package ca.ubc.cs.spl.aspectPatterns.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.aspectPatterns.
*
* For more details and the latest version of this code, please see:
* http://www.cs.ubc.ca/labs/spl/projects/aodps.html
*
* 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 1.1, 02/18/04
*/
public class PrinterSingleton {
/**
* counts the instances of this class
*/
protected static int objectsSoFar = 0;
/**
* stores this <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 instances created of that type.
* This constructor is protected to disallow it being called from other
* places but the factory method and this type's subtypes.
*
* Unfortunately, this still allows other types in the same package
* to access the constructor.
*
* Choosing to make it <code>private</code> would prevent that problem,
* but would make it impossible to subclass the type properly
* (as subtypes then could not use <code>super(..)</code> in their
* constructor.
*/
protected PrinterSingleton() {
id = ++ objectsSoFar;
}
/**
* Factory method that provides access to the <i>Singleton</i> instance.
* Uses creation-on-demand.
*
* @return 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);
}
}