싱글톤 생성법(늦은 생성)

1. public class Singleton {
   // Private constructor prevents instantiation from other classes
2.    private Singleton() {}
3. /**
4. * SingletonHolder is loaded on the first execution of Singleton.getInstance()
   * or the first access to SingletonHolder.INSTANCE, not before.
   */
5. private static class SingletonHolder {
6. private static final Singleton INSTANCE = new Singleton();
7. }
8. public static Singleton getInstance() {
      return SingletonHolder.INSTANCE;
   }
9. }