Thread-safe implementation

//Early instantiation using implementation with static field.
class Singleton
{
	private static final Singleton instance = new Singleton();

	private Singleton(){}

	public static synchronized Singleton getInstance(){    
		return instance;
	}
}

//Lazy instantiation using double chekced locking mechanism.
class Singleton
{
	private static volatile Singleton instance; 
  //자바 1.4이하의 메모리 관리 기술도 이해해야함

	private Singleton(){ }

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

		return instance;
	}
}