A HashMap cannot contain duplicate keys. HashMap不能包含重复的键 Java HashMap allowsnullvalues and thenullkey. Java Hash Map允许使用null值和null键 HashMap is an unordered collection. It does not guarantee any specific order of the elements. HashMap是一个无序集合。它不保证元素的任何特定顺序 J...
1、值没有在集合中，所以返回的结果就是 null （空）；2、值就是 null（空），所以返回的结果就是...
> It is very difficult to check for null keys and values > in my entire application . > Would it be easier to declare somewhere static final Object NULL = new Object(); and replace all use of nulls in uses of maps with NULL? -Doug 原作者认为,在ConcurrentMaps (ConcurrentHashMaps, Con...
原作者认为,在ConcurrentMaps (ConcurrentHashMaps, ConcurrentSkipListMaps)上,不允许null值的出现的主要原因是他可能会在并发的情况下带来难以容忍的二义性。如果在HashMap等非并发容器中,你可以通过contains方法来判断,这个key是究竟不存在,还是本来就是null。但是在并发容器中,如果允许空值的存在的话,你就没法判断真...
Hi, Could you please advice the reason behind why Hashtable doesn't allow null value and HashMap allows only one null as key
I forgot that hash map allows null keys. But what if it is hash table..? Thanks, E. Rajasekar. SCJP 1.4 Christophe Verré Sheriff Posts: 14691 16 I like... posted 18 years ago get() would also be ok with HashTable, but it may be good practice to use containsKey() instead. (...
个人认为在实现上是可以支持的,但是作者的设计风格是想尽量避免不必要的隐藏异常。但是如果你确实需要在ConcurrentHashMap中使用null怎么办呢? 三、替代方案 可以使用一个特殊的静态空对象来代替null。 publicstaticfinalObject NULL=newObject(); 参考 Why does ConcurrentHashMap prevent null keys and values?
Collections that Josh Bloch andIhave long disagreed about.)>>It is very difficult to checkfornullkeys and values>inmy entire application.>Would it be easier to declare somewherestaticfinal ObjectNULL=newObject();and replace all useofnullsinusesofmapswithNULL?-Doug ...
Java's HashMap doesn't actually allow multiple keys - it just allows you to replace the value for a particular key, which is the same for Hashtable in .NET, if you use the indexer rather than the Add method. As for supporting a null key - you could implement your own IDictionary whi...
This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map. (A key k is reinserted into a map m if m.put(k, v) is invoked...