GoF Design Patterns

Motivation and Purpose

• Encapsulate internal state of objects.
• Distinguish intrinsic state from extrinsic state.
  
• Decompose a system into abstractions.
• Delegation works best when used in highly stylized ways (i.e. design patterns).
  
• Principles of reusable object-oriented design:
• Program to an interface, not an implementation. (Use loose coupling.)
  
• Favor object composition over class inheritance. (Use delegation.)
  
• Techniques for designing for change:
• Avoid dependency on type of implementation class.
• Avoid dependency on specific operation dispatch.
• Increase information hiding.
• Increase algorithm hiding.
• Decrease tight coupling; increase loose coupling.
• Increase flexibility by using object composition in general and delegation in particular instead of subclassing.
• Alter behavior of a library class without modifying the library.
  

Examples of design patterns in Java:

Creational Patterns

• Abstract Factory -- BorderFactory, Collections, org.w3c.dom.Document (create methods)
  
• Builder -- StringBuilder, ProcessBuilder
  
• Factory Method -- URLStreamHandlerFactory, InitialContextFactory
  
• Prototype -- (copyable state objects for supporting cancelable UI)
  
• Singleton -- java.lang.Runtime
  

Structural Patterns

• Adapter -- InputStreamReader, OutputStreamWriter
  
• Bridge -- Swing PLAF
  
• Composite -- java.awt.Component/Container
  
• Decorator -- BufferedReader/Writer, ZipInputStream, other java.io classes
  
• Facade -- what Swing is to java.awt.Graphics
  
• Flyweight -- Icon (when cached)
  
• Proxy -- RMI, WeakReference, ThreadLocal
  

Behavioral Patterns

• Chain of Responsibility -- KeyEvent dispatch in AWT/Swing (consume() method stops chain)
  
• Command -- javax.swing.Action (not quite - need a better example)
  
• Interpreter -- java.util.regex.Pattern
  
• Iterator
  
• Mediator -- event cascades
  
• Memento --javax.swing.undo.UndoableEdit, java.awt.datatransfer.Transferable, java.beans.XMLEncoder/Decoder
  
• Observer -- Swing/AWT event listeners
  
• State -- graphical tools (e.g. from a component palette) used in a visual editor
  
• Strategy -- dispatch of Undo; typical pattern for View-Controller interaction
  
• Template Method -- java.io.Reader/Writer
  
• Visitor -- SAX API (org.xml.sax)