IoDH单例模式

class SingletonIoDH {
    private Singleton() {
    }

    private static class HolderClass {
            private final static Singleton instance = new Singleton();
    }

    public static SingletonIoDH getInstance() {
        return HolderClass.instance;
    }

    public static void main(String args[]) {
        SingletonIoDH s1, s2;
            s1 = SingletonIoDH.getInstance();
        s2 = SingletonIoDH.getInstance();
        System.out.println(s1==s2);
    }
}