Inspect java serialized object without defining the object

I want to inherit and modify a Java serialized object (in a file). The file has been serialized to ObjectOutputStream

or similar. The file was not written by me and I have no class definition.

There is a python module I found that can do this , but it's broken. Is there a way to do this using Java?

the Q&A here is about reading and writing serialized objects of instances of classes you defined , but I couldn't find anything about classes / objects external to the program.