code: private T doRemove(int index) {
          keys[index]   = 0;
          states[index] = REMOVED;
          final Object previous = values[index];
          values[index] = missingEntries;
          --size;
          ++count;
          return (T)previous;
      }
docs: Remove an element at specified index.
  @param index index of the element to remove
  @return removed value