The main reason that nulls aren't allowed in ConcurrentMaps
(ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that may be just barely tolerable in non-concurrent maps can't be accommodated. The main one is that if map.get(key) returns null, you can't detect whether the key explicitly maps to null vs the key isn't mapped.In a non-concurrent map, you can check this via map.contains(key),but in a concurrent one, the map might have changed between calls.
Further digressing: I personally think that allowingnulls in Maps (also Sets) is an open invitation for programsto contain errors that remain undetected untilthey break at just the wrong time. (Whether to allow nulls evenin non-concurrent Maps/Sets is one of the few design issues surroundingCollections that Josh Bloch and I have long disagreed about.)
It is very difficult to check for null keys and valuesin my entire application . Would it be easier to declare somewherestatic final Object NULL = new Object();and replace all use of nulls in uses of maps with NULL? -Dougv