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Wed Mar 24 16:00:00 GMT 2004
PrinterSingleton
package examples.singleton.java; /* -*- Mode: Java; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- * * This file is part of the design patterns project at UBC * * The contents of this file are subject to the Mozilla Public License * Version 1.1 (the "License"); you may not use this file except in * compliance with the License. You may obtain a copy of the License at * either http://www.mozilla.org/MPL/ or http://aspectj.org/MPL/. * * Software distributed under the License is distributed on an "AS IS" basis, * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License * for the specific language governing rights and limitations under the * License. * * The Original Code is ca.ubc.cs.spl.patterns. * * Contributor(s): */ /** * Implements a sample <i>Singleton</i> class. The class's functionality * is to store an instance-specific ID and provide a <code>print()</code> * method that shows an object's ID. * * Note that in this implementation the <i>Singleton</i> class has to know * that it implements the pattern (i.e. has to have appropriate code in it). * * @author Jan Hannemann * @author Gregor Kiczales * @version 1.0, 05/13/02 * * @see PrinterSubclass */ public class PrinterSingleton { /** * Counts the instances of this class */ protected static int objectsSoFar = 0; /** * Stores the <i>Singleton</i>'s only instance */ protected static PrinterSingleton onlyInstance = null; /** * Each instance has an ID to distinguish them. */ protected int id; /** * Creates a new <code>PrinterSingleton</code>. The new instance gets * an ID equal to the total number of instacnes of that type created +1. */ protected PrinterSingleton() { id = ++ objectsSoFar; } /** * Factory method that provides access to the <i>Singleton</i> instance. * * returns the unique <i>Singleton</i> instance */ public static PrinterSingleton instance() { if(onlyInstance == null) { onlyInstance = new PrinterSingleton(); } return onlyInstance; } /** * Prints the instance's ID to <code>System.out</code>. */ public void print() { System.out.println("\tMy ID is "+id); } }
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