Bi-directional Map in Java? [duplicate]

I have a simple integer-to-string mapping in Java, but I need to be able to easily retrieve string from integer, and also integer from string. I've tried Map, but it can retrieve only string from integer, it's one way:

private static final Map<Integer, String> myMap = new HashMap<Integer, String>();
// This works one way:
String myString = myMap.get(myInteger);

// I would need something like:
Integer myInteger = myMap.getKey(myString);

Is there a right way to do it to have it both directions?

Another problem is that I only have a few constant values that don't change (1->"low", 2->"mid", 3->"high", so it wouldn't be worth to go for a complicated solution.

Solution 1:

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.

Solution 2:

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"));
  }
}

Solution 3:

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