--- title: "GoF Design Patterns in Java's core libraries" description: "GoF Design Patterns in Java's core libraries" author: "Anonymous User" published: 2015-05-13 updated: 2015-05-14 canonical: https://www.mindstick.com/forum/23221/gof-design-patterns-in-java-s-core-libraries category: "java" tags: ["java", "oops"] reading_time: 5 minutes --- # GoF Design Patterns in Java's core libraries I am [learning](https://www.mindstick.com/articles/126221/instructional-design-for-elearning-why-it-is-so-important) GoF [Java](https://www.mindstick.com/articles/12214/web-development-company-in-india-laid-on-the-foundation-of-concrete-java-programming) [Design Patterns](https://www.mindstick.com/articles/337478/the-role-of-design-patterns-in-enhancing-code-reusability) 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](https://answers.mindstick.com/qa/49244/which-company-is-ahead-in-classification-of-data-for-libraries-innovations)? ## Replies ### Reply by Anonymous User [Creational patterns](http://en.wikipedia.org/wiki/Creational_pattern) [Abstract factory](http://en.wikipedia.org/wiki/Abstract_factory_pattern) (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](http://en.wikipedia.org/wiki/Builder_pattern) (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 [Factory method](http://en.wikipedia.org/wiki/Factory_method_pattern) (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) [Prototype](http://en.wikipedia.org/wiki/Prototype_pattern) (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](http://en.wikipedia.org/wiki/Singleton_pattern) (recognizeable by creational methods returning the same instance (usually of itself) everytime) 1. java.lang.Runtime#getRuntime() 2. java.awt.Desktop#getDesktop() \ **[Structural patterns](http://en.wikipedia.org/wiki/Structural_pattern)** ## [Adapter](http://en.wikipedia.org/wiki/Adapter_pattern) (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.io.InputStreamReader(InputStream) (returns a Reader) - java.io.OutputStreamWriter(OutputStream) (returns a Writer) - javax.xml.bind.annotation.adapters.XmlAdapter#marshal() and #unmarshal() ## [Bridge](http://en.wikipedia.org/wiki/Bridge_pattern) (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, List) which returns an unmodifiable linked map which doesn't clone the items, but usesthem. The java.util.Collections#newSetFromMap() and singletonXXX() methods however comes close. \ [Composite](http://en.wikipedia.org/wiki/Composite_pattern) (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](http://en.wikipedia.org/wiki/Decorator_pattern) (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 ### [Facade](http://en.wikipedia.org/wiki/Facade_pattern) (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](http://en.wikipedia.org/wiki/Flyweight_pattern) - (recognizeable by creational methods returning a cached instance, a bit the "multiton" idea) - `java.lang.Integer#valueOf(int)` (also on `Boolean`, `Byte`, `Character`, `Short` and `Long`) ### [Proxy](http://en.wikipedia.org/wiki/Proxy_pattern) (recognizable 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.*`, 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](http://en.wikipedia.org/wiki/Behavioral_pattern) ## [Chain of responsibility](http://en.wikipedia.org/wiki/Chain_of_responsibility_pattern) (recognizeable by behavioral methods which (indirectly) invokes the same method inanother implementation of same abstract/interface type in a queue) - `java.util.logging.Logger#log()` - `javax.servlet.Filter#doFilter()` ### [Command](http://en.wikipedia.org/wiki/Command_pattern) (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](http://en.wikipedia.org/wiki/Interpreter_pattern) (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](http://en.wikipedia.org/wiki/Iterator_pattern) (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](http://en.wikipedia.org/wiki/Mediator_pattern) (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](http://en.wikipedia.org/wiki/Memento_pattern) (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)](http://en.wikipedia.org/wiki/Observer_pattern) \ (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](http://en.wikipedia.org/wiki/State_pattern) (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](http://en.wikipedia.org/wiki/Strategy_pattern) \ (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](http://en.wikipedia.org/wiki/Template_method_pattern) ### ##### (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.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 "Method Not Allowed" error to the response. You're free to implement none or any of them. ## [Visitor](http://en.wikipedia.org/wiki/Visitor_pattern) (recognizeable by two different abstract/interface types which has methods definied which takes each the otherabstract/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 --- Original Source: https://www.mindstick.com/forum/23221/gof-design-patterns-in-java-s-core-libraries Copyright © MindStick Software Pvt. Ltd. This Markdown version is provided for developers, AI systems, and offline reading.