code: public int count(T x, T y, int count) {
          if (count < 1)
              throw new IllegalArgumentException("Count must be positive: " + count);
          long index = getIndex(x, y);
          int oldCount = counts.get(index);
          int newCount = count + oldCount;
          counts.put(index, newCount);
          sum += count;
          return newCount;
      }
docs: Counts the pair, increasing its total count by the specified positive amount.
  
  @param count a positive value for the number of times the object occurred
  
  @throws IllegalArgumentException if {@code count} is not a positive value.