Change to HashMap hash function in Java 8

Question

In java 8 java.util.Hashmap I noticed a change from:

static int hash(int h) {
    h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>>          


        

Answer 1

As Java documentation says that idea is to handle the situation where old implementation of Linked list performs O(n) instead of O(1). This happens when same hash code is generated for large set of data being inserted in HashMap.

This is not normal scenario though. To handle a situation that once the number of items in a hash bucket grows beyond a certain threshold, that bucket will switch from using a linked list of entries to a binary tree. In the case of high hash collisions, this will improve search performance from O(n) to O(log n) which is much better and solves the problem of performance.