How to use and set appropriately concurrency level for ConcurrentHashMap?

Question

I am working with around 1000 elements in concurrenthashmap . Default concurrency level is 16 . can anyone help me with some algorithm or factors from which i can identify t

Answer 1

Java 8:

Now the ConcurrentHashMap does not use a fixed lock striping scheme at all, instead each bucket serves as a “stripe” using intrinsic synchronization.

Code from source:

/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    ...
    Node f; int n, i, fh;
    ...
    else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
    ...
       synchronized (f) {
           ...
       }
}

And the constructor has the parameter just use it as a size hint as docs say.

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

And the source:

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor, int concurrencyLevel) {
    if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
        throw new IllegalArgumentException();
    if (initialCapacity < concurrencyLevel)   // Use at least as many bins
        initialCapacity = concurrencyLevel;   // as estimated threads
    long size = (long)(1.0 + (long)initialCapacity / loadFactor);
    int cap = (size >= (long)MAXIMUM_CAPACITY) ?
        MAXIMUM_CAPACITY : tableSizeFor((int)size);
    this.sizeCtl = cap;
}

So you don't need to consider it by yourself, ConcurrentHashMap will handle it for you.