How does Java 8's HashMap degenerate to balanced trees when many keys have the same hash code?

Question

How does Java 8\'s HashMap degenerate to balanced trees when many keys have the same hash code? I read that keys should implement Comparable to define an order

Answer 1

HashMap has it's own hash method that applies a supplemental 2 bit lenght hash to the objects inside in order to avoid this problems:

Applies a supplemental hash function to a given hashCode, 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.

If you want to see how it's done, take a look is inside the source of the HashMap class.

static int hash(int h) {
    // This function ensures that hashCodes that differ only by
    // constant multiples at each bit position have a bounded
    // number of collisions (approximately 8 at default load factor).
    h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>> 7) ^ (h >>> 4);
}