这个方法返回一个同步的Map,这个Map封装了底层的HashMap的所有方法,使得底层的HashMap即使是在多线程的环境中也是安全的。 2、HashMap可以接受null(HashMapallows one null key and any number ofnullvalues.而Hashtable则不行)。这就是说,HashMap中如果在表中没有发现搜索键,或者如果发现了搜索键,但它是一个空...
Lisa's city : null Obtaining the entrySet, keySet, and values from a HashMap 从HashMap获取entrySet, keySet和values TheMapinterface provides methods to retrieve the set of entries (key-value pairs), the set of keys, and the collection of values. Map接口提供了检索条目集(键-值对),键集和值...
ConcurrentHashMap在源码中加入不允许插入 null （空） 值的设计，主要目的是为了防止并发场景下的歧义问...
If the application allows us to put null values in the map, then we may verify if the key is absent or the mapped value is null using the method containsKey(). 3.3. Removing Entries by Key (remove) The HashMap.remove() method removes the key-value pair for the specified key, if pre...
方法/步骤 1 HashMap几乎可以等价于Hashtable，除了HashMap是非synchronized的，并可以接受null(HashMap allows one null key and any number of null values.，而Hashtable则不行)。这就是说，HashMap中如果在表中没有发现搜索键，或者如果发现了搜索键，但它是一个空的值，那么get()将返回null。如果有必要，用...
HashMap几乎可以等价于Hashtable,除了HashMap是非synchronized的,并可以接受null(HashMapallowsonenullkeyandanynumberofnullvalues.,而Hashtable则不行)。这就是说,HashMap中如果在表中没有发现搜索键,或者如果发现了搜索键,但它是一个空的值,那么get()将返回null。如果有必要,用containKey()方法来区别这两种情况。
> without any issues as HashMap allows them. > > But ConcurrentHashMap does not allow any null key and > values . > Try to take Holger's advice. As mostly an aside though... The main reason that nulls aren't allowed in ConcurrentMaps ...
which are prone to multi threading issues>withConCurrentHashMap.>>Currently we keepnullkey and valuesinhashmap>without any issuesasHashMap allows them.>>But ConcurrentHashMap does not allow anynullkey and>values.>Try to take Holger's advice.As mostly an aside though...The main reason that ...
HashMapallows one null key(for othernullkeys, the existing value will simply be overwritten with a new value) and any number ofnullvalues. // Putting null key in TreeMapTreeMap<String,String>map=newTreeMap<>();map.put(null,"value");//Exception in thread "main" java.lang.NullPointerExc...
Similar toHashMap, SynchronizedHashMapallows one null key and multiple null values as it just wraps the methods ofHashMapwith synchronized keyword. Rest of the behavior remains same as original collection. You can also checkSynchronizedSortedMapwhich is similar data structure to convert thread-unsafe...