[xxs]

>you can only do so much with Java being a reference heavy language.

This is definitely not true, implementing array based hashtable is trivial and outperforms in most cases java.utl.HashMap. It's just the decision to have LinkedHashMap (in java 1.2) extending HashMap crippled the latter. The issue has been discussed quite a few times in java core mailing list.

[revertts]

I agree, HashMap is unnecessarily hamstrung because the API requirements push it into a bucket chained implementation (C++ also made this mistake and is one of the downsides of std::unordered_map). And you can definitely write a faster implementation with an array based hashtable.

But I still stand by "you can only do so much being a reference heavy language." Unless you stick to purely primitive types, implement the hash table off heap, or Project Valhalla bears fruit, it's hard to get the data layout you'd want for a really good implementation. So I agree it can be better, but it's going to be hard to get to best - hence my comment.

[xxs]

I do agree data layout is hard - pretty much direct byte buffers (off heap) and some poor man's memory manager (plus serialization/desirilization code).

Project Valhalla and structs is something people have been asking for since Java 1.4 or so. Still nowhere near. And indeed, "best" would take a custom implementation, case by case. It's doable, but usually very far from pretty.