What operation is performed on hashCode before determining the bucket (bitwise shift)?

In Java, for example in the implementation of HashMap, a mixing operation (hash spreading) of the hash code is performed before determining the bucket, using bitwise shifts and XOR. This is done to reduce collisions and achieve a more uniform distribution of elements across buckets.

Specifically, a common expression used is:

int hash = key.hashCode();
hash ^= (hash >>> 16);

Here, >>> is an unsigned right shift by 16 bits, and ^= is the XOR operation. This operation mixes the high and low bits of the hash code to avoid situations where keys differ only in the high bits, which improves the distribution quality of keys across buckets and reduces the likelihood of collisions.