Design Patterns are reusable solutions to common problems in object-oriented design. The 23 classic patterns were systematized by the Gang of Four (GoF: Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides) in their 1994 book Design Patterns: Elements of Reusable Object-Oriented Software.

This article is compiled from a blog post by smallnest and a StackOverflow discussion on GoF design pattern implementations in the Java API.

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Design patterns are not code libraries — they are conceptual frameworks for solving problems. The key is understanding when to use a pattern, not just how to implement it.

Creational Patterns

Creational patterns abstract the instantiation process, making a system independent of how its objects are created, composed, and represented.

Abstract Factory

Recognizable by: creational methods returning the factory itself, which in turn creates another abstract/interface type.

Java API Examples:

  • javax.xml.parsers.DocumentBuilderFactory#newInstance()
  • javax.xml.transform.TransformerFactory#newInstance()
  • javax.xml.xpath.XPathFactory#newInstance()

Builder

Recognizable by: creational methods returning the instance itself (fluent API).

Java API Examples:

  • java.lang.StringBuilder#append() (unsynchronized)
  • java.lang.StringBuffer#append() (synchronized)
  • java.nio.ByteBuffer#put() (also CharBuffer, ShortBuffer, IntBuffer, LongBuffer, FloatBuffer, DoubleBuffer)
  • javax.swing.GroupLayout.Group#addComponent()
  • All implementations of java.lang.Appendable

Factory Method

Recognizable by: creational methods returning an implementation of an abstract/interface type.

Java API Examples:

  • java.util.Calendar#getInstance()
  • java.util.ResourceBundle#getBundle()
  • java.text.NumberFormat#getInstance()
  • java.nio.charset.Charset#forName()
  • java.net.URLStreamHandlerFactory#createURLStreamHandler(String) (returns singleton per protocol)

Prototype

Recognizable by: creational methods returning a different instance of itself with the same properties.

Java API Examples:

  • java.lang.Object#clone() (class must implement java.lang.Cloneable)

Singleton

Recognizable by: creational methods returning the same instance every time.

Java API Examples:

  • java.lang.Runtime#getRuntime()
  • java.awt.Desktop#getDesktop()

Structural Patterns

Structural patterns focus on how classes and objects are composed to form larger structures.

Adapter

Recognizable by: creational methods taking an instance of a different abstract/interface type and returning an implementation of own/another abstract/interface type that decorates/overrides the given instance.

Java API Examples:

  • 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

Recognizable by: creational methods taking an instance of a different abstract/interface type and returning an implementation that delegates/uses the given instance.

Java API Examples:

JDBC, JNDI, JCE are the classic examples. No direct JDK class comes to mind. java.util.Collections#newSetFromMap() and singletonXXX() methods come close.

Composite

Recognizable by: behavioral methods taking an instance of the same abstract/interface type into a tree structure.

Java API Examples:

  • java.awt.Container#add(Component) (all over Swing)
  • javax.faces.component.UIComponent#getChildren() (all over JSF)

Decorator

Recognizable by: creational methods taking an instance of the same abstract/interface type and adding additional behavior.

Java API Examples:

  • All subclasses of java.io.InputStream, OutputStream, Reader, Writer have constructors taking the same type
  • java.util.Collections checkedXXX(), synchronizedXXX(), unmodifiableXXX() methods
  • javax.servlet.http.HttpServletRequestWrapper and HttpServletResponseWrapper

Facade

Recognizable by: behavioral methods that internally use instances of different independent abstract/interface types.

Java API Examples:

  • javax.faces.context.FacesContext — internally uses LifeCycle, ViewHandler, NavigationHandler, etc.
  • javax.faces.context.ExternalContext — internally uses ServletContext, HttpSession, HttpServletRequest, etc.

Flyweight

Recognizable by: creational methods returning a cached instance (the “multiton” idea).

Java API Examples:

  • java.lang.Integer#valueOf(int) (also Boolean, Byte, Character, Short, Long)
  • String constant pool

Proxy

Recognizable by: creational methods returning an implementation that delegates/uses a different implementation.

Java API Examples:

  • java.lang.reflect.Proxy
  • java.rmi.* — the entire API

Behavioral Patterns

Behavioral patterns focus on how objects distribute responsibilities and interact.

Chain of Responsibility

Recognizable by: behavioral methods that (indirectly) invoke the same method in another implementation of the same type in a queue.

Java API Examples:

  • java.util.logging.Logger#log()
  • javax.servlet.Filter#doFilter()

Command

Recognizable by: behavioral methods in an abstract/interface type that invoke a method in a different type, encapsulated during creation.

Java API Examples:

  • All implementations of java.lang.Runnable
  • All implementations of javax.swing.Action

Interpreter

Recognizable by: behavioral methods returning a structurally different instance/type of the given instance/type.

Java API Examples:

  • java.util.Pattern
  • java.text.Normalizer
  • All subclasses of java.text.Format
  • All subclasses of javax.el.ELResolver

Iterator

Recognizable by: behavioral methods sequentially returning instances of a different type from a queue.

Java API Examples:

  • All implementations of java.util.Iterator
  • All implementations of java.util.Enumeration

Mediator

Recognizable by: behavioral methods taking an instance of a different abstract/interface type and delegating/using it.

Java API Examples:

  • java.util.Timer (all scheduleXXX() methods)
  • java.util.concurrent.Executor#execute()
  • java.util.concurrent.ExecutorService (invokeXXX() and submit() methods)
  • java.util.concurrent.ScheduledExecutorService (all scheduleXXX() methods)
  • java.lang.reflect.Method#invoke()

Memento

Recognizable by: behavioral methods that internally change the state of the whole instance.

Java API Examples:

  • java.util.Date (setters change the internal long value)
  • All implementations of java.io.Serializable
  • All implementations of javax.faces.component.StateHolder

Observer (Publish/Subscribe)

Recognizable by: behavioral methods that invoke a method on another abstract/interface type instance, depending on own state.

Java API Examples:

  • java.util.Observer / java.util.Observable (rarely used in practice)
  • All implementations of java.util.EventListener (all over Swing)
  • javax.servlet.http.HttpSessionBindingListener
  • javax.servlet.http.HttpSessionAttributeListener
  • javax.faces.event.PhaseListener

State

Recognizable by: behavioral methods that change behavior depending on the instance’s state, which can be controlled externally.

Java API Examples:

  • javax.faces.lifecycle.LifeCycle#execute() (behavior depends on the current JSF lifecycle phase)

Strategy

Recognizable by: behavioral methods in an abstract/interface type that invoke a method in a different abstract/interface type passed in as a method argument.

Java API Examples:

  • java.util.Comparator#compare() (executed by Collections#sort())
  • javax.servlet.http.HttpServlet — service() and all doXXX() methods
  • javax.servlet.Filter#doFilter()

Template Method

Recognizable by: behavioral methods in an abstract type that already have a “default” behavior defined.

Java API Examples:

  • All non-abstract methods of java.io.InputStream, OutputStream, Reader, Writer
  • All non-abstract methods of java.util.AbstractList, AbstractSet, AbstractMap
  • javax.servlet.http.HttpServlet — all doXXX() methods default to HTTP 405

Visitor

Recognizable by: two different abstract/interface types that each have methods taking the other type; one calls the other’s method to execute the desired strategy.

Java API Examples:

  • javax.lang.model.element.AnnotationValue and AnnotationValueVisitor
  • javax.lang.model.element.Element and ElementVisitor
  • javax.lang.model.type.TypeMirror and TypeVisitor

References