Does the unmodifiable wrapper for java collections make them thread safe?

Question

I need to make an ArrayList of ArrayLists thread safe. I also cannot have the client making changes to the collection. Will the unmodifiable wrapper make it thread safe or d

Answer 1

Not sure if I understood what you are trying to do, but I'd say the answer in most cases is "No".

If you setup an ArrayList of ArrayList and both, the outer and inner lists can never be changed after creation (and during creation only one thread will have access to either inner and outer lists), they are probably thread safe by a wrapper (if both, outer and inner lists are wrapped in such a way that modifying them is impossible). All read-only operations on ArrayLists are most likely thread-safe. However, Sun does not guarantee them to be thread-safe (also not for read-only operations), so even though it might work right now, it could break in the future (if Sun creates some internal caching of data for quicker access for example).