集合名称	key	value	说明

HashMap	            允许为null	允许为null	线程不安全
TreeMap	            不允许为null	允许为null	线程不安全
HashTable	        不允许为null	不允许为null	线程安全
ConcurrentHashMap	不允许为null	不允许为null	线程安全

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.

ConcurrentHashmap 和 Hashtable 支持并发，通过get(k)获取对应的value时，如果获取到的是null时，无法判断value为null，还是key不存在。
HashMap是非并发的，可以通过contains(key)来做判断。支持并发的Map在调用m.contains（key）和m.get(key) 值可能已经变了。


If the map can hold nulls, there is no way to tell get is returning a null 
because there was no key for that value, or just because the value was null.


https://stackoverflow.com/questions/698638/why-does-concurrenthashmap-prevent-null-keys-and-values