My find method is not synchronized. If you have designed your Data class without find being synchronized, then you shouldn't worry about somebody else's decision and second guess yourself.
If find is synchronized, then this implies that only one thread at a time can be running it. Is that the behavior you want? I personally allow mutliple threads into the find, and only down deep within the find implementation (actually, another class that does record managing), when it comes to reading the file (via classes in java.io package) do I synchronize.
I think the logic behind synchronizing the find method, is that doing so means that records cannot have been added/deleted/modified while the find method is running.
My personal opinion is that this is unnecessary - all the find method returns is a set of record numbers, so the records could be added/deleted/modified between when the find method returns and when the records are read. So you are not really saving yourself anything.