GoF Design Patterns

Examples of GoF Design Patterns in core Java. Feel free to suggest new examples or mail corrections!

Creational Patterns

In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation.

  1. 1 Abstract factory 379
  2. 2 Builder 97
  3. 3 Factory method 96
  4. 4 Prototype 57
  5. 5 Singleton 82

Abstract factory

Recognizeable by creational methods returning the factory itself which in turn can be used to create another abstract/interface type.

  1. 1 java.sql.DriverManager#getConnection() 154
  2. 2 java.sql.Connection#createStatement() 21
  3. 3 java.sql.Statement#executeQuery() 12
  4. 4 javax.xml.parsers.DocumentBuilderFactory 11
  5. 5 javax.xml.transform.TransformerFactory 6
  6. 6 javax.xml.xpath.XPathFactory 6

Builder

Recognizeable by creational methods returning the instance itself.

  1. 1 java.lang.Appendable 47
  2. 2 java.nio.Buffer 21
  3. 3 javax.swing.GroupLayout.Group 9

Factory method

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

  1. 1 java.lang.Class#forName() 37
  2. 2 java.text.NumberFormat#getInstance() 11
  3. 3 java.util.Calendar#getInstance() 14
  4. 4 java.util.ResourceBundle#getBundle() 5

Prototype

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

  1. 1 java.lang.Object#clone() 21

Singleton

Recognizeable by creational methods returning the same instance (usually of itself) everytime.

  1. 1 java.awt.Desktop#getDesktop() 55
  2. 2 java.lang.Runtime#getRuntime() 19
  3. 3 java.lang.System#getSecurityManager() 2

Structural Patterns

In software engineering, structural design patterns are design patterns that ease the design by identifying a simple way to realize relationships between entities.

  1. 1 Adapter 101
  2. 2 Bridge 47
  3. 3 Composite 44
  4. 4 Decorator 45
  5. 5 Facade 44
  6. 6 Flyweight 53
  7. 7 Proxy 33

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.

  1. 1 java.io.InputStreamReader 54
  2. 2 java.io.OutputStreamWriter 7
  3. 3 java.util.Arrays#asList() 24

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.

  1. 1 java.util.Collections#newSetFromMap() 40

Composite

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

  1. 1 java.awt.Container 24
  2. 2 javax.faces.component.UIComponent 6

Decorator

Recognizeable by creational methods taking an instance of same abstract/interface type which adds additional behaviour.

  1. 1 java.io.InputStream 25
  2. 2 java.io.OutputStream 4
  3. 3 java.io.Reader 8
  4. 4 java.io.Writer 7
  5. 5 java.util.Collections#checkedXxx() 5
  6. 6 java.util.Collections#synchronizedXxx() 3
  7. 7 java.util.Collections#unmodifiableXxx() 1
  8. 8 javax.s.h.HttpServletRequestWrapper 2

Facade

Recognizeable by behavioral methods which internally uses instances of different independent abstract/interface types.

  1. 1 javax.faces.context.FacesContext 24
  2. 2 javax.faces.context.ExternalContext 7

Flyweight

Recognizeable by creational methods returning a cached instance, a bit the “multiton” idea.

  1. 1 java.lang.Long#valueOf(long) 38
  2. 2 java.lang.Integer#valueOf(int) 6
  3. 3 java.lang.Short#valueOf(short) 2
  4. 4 java.lang.Character#valueOf(char) 1
  5. 5 java.lang.Byte#valueOf(byte) 1
  6. 6 java.lang.Boolean#valueOf(boolean) 2

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.

  1. 1 java.lang.reflect.Proxy 16
  2. 2 java.rmi.* 1
  3. 3 javax.inject.Inject 6
  4. 4 javax.ejb.EJB 10
  5. 5 javax.persistence.PersistenceContext 4

Behavioral Patterns

In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication.

  1. 1 Chain of responsibility 65
  2. 2 Command 33
  3. 3 Interpreter 15
  4. 4 Iterator 7
  5. 5 Mediator 18
  6. 6 Memento 21
  7. 7 Observer 28
  8. 8 State 21

Chain of responsibility

Recognizeable by behavioral methods which (indirectly) invokes the same method in another implementation of same abstract/interface type in a queue.

  1. 1 java.util.logging.Logger 43
  2. 2 javax.servlet.Filter 14

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.

  1. 1 java.lang.Runnable 18
  2. 2 java.util.concurrent.Callable 6
  3. 3 javax.swing.Action 5
  4. 4 javax.servlet.FilterChain 3

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 Interpreter pattern, determining the given pattern and how to apply it is.

  1. 1 java.text.Format 8
  2. 2 java.text.Normalizer 3
  3. 3 java.util.regex.Pattern 2
  4. 4 javax.el.ELResolver 3

Iterator

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

  1. 1 java.util.Enumeration 7
  2. 2 java.util.Iterator 4

Mediator

Recognizeable by behavioral methods taking an instance of different abstract/interface type (usually using the command pattern) which delegates/uses the given instance.

  1. 1 java.lang.reflect.Method 3
  2. 2 java.util.Timer 9
  3. 3 java.util.concurrent.Executor 4

Memento

Recognizeable by behavioral methods which internally changes the state of the whole instance.

  1. 1 java.util.Date 14
  2. 2 java.io.Serializable 6
  3. 3 javax.swing.undo.UndoManager 0
  4. 4 javax.faces.component.StateHolder 0

Observer (Publish/Subscribe)

Recognizeable by behavioral methods which invokes a method on an instance of another abstract/interface type, depending on own state.

  1. 1 java.util.EventListener 19
  2. 2 javax.s.h.HttpSessionBindingListener 4
  3. 3 javax.s.h.HttpSessionAttributeListener 2
  4. 4 javax.faces.event.FacesListener 1
  5. 5 javax.faces.event.PhaseListener 1

State

Recognizeable by behavioral methods which changes its behaviour depending on the instance’s state which can be controlled externally.

  1. 1 javax.faces.lifecycle.Lifecycle 24

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.

  1. 1 java.util.Comparator#compare() 35
  2. 2 javax.servlet.http.HttpServlet#doXxx() 6
  3. 3 javax.servlet.Filter#doFilter() 7

Template method

Recognizeable by behavioral methods which already have a “default” behaviour definied by an abstract type.

  1. 1 java.util.AbstractList 24
  2. 2 java.util.AbstractMap 2
  3. 3 java.util.AbstractSet 1
  4. 4 javax.servlet.http.HttpServlet 6

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.

  1. 1 java.nio.file.FileVisitor 17
  2. 2 javax.faces.component.visit.VisitCallback 11
  3. 3 javax.l.m.e.AnnotationValueVisitor 5
  4. 4 javax.l.m.e.ElementVisitor 5
  5. 5 javax.l.m.t.TypeVisitor 4
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