Best Practice

public class Singleton{

	public static final Singleton INSTANCE = new Singleton();

	private Singleton() {}
}

//inner static class 활용
class Singleton
{
	private Singleton() {}

  private static class SingletonHolder {
		private static final Singleton INSTANCE = new Singleton();
	}

	public static Singleton getInstance() {
			return SingletonHolder.Instance;
	}
}

public enum Elvis {
	INSTANCE;

	...
}