java - 为什么 ConcurrentHashMap 会阻止 null 键和值？

Question

ConcurrentHashMap 的 JavaDoc是这样说的:

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

我的问题:为什么？

第二个问题:为什么Hashtable不允许null？

我使用了很多 HashMap 来存储数据。但是当更改为 ConcurrentHashMap 时，由于 NullPointerExceptions，我遇到了几次麻烦。

Answer 1

From the author of ConcurrentHashMap himself (Doug Lea) :

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.