Here is an example of the Singleton creational pattern in Java:
/*
Copyright (c) 2007 Mike Quentel
Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation
files (the "Software"), to deal in the Software without
restriction, including without limitation the rights to use,
copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following
conditions:
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.
*/
public class SingletonExample {
/**
* The Singleton contains a private static data member
* representing itself.
*/
private static SingletonExample singleton;
/**
* Private constructor.
*/
private SingletonExample() { }
/**
* The getSelf() accessor lazily instantiates
* the Singleton.
* The synchronized qualifier is
* used to prevent multiple threads
* from creating multiple instances.
*/
public static synchronized SingletonExample getSelf() {
if (singleton == null) {
singleton = new SingletonExample();
} //if (singleton == null)...
return singleton;
} //public static synchronized...
/**
* Clone() overridden to prevent cloning.
*/
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
} //public Object clone...
} //public class SingletonExample...
