并发集合的包含方法是无用的吗？ [英] Is concurrent collection's contain-like method useless?

问题描述

  if（concurrentHashMap.containKey（key））
 {
 // oops，v在当前线程之后的另一个线程中被删除
 / / complete containsKey调用
值v = concurrentHashMap.get（key）; 
 //在v上做某事//空指针异常
} 
  

似乎并发集合的包含类方法是无用的，要解决上述问题：

  Vaule v = concurrentHashMap.get（key）; 
 if（v！= null）
 {
 //确定，保存v的引用
 //在v 
上执行操作} 
  
 
 
 
我是对吗？
解决方案
如注释线程中所述：否， containsKey 不会以任何方式保持条目锁定，因此稍后调用 get（key）可能返回 null 。也就是说，如果你只对布尔感兴趣 - 是地图中的键？ -  
 
 
 
此外，您还可以使用 containsKey 接口的 containsKey 是必需的，所以它不像它不会有。
 
if(concurrentHashMap.containKey(key))
{
    // oops, v has been removed in another thread right after current thread 
    // complete containKey calling
    Value v = concurrentHashMap.get(key); 
    // do something on v                  // null pointer exception
}
It seems concurrent collection's contain-like method is useless, to fix above problem:
Vaule v = concurrentHashMap.get(key);
if(v != null)
{
    // ok, hold v's reference
    // do something on v
}
Am I right?
解决方案
As stated in the comment thread: no, containsKey does not keep the entry locked in any way, so a later call to get(key) might return null.  That said, if you're only interested in the boolean -- is the key in the map? -- and you don't need to get the key later on, then containsKey is fine.

Additionally, containsKey is required by the Map interface, so it's not like it wouldn't be there anyway.

                        
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