Fastest way to check if a List contains a unique String

Question

Basically I have about 1,000,000 strings, for each request I have to check if a String belongs to the list or not.

I\'m worried about the performance, so what\'s the

Answer 1

In general, a HashSet will give you better performance, since it does not have to look through each element and compare, like an ArrayList does, but typically compares at most a few elements, where the hashcodes are equal.

However, for 1M strings, the performance of hashSet may still not be optimal. A lot of cache misses will slow down searching the set. If all strings are equally likely, then this is unavoidable. However, if some strings are more often requested than others, then you can place the common strings into a small hashSet, and check that first, before checking the larger set. The small hashset should be sized to fit in cache (e.g. a few hundred K at most). Hits to the small hashset will then be very fast, while hits to the larger hashset proceed at speed limited by the memory bandwidth.