Hello, I think it was topic before, but I just wanted to point out, that it is still an topic on the internetz. :)
Motivated by a StackOverflow question regarding HashMap performance regression in Java 8 http://stackoverflow.com/questions/27759527/using-java-7-hashmap-in-java-8/27760442 I made a JMH test and compared 7 and 8 speed. (the test is not very scientific as I dont really know how to squeeze the longrunning loop which alters state into the harness, but the results seem to be consitent wth theory and stopwatch testing) https://gist.github.com/ecki/9f69773eb29428a36077 What can be seen is, that with a good distribution of hash keys 8 looks faster than 7, and with a bad distribution of hash keys Java 7 is faster (unless you supply a Comparator for the key). (and a good distributed hashkey with comparable seems to be a bit slower) I think the regression is somewhat expected, but I guess its not widely known. (I do not use a cached hashcode, but it has a nearly trivial implementation just to make it more life like. the tests also compares different initial sizes, but they do not have an measurable effect on the performance, I show only default size below:) java version "1.7.0_72" Benchmark (initialSize) Mode Samp Score Error Units n.e.j.h.HashMapCollision.badDistNoComp 16 avgt 4 10847,318 ± 5596,690 ms/op n.e.j.h.HashMapCollision.badDistWithComp 16 avgt 4 10761,430 ± 5376,975 ms/op n.e.j.h.HashMapCollision.goodDistNoComp 16 avgt 4 3613,923 ± 254,823 ms/op n.e.j.h.HashMapCollision.goodDistWithComp 16 avgt 4 3656,229 ± 274,350 ms/op java version "1.8.0_25" Benchmark (initialSize) Mode Samp Score Error Units n.e.j.h.HashMapCollision.badDistNoComp 16 avgt 4 14309,880 ± 1811,709 ms/op <-slower n.e.j.h.HashMapCollision.badDistWithComp 16 avgt 4 8232,037 ± 5974,530 ms/op n.e.j.h.HashMapCollision.goodDistNoComp 16 avgt 4 3304,698 ± 116,866 ms/op n.e.j.h.HashMapCollision.goodDistWithComp 16 avgt 4 3425,762 ± 107,659 ms/op Greetings Bernd
