code: String serializeToString() {
      final StringBuilder sb = new StringBuilder();
      sb.append(String.format("%d,%d,", numActive, keys.length));
  
      for (int i = 0; i < keys.length; i++) {
        if (states[i] != 0) {
          sb.append(String.format("%d,%d,", keys[i], values[i]));
        }
      }
      return sb.toString();
    }
docs: Returns a String representation of this hash map.
  
  @return a String representation of this hash map.