实现序列号接口（Serializable），标记这个类可以被序列化。 “I’m fine to be serialized”.
当你尝试序列化没有实现Serializable接口的对象时，将会抛出异常 NotSerializableException.
The idea is to be able to transparently serialize instances of the class – potentially for caching or other purposes, I’m not sure in this case. The serialVersionUID field is just part of the versioning that Java binary serialization uses.
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