Bi-directional Map in Java?

You can use the Google Collections API for that, recently renamed to Guava, specifically a BiMap

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.

You could insert both the key,value pair and its inverse into your map structure, but would have to convert the Integer to a string:

map.put("theKey", "theValue");
map.put("theValue", "theKey");

Using map.get("theValue") will then return "theKey".

It's a quick and dirty way that I've made constant maps, which will only work for a select few datasets:

• Contains only 1 to 1 pairs
• Set of values is disjoint from the set of keys (1->2, 2->3 breaks it)

If you want to keep <Integer, String> you could maintain a second <String, Integer> map to "put" the value -> key pairs.

There is no bidirectional map in the Java Standard API. Either you can maintain two maps yourself or use the BidiMap from Apache Collections.

Creating a Guava BiMap and getting its inverted value is not so trivial.

A simple example:

import com.google.common.collect.BiMap;
import com.google.common.collect.HashBiMap;

public class BiMapTest {

  public static void main(String[] args) {

    BiMap<String, String> biMap = HashBiMap.create();

    biMap.put("k1", "v1");
    biMap.put("k2", "v2");

    System.out.println("k1 = " + biMap.get("k1"));
    System.out.println("v2 = " + biMap.inverse().get("v2"));
  }
}