Generic HashMap that takes two keys

TwoKeyHashMap.java

public class TwoKeyHashMap<K1, K2, V> extends HashMap<K1, HashMap<K2, V>> {

    public void put(K1 key_1, K2 key_2, V value) {
        if (containsKey(key_1))
            get(key_1).put(key_2, value);
        else {
            HashMap<K2, V> inner = new HashMap<>();
            inner.put(key_2, value);
            put(key_1, inner);
        }
    }

    public V get(K1 key_1, K2 key_2) {
        if (contains(key_1, key_2))
            return get(key_1).get(key_2);
        return null;
    }

    public boolean contains(K1 key_1, K2 key_2) {
        HashMap<K2, V> inner = get(key_1);
        if (inner == null)
            return false;
        return inner.containsKey(key_2);
    }
}