为什么 ConcurrentHashMap 防止空键和值? [英] Why does ConcurrentHashMap prevent null keys and values?

问题描述

ConcurrentHashMap<的JavaDoc/code> 是这样说的:

与Hashtable 类似，但与HashMap 不同的是，此类不允许允许将null 用作键或价值.

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 不允许为空?

2nd question: Why doesn't Hashtable allow 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.

I've used a lot of HashMaps for storing data. But when changing to ConcurrentHashMap I got several times into trouble because of NullPointerExceptions.

推荐答案

来自ConcurrentHashMap 的作者本人(Doug Lea):

From the author of ConcurrentHashMap himself (Doug Lea):

ConcurrentMaps 中不允许空值的主要原因(ConcurrentHashMaps, ConcurrentSkipListMaps) 就是那种歧义可能在非并发映射中勉强可以忍受 不能容纳.主要是如果 map.get(key) 返回 null，你无法检测密钥是否显式映射到 null 与密钥不是映射.在非并发地图中，您可以通过map.contains(key)，但在并发中，地图可能已更改通话之间.

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.

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