Bidirectional Map

Question

Can you suggest a kind of map or similar data structure where we can get both the value and key from each other at equal ease. That is to say, that each may be used to find other.

Answer 1

Java doesn't have a bidirectional map in its standard library.

Use for example BiMap<K, V> from Google Guava .

Answer 2

If you feel it pain importing some third party library. How about this simple class.

public class BiMap<K,V> {

    HashMap<K,V> map = new HashMap<K, V>();
    HashMap<V,K> inversedMap = new HashMap<V, K>();

    void put(K k, V v) {
        map.put(k, v);
        inversedMap.put(v, k);
    }

    V get(K k) {
        return map.get(k);
    }

    K getKey(V v) {
        return inversedMap.get(v);
    }

}

Make sure K and V class has proper hashCode implementation.

Answer 3

The most common solution is using two maps. You can easily encapsulate them in a class with a friendly interface by extending AbstractMap. (Update: This is how Guava's HashBiMap is implemented: two maps)

Creating a new data structure using nothing but arrays and custom classes has few advantages. The map implementations are lightweight wrappers of a data structure that indexes the keys. Since you need two indexes you might as well use two complete maps.