How ConcurrenthashMap manages if Concurrency level is HIGHER than the number of Buckets?

Question

The ConcurrentHashMap uses segment level locking mechanism for supporting concurrent modifications. It has three basic parameters

• Number of Buckets. (Default initial size = 16)
• Concurrency Level. (Default size = 16)
• Load Factor.(Default size = 0.75)

In default case we have Single Lock per Bucket . If the number of buckets are made as 32 then we will have Single Lock per Two Buckets .

How is it managed if number of buckets are Less than the Concurrency Level, ie if

Map cMap = new ConcurrentHashMap(16,1,32); 

Usually a bucket uses a Collection (Linked List) for the items with Hash-codes colliding in same bucket. Do we have in above case Two Locks per Bucket, if yes then how it is managed, (does half of the collection in a bucket uses one lock and other half uses second lock ?)

I have searched and was able to see the answers if the ConcurrentHashMap is resized to have more buckets than the number of Locks but I was not able to get an answer what if it was the reverse case, ie :

How ConcurrenthashMap manages if Concurrency level is HIGHER than the number of Buckets ?

Answer 1

This code in the constructor should answer your question:

if (initialCapacity < concurrencyLevel)   // Use at least as many bins
    initialCapacity = concurrencyLevel;   // as estimated threads

See also the documentation :

concurrencyLevel the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.