code: public void sumUp() {
          int offset=0;
          int tmp=0;
          for (int i = 0; i < numChunks; i++) {
              int[] chunk = chunks[i];
              if (chunk==null) throw new IllegalStateException("Chunks are not continous, null fragement at offset "+i);
              for (int j = 0; j < chunkSize; j++) {
                  tmp = chunk[j];
                  chunk[j] = offset;
                  offset += tmp;
              }
          }
      }
docs: special operation implemented inline to compute and store sum up until here