[java] Bi-directional Map in Java?

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.

Use Google's BiMap

It is more convenient.

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"));
  }
}

Apache commons collections has a BidiMap