Definition: The singleton pattern ensures that a class has at most one instance and that there is a global point of access to it.

Usage: Use singleton whenever you need exactly one instance of a class. Also, a class containing only static methods may be considered a singleton in some cases.

Example

class Singleton {
	public static Singleton getInstance() {
		if(uniqueInstance == null) {
			synchronized(Singleton.class) {
				if(uniqueInstance == null)
					uniqueInstance = new Singleton();
			}
		}
	}

	private Singleton() {}

	private volatile static Singleton uniqueInstance;
}

Note that this getInstance method won't work on Java 1.4 or earlier because the volatile keyword didn't work properly.