This is going to throw a NullPointerException ?. I see this from the javadocs
Except that the action is performed atomically.
Specified by: putIfAbsent in interface ConcurrentMap<K,V>
key - key with which the specified value is to be associated.
value - value to be associated with the specified key.
previous value associated with specified key, or null if there was no mapping for key.
NullPointerException - if the specified key or value is null.