Question

How is hashCode() implemented?

My assumption is that it uses the object memory location as the initial number (the seed) on which it runs the hash function. However, this is not the case.

I’ve also looked at Hash : How does it work internally? but it does not answer my question.

Yes I could download the SDK, but before I do that and look at the code, perhaps someone else already has knowledge of it.

Thanks 🙂

EDIT:
I know it should be overridden and such, so please try to stay on topic 🙂

Answer 1

Of course it is implementation specific, but generally the hash code for an object will be computed lazily and stored in the object header. Odd things are done with headers to keep them small whilst allowing complex locking algorithms.

In the OpenJDK/Oracle JVM the usual method of computing the initial hash code is based on the memory address at the time of the first request. Objects move about in memory, so using the address each time would not be a good choice. The hash code isn’t the actual address – that would typically be a multiple of eight which isn’t great for using straight in a hash table particularly with a power of two size. Note identity hash codes are not unique.

HotSpot has build time options to always use zero or use a secure random number generator (SRNG) for testing purposes.