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();