if (lastKnownIndex == -1
          || lastKnownIndex >= size()
          || !Objects.equal(key, keys[lastKnownIndex])) {
        lastKnownIndex = indexOf(key);
      }
    }

    @SuppressWarnings("unchecked") // values only contains Vs
    @Override
    public int getCount() {
      updateLastKnownIndex();
      return (lastKnownIndex == -1) ? 0 : values[lastKnownIndex];
    }