Effective Java Item3:Enforce the singleton property with a private constructor or an enum type

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();