ConcurrentModificationException when using stream with Maps key set

Question

I want to remove all items from someMap which keys are not present in someList. Take a look into my code:

someMap.keySet().stream().filter(v -> !someList.contains(v)).forEach(someMap::remove); 

I receive java.util.ConcurrentModificationException. Why? Stream is not parallel. What is the most elegant way to do this?

Answer 1

@Eran already explained how to solve this problem better. I will explain why ConcurrentModificationException occurs.

The ConcurrentModificationException occurs because you are modifying the stream source. Your Map is likely to be HashMap or TreeMap or other non-concurrent map. Let's assume it's a HashMap. Every stream is backed by Spliterator. If spliterator has no IMMUTABLE and CONCURRENT characteristics, then, as documentation says:

After binding a Spliterator should, on a best-effort basis, throw ConcurrentModificationException if structural interference is detected. Spliterators that do this are called fail-fast.

So the HashMap.keySet().spliterator() is not IMMUTABLE (because this Set can be modified) and not CONCURRENT (concurrent updates are unsafe for HashMap). So it just detects the concurrent changes and throws a ConcurrentModificationException as spliterator documentation prescribes.

Also it worth citing the HashMap documentation:

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

While it says about iterators only, I believe it's the same for spliterators.

Answer 2

You don't need the Stream API for that. Use retainAll on the keySet. Any changes on the Set returned by keySet() are reflected in the original Map.

someMap.keySet().retainAll(someList);