor, lazy loading has actually completely nothing to do with the proxy pattern at all.
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)
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.Patternjava.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 under each alsojava.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(allscheduleXXX()methods)java.util.concurrent.Executor#execute()java.util.concurrent.ExecutorService(theinvokeXXX()andsubmit()methods)java.util.concurrent.ScheduledExecutorService(allscheduleXXX()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,Dateis internally represented by alongvalue)- 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.HttpSessionBindingListenerjavax.servlet.http.HttpSessionAttributeListenerjavax.faces.event.PhaseListener
State (recognizeable by behavioral methods which changes its behaviour depending on the instance's state which can be controlled externally)
- All implementations of
java.util.Iterator javax.faces.lifecycle.LifeCycle#execute()(controlled byFacesServlet, 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 under eachCollections#sort().javax.servlet.http.HttpServlet, theservice()and alldoXXX()methods takeHttpServletRequestandHttpServletResponseand 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 "default" behaviour definied by an abstract type)
- All non-abstract methods of
java.io.InputStream,java.io.OutputStream,java.io.Readerandjava.io.Writer. - All non-abstract methods of
java.util.AbstractList,java.util.AbstractSetandjava.util.AbstractMap. javax.servlet.http.HttpServlet, all thedoXXX()methods by default sends a HTTP 405 "Method Not Allowed" 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 definied 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)
BalusC
2010-04-25 04:51:25
impressive.. :) +1. `javax.lang.model.element` defines visitors ;) I'm not quite sure whether `doXXX` and `doFilter` are "strategies".
Bozho
2010-04-26 13:14:48
@Bozho: Thank you for the visitor example! (that whole API is actually new to me btw). In the `doXXX()` and `doFilter()` you basically write the strategy algorithm alone. The server loads and executes it.
BalusC
2010-04-26 13:24:33
hm, yes.. my impression is that the strategies have to be objects conforming to a strategy interface, and thus interchangeable at runtime
Bozho
2010-04-26 13:31:18