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You need a definition for the class that the jvm can use to build the object from the serialized object.

Read the serialized object into an instance of the Object class and call getClass() on that object.
Something like:
Object obj = readObject()
S.o.p(obj.getClass())


You will need code to...

For debugging you could use the getClass() method.
Did you try using instanceof to test the object that was read?