In jdk1.1.x, when a Serializable class is evolved to have a new Serializable superclass, this evolved class is no longer bidirectionally compatible with the original class.

Specifically, when the original class tries to read an object serialized by
the evolved class, a core dump occurs. The evolved class does not have trouble
reading an object that has been written by the original class.

In jdk1.2, it works fine.