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

An immutable object is by definition thread safe (assuming no-one retains references to the original collections), so synchronization is not necessary.

Wrapping the outer ArrayList using Collections.unmodifiableList() prevents the client from changing its contents (and thus makes it thread safe), but the inner ArrayLists are still mutable.

Wrapping the inner ArrayLists using Collections.unmodifiableList() too prevents the client from changing their contents (and thus makes them thread safe), which is what you need.

Let us know if this solution causes problems (overhead, memory usage etc); other solutions may be applicable to your problem. :)

EDIT: Of course, if the lists are modified they are NOT thread safe. I assumed no further edits were to be made.