Java: A “prime” number or a “power of two” as HashMap size?

Question

Many books and tutorials say that the size of a hash table must be a prime to evenly distribute the keys in all the buckets. But Java\'s HashMap always uses a s

Answer 1

The standard HashMap implementation has a hash method which rehashes your object's hashcode to avoid that pitfall. The comment before the hash() method reads:

/**
 * Retrieve object hash code and applies a supplemental hash function to the
 * result hash, which defends against poor quality hash functions.  This is
 * critical because HashMap uses power-of-two length hash tables, that
 * otherwise encounter collisions for hashCodes that do not differ
 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
 */