Question

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?

Answer 1

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.