The return value of "putIfAbsent" must be used - really?

Question

i've got a ConcurrentMap of ConcurrentMaps like this...

ConcurrentMap<String, ConcurrentMap<K, V>> mapsMap = new ConcurrentHashMap<>();

Now in some method, I would like to prevent NPE by making sure a map exists for a certain key like this...

 mapsMap.putIfAbsent(someKey, new ConcurrentHashMap<K, V>());

...so I can safely call stuff like...

 mapsMap.get(someKey).put(...);

...without worrying about null values here.

Now, Sonarqube is telling me, that this violates the rule RSPEC-2201...

Return values from functions without side effects should not be ignored [..] and also on ConcurrentMap.putIfAbsent calls ignored return value.

Is this just SonarQube not detecting that the side effect of the method is enough for me here (and the return value would not add any information) or am I missing an important point about the putIfAbsent contract?

Answer 1

If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.

You can safely ignore the returned value, if you don't need it.
Sonarqube is pretty invasive with certain rules, so just disable it and go on with your business.

Imho, that specific rule is mostly set for common patterns on a project where code is written by multiple developers.