Like you suggest you can make a cheap check that sometimes triggers an exception. Or an expensive check that always triggers on all concurrent access. As far as I can see in the java HashMap source, the concurrent modification check is actually only done for the enumerator (?) so the problem of multiple concurrent insertions, or even reading-while-inserting will never be caught anyway. Just like Go, or .NET.<p>A check for single thread access (which is even stricter than non-concurrent access which allows several threads as long as they aren&#x27;t used concurrently) would be pretty cheap: store the thread ID on creation and then verify on reads and writes.<p>Failing on concurrent access otherwise on all reads and writes would basically mean that you add a lock to the write operation, and fail any reads while anyone is writing. This however is close enough to making it a full concurrent map, so it isn&#x27;t worth it.