Getting a value's key instead of getting a keys value?

Question

How would I be able to get a specific key from its value. Instead of putting in the key and getting the value how would I be able to input a value and get a key?

There may be multiple keys for the same value; which would you want? — Scott Hunter, Sep 20 '15 at 00:09

score 2 · Accepted Answer · answered Sep 20 '15 at 00:13

You cannot do this with a simple Map (at least not in an efficient manner), because a key can have multiple values and the values have not been stored in a way that makes them quick to retrieve. Using just a regular Map, you would need to iterate over all the entries to find the corresponding keys for a value. However, you can create your own data structure that includes two maps (one from A to B, and the second from B to A) to create a bidirectional map with a 1:1 mapping, where you can lookup the A from B or B from A in O(1) time (I'm not using "key" and "value", because in a bidirectional map, both types are both). Check out BiMap (HashBiMap source code) from Guava for an example of this.

Getting a value's key instead of getting a keys value?

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