Seeing sporadic OptionalDataExceptions in your Java code?

This is basically to uphold a valuable mental note I made today:

I was seeing very sporadic OptionalDataExceptions while doing deserialization of certain objects. Have wondered for a looong time why it was, and today I took the time (many hours) to sit down and figure out what it was.

After spending a lot of time creating custom ObjectInputStreams, debugging readObject() methods and more, I finally tracked it down to being an elementary (but extremely hard to identify) issue: Some of my collections (HashMaps, HashSets and more) were exposed to multiple threads and since these basic collection type are neither concurrent nor synchronized, this was the issue.

In essense: If you're seeing this issue, double-check all your HashSets, HashMaps, LinkedHashMaps, LinkedHashSets and IdentityHashMaps! If they're exposed to multithreading, then make them concurrent (or synchronized).