public V get(K1 key1, K2 key2) {
        checkNotNull(key1, "Key1 cannot be null");
        checkNotNull(key2, "Key2 cannot be null");

        Map<K2, V> innerMap = backingMap.get(key1);
        if (innerMap == null) {
            return null;
        }
        return innerMap.get(key2);
    }
@Test
    public void givenEmpty_whenGet_thenReturnNull() {
        String value = map.get("key1", "key2");
        assertNull(value);
    }