別的不說，singleton 沒答好，有點兒說不過去。

In Java, the latest Singleton pattern, use enum instead of a regular class. It's that simple.

The reasoning based on Joshua's is that serialization can cause issues. If a class implements Serializable, then after serialized and deserialized, a new instance can be created if the instance variables are not transient.

If you can answer the above, you will be guaranteed to pass the Singleton question.

Good luck.