data structures - How to create a 2 way map in java

Question

I need a data structure to store string-int value pairs in an 1:1 relationship, and being able too look up from either way their counterpart.

I wrote a class with a Hashtable and a String array and stored the data 2 times and used the built in functions for lookup.

My question is that is there a nicer way to accomplish this? And by nicer I mean being efficient and not storing the data 2 times, and preferably without writing a ton of code either :P.

Answer 1

It seems like you may be looking for a bimap.

The Google Collections (now a part of Guava) contains an BiMap interface with a few implementations.

From the BiMap documentation:

A bimap (or "bidirectional map") is a map that preserves the uniqueness of its values as well as that of its keys. This constraint enables bimaps to support an "inverse view", which is another bimap containing the same entries as this bimap but with reversed keys and values.

The BiMap.inverse method appears to return a Map with the values as the keys, and the keys as the values, so that Map can be used to call get on the value and retrieve a key.

In addition the Map returned by inverse is a view of the underlying data, so it does not have to make extra copies of the original data.

From the BiMap.inverse method documentation:

Returns the inverse view of this bimap, which maps each of this bimap's values to its associated key. The two bimaps are backed by the same data; any changes to one will appear in the other.