使用单元素的枚举类型实现Singleton

从java1.5版本开始，实现singleton出现了第三种方式：

public enum SingleTon {
    INSTANCE;
    public void speak() {
        System.out.println(this + " is speaking! ");
    }
}  

测试代码：

public class SingleTonTest {  

    public static void main(String[] args) throws Exception {
        SingleTon s1 = SingleTon.INSTANCE;
        s1.speak();
        SingleTon s2 = SingleTon.INSTANCE;
        s2.speak();
        System.out.println(s1 == s2);
    }
}  

测试结果：
INSTANCE is speaking!
INSTANCE is speaking!
true