java - One-to-one mapping data structure (A,B) with getKey(B) in O(1)? - Stack Overflow

java - One-to-one mapping data structure (A,B) with getKey(B) in O(1)? - Stack Overflow

If you're willing to use third-party libraries, Guava provides a nice API for this as BiMap. Instead of a getKey method, it provides an inverse() view which returns a BiMap<V, K>. (It does, of course, provide constant-time containsValue.)

At the moment, HashBiMap is basically internally two HashMaps kept in sync -- though it's very consistent about how it keeps them matching each other -- but the implementation might get smarter in the future.

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