The Singleton pattern is used for classes which should have only one instance in the application. The other classes can't invoke the constructor directly because it is declared private and some public static method is provided which returns the single instance. This pattern makes the instance available globaly and thus should be used carefully.

e.g.

class InstanceOne{
InstanceOne one;
private InstanceOne(){}
public static InstanceOne getInstance(){
	if(one != null) {
		return one;
	} else {
		one = new InstanceOne();
	}
}
}