Java/Design Pattern/Singleton Pattern

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A static class implementation of Singleton pattern

   <source lang="java">

/* The Design Patterns Java Companion Copyright (C) 1998, by James W. Cooper IBM Thomas J. Watson Research Center

  • /

final class Spooler { 

 static public void print(String s) {
   System.out.println(s);
 }

}

public class StaticSpool { 

 public static void main(String argv[]) {
   Spooler.print("here it is");
 }

} 

      </source>



Singleton

   <source lang="java">

/* 

* Copyright (c) Ian F. Darwin, http://www.darwinsys.ru/, 1996-2002.
* All rights reserved. Software written by Ian F. Darwin and others.
* $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
*    notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
*    notice, this list of conditions and the following disclaimer in the
*    documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS""
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
* TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
* 
* Java, the Duke mascot, and all variants of Sun"s Java "steaming coffee
* cup" logo are trademarks of Sun Microsystems. Sun"s, and James Gosling"s,
* pioneering role in inventing and promulgating (and standardizing) the Java 
* language and environment is gratefully acknowledged.
* 
* The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
* inventing predecessor languages C and C++ is also gratefully acknowledged.
*/

/** An example of a Singleton implementation in Java. 

* The Singleton design pattern is described in GOF; the idea is to ensure
* that only one instance of the class will exist in a given application.
* @author Ian F. Darwin, http://www.darwinsys.ru/
* @version $Id: Singleton.java,v 1.7 2004/02/09 03:33:58 ian Exp $
*/

public class Singleton { 

 private static Singleton singleton = new Singleton();
 /** A private Constructor prevents any other class from instantiating. */
 private Singleton() {
 }
 /** Static "instance" method */
 public static Singleton getInstance() {
   return singleton;
 }
 // other methods protected by singleton-ness would be here...
 /** A simple demo method */
 public String demoMethod() {
   return "demo";
 }

} 


      </source>



Singleton Pattern 2

   <source lang="java">

//[C] 2002 Sun Microsystems, Inc.--- import java.awt.Container; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.awt.event.WindowAdapter; import java.awt.event.WindowEvent; import java.util.ArrayList; import java.util.Collections; import java.util.List; import javax.swing.BoxLayout; import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.JTextArea; public class RunSingletonPattern { 

 public static void main(String[] arguments) {
   System.out.println("Example for Singleton pattern");
   System.out.println();
   System.out.println("This example will demonstrate the use of");
   System.out.println(" the Singleton pattern by creating two GUI");
   System.out.println(" editors, both of which will reference the");
   System.out.println(" same underlying history list.");
   System.out.println("Creating the first editor");
   System.out.println();
   SingletonGui editor1 = new SingletonGui();
   editor1.createGui();
   System.out.println("Creating the second editor");
   System.out.println();
   SingletonGui editor2 = new SingletonGui();
   editor2.createGui();
 }

} class SingletonGui implements ActionListener { 

 private JFrame mainFrame;
 private JTextArea display;
 private JButton newContact, newAppointment, undo, refresh, exit;
 private JPanel controlPanel, displayPanel;
 private static int historyCount;
 public void createGui() {
   mainFrame = new JFrame("Singleton Pattern Example");
   Container content = mainFrame.getContentPane();
   content.setLayout(new BoxLayout(content, BoxLayout.Y_AXIS));
   displayPanel = new JPanel();
   display = new JTextArea(20, 60);
   display.setEditable(false);
   displayPanel.add(display);
   content.add(displayPanel);
   controlPanel = new JPanel();
   newContact = new JButton("Create contact");
   newAppointment = new JButton("Create appointment");
   undo = new JButton("Undo");
   refresh = new JButton("Refresh");
   exit = new JButton("Exit");
   controlPanel.add(newContact);
   controlPanel.add(newAppointment);
   controlPanel.add(undo);
   controlPanel.add(refresh);
   controlPanel.add(exit);
   content.add(controlPanel);
   newContact.addActionListener(this);
   newAppointment.addActionListener(this);
   undo.addActionListener(this);
   refresh.addActionListener(this);
   exit.addActionListener(this);
   mainFrame.addWindowListener(new WindowCloseManager());
   mainFrame.pack();
   mainFrame.setVisible(true);
 }
 public void refreshDisplay(String actionMessage) {
   display.setText(actionMessage + "\nCOMMAND HISTORY:\n"
       + HistoryList.getInstance().toString());
 }
 public void actionPerformed(ActionEvent evt) {
   Object originator = evt.getSource();
   if (originator == newContact) {
     addCommand(" New Contact");
   } else if (originator == newAppointment) {
     addCommand(" New Appointment");
   } else if (originator == undo) {
     undoCommand();
   } else if (originator == refresh) {
     refreshDisplay("");
   } else if (originator == exit) {
     exitApplication();
   }
 }
 private class WindowCloseManager extends WindowAdapter {
   public void windowClosing(WindowEvent evt) {
     exitApplication();
   }
 }
 private void addCommand(String message) {
   HistoryList.getInstance().addCommand((++historyCount) + message);
   refreshDisplay("Add Command: " + message);
 }
 private void undoCommand() {
   Object result = HistoryList.getInstance().undoCommand();
   historyCount--;
   refreshDisplay("Undo Command: " + result);
 }
 private void exitApplication() {
   System.exit(0);
 }

} class HistoryList { 

 private List history = Collections.synchronizedList(new ArrayList());
 private static HistoryList instance = new HistoryList();
 private HistoryList() {
 }
 public static HistoryList getInstance() {
   return instance;
 }
 public void addCommand(String command) {
   history.add(command);
 }
 public Object undoCommand() {
   return history.remove(history.size() - 1);
 }
 public String toString() {
   StringBuffer result = new StringBuffer();
   for (int i = 0; i < history.size(); i++) {
     result.append("  ");
     result.append(history.get(i));
     result.append("\n");
   }
   return result.toString();
 }

} 


      </source>



Singleton pattern in Java 1

   <source lang="java">

/* The Design Patterns Java Companion Copyright (C) 1998, by James W. Cooper IBM Thomas J. Watson Research Center

  • /

public class InstanceSpooler { 

 static public void main(String argv[]) {
   Spooler pr1, pr2;
   //open one printer--this should always work
   System.out.println("Opening one spooler");
   pr1 = Spooler.Instance();
   if (pr1 != null)
     System.out.println("got 1 spooler");
   //try to open another printer --should fail
   System.out.println("Opening two spoolers");
   pr2 = Spooler.Instance();
   if (pr2 == null)
     System.out.println("no instance available");
   //fails because constructor is privatized
   //iSpooler pr3 = new iSpooler();
 }

} class Spooler { 

 //this is a prototype for a printer-spooler class
 //such that only one instance can ever exist
 static boolean instance_flag = false; //true if 1 instance
 private Spooler() {
 }
 static public Spooler Instance() {
   if (!instance_flag) {
     instance_flag = true;
     return new Spooler();
   } else
     return null;
 }
 public void finalize() {
   instance_flag = false;
 }

} 


      </source>



Singleton pattern in Java 2

   <source lang="java">

/* The Design Patterns Java Companion Copyright (C) 1998, by James W. Cooper IBM Thomas J. Watson Research Center

  • /

class Spooler { 

 //this is a prototype for a printer-spooler class
 //such that only one instance can ever exist
 static boolean instance_flag = false; //true if 1 instance
 public Spooler() throws SingletonException {
   if (instance_flag)
     throw new SingletonException("Only one printer allowed");
   else
     instance_flag = true; //set flag for 1 instance
   System.out.println("printer opened");
 }
 public void finalize() {
   instance_flag = false;
 }

} class SingletonException extends RuntimeException { 

 //new exception type for singleton classes
 public SingletonException() {
   super();
 }
 public SingletonException(String s) {
   super(s);
 }

} public class SingleSpooler { 

 static public void main(String argv[]) {
   Spooler pr1, pr2;
   //open one printer--this should always work
   System.out.println("Opening one spooler");
   try {
     pr1 = new Spooler();
   } catch (SingletonException e) {
     System.out.println(e.getMessage());
   }
   //try to open another printer --should fail
   System.out.println("Opening two spoolers");
   try {
     pr2 = new Spooler();
   } catch (SingletonException e) {
     System.out.println(e.getMessage());
   }
 }

} 

      </source>



Singleton pattern in Java 3

   <source lang="java">

/* The Design Patterns Java Companion Copyright (C) 1998, by James W. Cooper IBM Thomas J. Watson Research Center

  • /

class Printer { 

 //this is a prototype for a printer-spooler class
 //such that only one instance can ever exist
 static boolean instance_flag = false; //true if 1 instance
 public Printer() throws SingletonException {
   if (instance_flag)
     throw new SingletonException("Only one printer allowed");
   else
     instance_flag = true; //set flag for 1 instance
   System.out.println("printer opened");
 }
 public void finalize() {
   instance_flag = false;
 }

} class SingletonException extends RuntimeException { 

 //new exception type for singleton classes
 public SingletonException() {
   super();
 }
 public SingletonException(String s) {
   super(s);
 }

} public class SinglePrinter { 

 static public void main(String argv[]) {
   Printer pr1, pr2;
   //open one printer--this should always work
   System.out.println("Opening one printer");
   try {
     pr1 = new Printer();
   } catch (SingletonException e) {
     System.out.println(e.getMessage());
   }
   //try to open another printer --should fail
   System.out.println("Opening two printers");
   try {
     pr2 = new Printer();
   } catch (SingletonException e) {
     System.out.println(e.getMessage());
   }
 }

} 

      </source>
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