Should you check if the map containsKey before using ConcurrentMap’s putIfAbsent

Concurrency is hard. If you are going to bother with concurrent maps instead of straightforward locking, you might as well go for it. Indeed, don’t do lookups more than necessary.

Set<X> set = map.get(name);
if (set == null) {
    final Set<X> value = new HashSet<X>();
    set = map.putIfAbsent(name, value);
    if (set == null) {
        set = value;
    }
}

(Usual stackoverflow disclaimer: Off the top of my head. Not tested. Not compiled. Etc.)

Update: 1.8 has added computeIfAbsent default method to ConcurrentMap (and Map which is kind of interesting because that implementation would be wrong for ConcurrentMap). (And 1.7 added the “diamond operator” <>.)

Set<X> set = map.computeIfAbsent(name, n -> new HashSet<>());

(Note, you are responsible for the thread-safety of any operations of the HashSets contained in the ConcurrentMap.)