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But you have not solved much more .... you are still opening the file in "append" mode. If you run the app 2 times, you corrupt the stream as I have described.

I can add an interesting thing. The main problem is given by that header. It's written or read/verified somewhere by the code into ObjectOutputStream / ObjectInputStream but fortunately this piece of...

Sorry, there's an important quibble with object serialization: it doesn't work in "append" mode.
The object serialization uses a well defined byte-level "protocol" and when you create an...