Two Way Mapping using single Data Structure

Question

I ran across some code recently at work (recreated to be similar to what I am dealing with) similar to the code below

Is there a way I can rework the code below to use one data structure (with performance in mind)?

Here is some code to illustrate what I mean:

public class ObjectMapper {

    private Map<UUID,Integer> uuidMap;
    private Map<Integer,UUID> indexMap;

    public ObjectMapper(){
        uuidMap = new HashMap<UUID,Integer>();
        indexMap = new HashMap<Integer,UUID>();
    }

    public void addMapping(int index, UUID uuid){
        uuidMap.put(uuid, index);
        indexMap.put(index, uuid);
    }


    .
    .
    .

    public Integer getIndexByUUID(UUID uuid){
        return uuidMap.get(uuid);
    }

    public UUID getUUIDByIndex(Integer index){
        return indexMap.get(index);
    }


}

Answer 1

This is answered here with the recommendation to use BiMap from Google Collections

Answer 2

Apache collections supports a BidiMap interface and a variety of fairly effective implementations.

Answer 3

You could use a single Map<Object,Object> to do both mappings. Ugly, sure. Performance should be roughly the same, or slightly better in the unlikely event that you have many ObjectMappers with few mapped values.

Answer 4

You can use BiMap from Eclipse Collections.

BiMap is a map that allows users to perform lookups from both directions. Both the keys and the values in a BiMap are unique.

The main implementation is HashBiMap.

inverse()

BiMap.inverse() returns a view where the position of the key type and value type are swapped.

MutableBiMap<Integer, String> biMap =
  HashBiMap.newWithKeysValues(1, "1", 2, "2", 3, "3");
MutableBiMap<String, Integer> inverse = biMap.inverse();
Assert.assertEquals("1", biMap.get(1));
Assert.assertEquals(1, inverse.get("1"));
Assert.assertTrue(inverse.containsKey("3"));
Assert.assertEquals(2, inverse.put("2", 4));

put()

MutableBiMap.put() behaves like Map.put() on a regular map, except it throws when a duplicate value is added.

MutableBiMap<Integer, String> biMap = HashBiMap.newMap();
biMap.put(1, "1"); // behaves like a regular put()
biMap.put(1, "1"); // no effect
biMap.put(2, "1"); // throws IllegalArgumentException

forcePut()

This behaves like MutableBiMap.put(), but it silently removes the map entry with the same value before putting the key-value pair in the map.

MutableBiMap<Integer, String> biMap = HashBiMap.newMap();
biMap.forcePut(1, "1"); // behaves like a regular put()
biMap.forcePut(1, "1"); // no effect
biMap.forcePut(1, "2"); // replaces the [1,"1"] pair with [1, "2"]
biMap.put(2, "2"); // removes the [1, "2"] pair before putting
Assert.assertFalse(biMap.containsKey(1));
Assert.assertEquals(HashBiMap.newWithKeysValues(2, "1"), biMap);

Note: I am a committer for Eclipse collections.