a singleton using enum

enum-singleton.java

```
public enum Elvis {
       INSTANCE;
       public void leaveTheBuilding() { ... }
}

static void main(){
    Elvis.INSTANCE.leaveTheBuilding();
}
```

<< Effective Java >> Item 3.

This approach is functionally equivalent to the public field approach, except that it is more concise, provides the serialization machinery for free, and provides an ironclad guarantee against multiple instantiation, even in the face of sophisticated serialization or reflection attacks. While this approach has yet to be widely adopted, a single-element enum type is the best way to implement a singleton.