Question

I write my implementation of HashMap in Java. I use open addressing for collision resolution. For better key distribution I want use a nice hash function for int hashcode of key. I dont know what hash function is better for it?

public int getIndex(K key) { return hash(key.hashCode()) % capacity; }

I need a hash function for hashcode of key.

Answer 1

The main problem with using % capacity is that it can return negative and positive values.

HashMap avoids this issue by using a power of 2 and uses the following approach

 public int getIndex(K key) { return hash(key.hashCode()) & (capacity-1); }

If the capacity is not a power of 2, you can ignore the high bit (which is often no so random)

 public int getIndex(K key) { return (hash(key.hashCode()) & 0x7FFFFFFF) % capacity; }

The hash function actually used can matter. HashMap uses the following

/**
 * 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.
 */
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);
}

I would use this, unless you have a good reason not to. E.g. for security reasons, if you have a service which could the subject of a denial of service attack, you will want to use a different hash to avoid a malicious user turning your HashMap into a LinkedList. Unfortunately you still have to use a different hashCode() as well as you can create a long list of Strings with the underlying hash code so mutating it later is too later.

Here is a list of strings with all have a hashCode() of 0, there is nothing a hash() function can do about that.

Why doesn't String's hashCode() cache 0?