What is the default value for HashMap in Java?

What does the private hash() function in the HashMap class do?

• I was studying the java.util.HashMap source code, and I found the implementation of hash() quite unclear. Could some Java expert explain the rationale behind it?     /**      * 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);     }

• Answer:

  The supplemental hash function simply mixes up the bits of the value returned by the object's hashCode() function. It does this to help avoid patterns (hashes should appear random to some extent) that the hashCode() function might have, because hashCode() is implemented by the programmer using the HashMap API, and that programmer might not be trusted to implement a suitable hashCode(). To take a somewhat concrete example, let's say you're using a version of HashMap that didn't do the supplemental hash, and your objects are students. Also, let's say student IDs all end with 000, and you just use student ID as the hash code. Then, all hashes turn out to be multiples of 8, and because the HashMap uses power-of-two length tables (let's say 32, for example), this will result in a lot of collisions: only buckets 0, 8, 16, and 24 would ever be used, and only the last five digits (really, only the two non-zero digits of the last five) of the student ID would be used in the hash value. Running the supplemental hash function mixes up the bits of the student ID, so all digits are used.