What is the Big-O of String.contains() in Java?

I'm w开发者_运维知识库orking on a project, and need to optimize the running time. Is String.contains() runtime the same as TreeSet.contains(), which is O(logN)?

The reason I'm asking is I'm building a TreeMap<String, TreeSet<Song>>, where Songs contain a String of lyrics. Depending on the efficiency, I am considering including a Set of the lyric words inside the Song and running searches on that rather than the String.

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One of the best known algorithms is the Boyer-Moore string searching algorithm which is O(n) although it can give sublinear performance in the best case.

Which algorithm is used in Java depends on which implemetation you download. It seems that for example OpenJDK uses a naive algorithm that runs in O(nm) and linear performance in the best case. See lines 1770-1806 here.

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.contains() definitely uses naive approach and equivalent to O(nm) time complexity.

• Boyer-moore takes O(nm) time in the worst case.
• KMP takes O(n) time in the worst case.

In one of the problems related to pattern matching, I used .contains() and it took 70 ms whereas replacing that line with patternSearch() //KMP search reduced the time to 14 ms.

Java source code | KMP search vs .contains()

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You could also try using a Trie instead of a TreeMap (although Java has no built-in Trie implementation)