The performance of the HashMap in JAVA8

Question

I had a question when I learned the HashMap source code in java8。

Source code is so complicated, how much efficiency?

So I wrote a code about the hash conflict。<

Answer 1

Your test scenario is non-optimal for Java 8 HashMap. HashMap in Java 8 optimizes collisions by using binary trees for any hash chains longer than a given threshold. However, this only works if the key type is comparable. If it isn't then the overhead of testing to see if the optimization is possible actually makes Java 8 HashMap slower. (The slow-down is more than I expected ... but that's another topic.)

Change your Test class to implement Comparable<Test> ... and you should see that Java 8 performs better than the others when the proportion of hash collisions is large enough.

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Note that the tree optimization should be considered as a defensive measure for the case where the hash function doesn't perform. The optimization turns O(N) worst-case performance to O(logN) worst-case.

If you want your HashMap instances to have O(1) lookup, you should make sure that you use a good hash function for the key type. If the probability of collision is minimized, the optimization is moot.

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Source code is so complicated, how much efficiency?

It is explained in the comments in the source code. And probably other places that Google can find for you :-)