JPerf is a perfect hash function generator for Java. The principle of perfect hashing is to reduce the average constant overhead of a hash table by precomputing a hash function which is optimal for the key set. Other advantages include a reduction in memory usage. Finding such a hash function is hard, especially in the general case. These run-time savings come at a cost of increased map creation time. JPerf can create a perfect hash function for a given set of keys and values.