code: private int getDataIndexOfKey(Object key) {
      int dataSize = this.size << 1;
      Object[] data = this.data;
      for (int i = 0; i < dataSize; i += 2) {
        Object k = data[i];
        if (key == null ? k == null : key.equals(k)) {
          return i;
        }
      }
      return -2;
    }
docs: Returns the data index of the given key or {@code -2} if there is no such key.