code: public void increment(int idx) {
      Preconditions.checkElementIndex(idx, this.values.size());
      Preconditions.checkArgument(this.values.get(idx) < Long.MAX_VALUE);
      this.values.set(idx, this.values.get(idx) + 1);
    }
docs: Increment the idx'th value. idx must be between 0 and size()-1.