Generic keying across maps: Map<K,V> from Map<T,K> and Map<T,V>

Question

I have a function which returns a one-sided intersection of values between two input maps:

Map<Key, Value> mergeMaps(Map aKeys<CompositeKey, Key>, 
         Map <CompositeKey, Value> aValues) {

    Map<Key, Value> myResult = Maps.newHashMap();
    for (CompositeKey myKey : aKeys.keySet()) {
        if (aValues.containsKey(myKey)) {
            myResult.put( aKeys.get(myKey), aValues.get(myKey));
        }
    }
    return myResult;
}

This is not a transitive mapping composition i.e.

T->K, K->V ===> T->V

but instead transforming

T->(K,V) ===> K->V

Is there a way in Java to make this function generic such that its signature is as follows?

Map<K, V> mergeMaps(Map aKeys<T, K>, Map <T, V> aValues)

Answer 1

I think this signature should do what you want:

<T, K, V> Map<K, V> mergeMaps(Map<T, K> aKeys, Map<T, V> aValues)

Answer 2

For a completely generic form you can have

public <T, K, V> Map<K, V> mergeMaps(Map<T, K> aKeys, Map<T, V> aValues)

If K and V are set in the class you can take them out.

For a case where you want to specify a relation between T and K, V you might want something like

public <K, V, T extends CompositeKey<K, V>> ...

possibly with some ? extends or ? super in there, but it depends a bit on the use case.