Saturday, August 8, 2009

is hashcode used as array index inside the hash table?

We know that 2 objects that both hash to 112233 will fall into the same bucket on an array of buckets, but is the hashcode of 112233 the real array index? To create an array of 112233 elements, system must reserve that much memory. That's the C/C++ array.

(A2: Since internal array size is always an exponential of 2, one economical algorithm is to take the lowest n bits of the 32-bit hashCode(). Throws away lots of information, but i guess can still be random.)

Alternatively, can each hash table in memory have an internal lookup table that translates hashcode to array indices? How do you use such a look-up? Remember internally most systems only has arrays and linked lists. All other data structures must be built on these.

Answer is in [[generics and collections]]. algorithm can be arrayIndex = hashcode%somePrimeNumber

Q2: A barc cap low-latency interview actually asked how to economically derive an array index value from a 32-bit hashCode() for a growing hash table. Note the hash table's internal array starts out fairly small, like 16-buckets. It usually doubles in size each time. Answer appears elsewhere in this post.

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