Hashmap in multithreaded environment when doing resizing

Question

I am following a tutorial and it basically explains about the cause of race condition which happens when resizing Hashmap in a multithreaded environment:

Answer 1

Actually there is at least one race condition related to rehashing. Look at this code fragment (it is from Sun JDK7):

boolean oldAltHashing = this.useAltHashing;
this.useAltHashing |= sun.misc.VM.isBooted() && (this.newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean rehash = oldAltHashing ^ this.useAltHashing;
transfer(newTable, rehash);
this.table = newTable;

Here It is possible that thread T1 ends up with rehash = true and thread T2 ends up with rehash = false (let's suppose T1 has already changed the value of this.useAltHashing).

Now, guess that which thread will write the this.table - you have no idea, either can. So, it's a question of luck whether you get a consistent internal state or not.

Anyway, as I mentioned in my comment by design, it is not supposed to use HashMap in a multithreaded environment. It will not work. Either because of this, or because of other reasons. The above was just one example why you shouldn't try to go against the contracts.