Best practice to use ConcurrentMap's putIfAbsent

http://stackoverflow.com/questions/3752194/best-practice-to-use-concurrentmaps-putifabsent

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I have been using Java's ConcurrentMap for a map that can be used from multiple threads. The putIfAbsent is a great method and is much easier to read/write than using standard map operations. I have some code that looks like this:

ConcurrentMap<String, Set<X>> map = new ConcurrentHashMap<String, Set<X>>();    // ...    map.putIfAbsent(name, new HashSet<X>());  map.get(name).add(Y);  

Readability wise this is great but it does require creating a new HashSet every time even if it is already in the map. I could write this:

if (!map.containsKey(name)) {      map.putIfAbsent(name, new HashSet<X>());  }  map.get(name).add(Y);  

With this change it loses a bit of readability but does not need to create the HashSet every time. Which is better in this case? I tend to side with the first one since it is more readable. The second would perform better and may be more correct. Maybe there is a better way to do this than either of these.

What is the best practice for using a putIfAbsent in this manner?

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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;      }  }