Singleton

http://www.javacoffeebreak.com/articles/designpatterns/index.html

A singleton is an class that can be instantiated once, and only once. This is a fairly unique property, but useful in a wide range of object designs. Creating an implementation of the singleton pattern is fairly straightforward - simple block off access to all constructors, provide a static method for getting an instance of the singleton, and prevent cloning.


public class SingletonObject
{
private SingletonObject()
{
// no code req'd
}

public static SingletonObject getSingletonObject()
{
if (ref == null)
// it's ok, we can call this constructor
ref = new SingletonObject();
return ref;
}

public Object clone()
throws CloneNotSupportedException
{
throw new CloneNotSupportedException();
// that'll teach 'em
}

private static SingletonObject ref;
}


So why would this be useful? Often in designing a system, we want to control how an object is used, and prevent others (ourselves included) from making copies of it or creating new instances. For example, a central configuration object that stores setup information should have one and one only instance - a global copy accessible from any part of the application, including any threads that are running. Creating a new configuration object and using it would be fairly useless, as other parts of the application might be looking at the old configuration object, and changes to application settings wouldn't always be acted upon.