Item3:Enforce the singleton property with a private constructor or an enum type
采用枚举类型（ENUM）实现单例模式。
public enum Elvis { INSTANCE; public void leaveTheBuiding(){ System.out.println("a single-element enum type is the best way to implement a singleton"); } }
使用方式：
Elvis.INSTANCE.leaveTheBuiding();
Effective Java Item3:Enforce the singleton property with a private constructor or an enum type
原文：http://www.cnblogs.com/yshyee/p/4299436.html