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Why does ConcurrentHashMap does not allow null key or values?

The main reason that null is not allowed in ConcurrentMaps such as ConcurrentHashMaps, ConcurrentSkipListMaps is to avoid ambiguities. If map.get(key) returns null, you cannot detect whether the key explicitly maps to null or the key itself is not mapped. In a non-concurrent map, you may check this using map.contains(key), but in a concurrent environment, the map might have changed between calls.

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