A perfect hash function for a set S is a hash function that maps distinct elements in S to a set of integers, with no collisions.
-- Wikipedia
The default HashMap implementation in Java is very well-written, fast, efficient and should be used by default in almost any application.
However, you know the difference between System.nanoTime() and System.currentTimeMillis(). You have already benchmarked and optimized the GC's behaviour, you have experimented with pinning the threads in your application to a fixed set of processors and measuring latency in milliseconds is simply too coarse for your domain.
Still reading?
You need the fastest possible hashmap implementation which is primarily used for read operations. You are probably writing a latency sensitive financial application where milliseconds can be transformed into dollars.
PerfectHashmap is optimized for fast get and contains operations.
It is required that the set of keys is finite, known in advance and the hashcodes for the keys are collision free.
Typically, it's well suited for small, fixed lookup tables. But you should of course benchmark the performance in your usage scenario.
The map is built with a builder object:
Map<Integer, String> map = PerfectHashMap.newBuilder().add(1, "a").add(2, "b").build();
Then all the usual methods can be invoked except new keys cannot be inserted. However, existing keys can have the values updated:
map.put(1, "c"); // Updates the value for the key 1.
map.put(9, "d"); // Throws IllegalArgumentException.
(The MIT License)
Copyright © 2013 Nicholas Schultz-Møller
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