code: public long getNextCluster(long cluster) {
          testCluster(cluster);
          long entry = entries[(int) cluster];
          if (isEofCluster(entry)) {
              return -1;
          } else {
              return entry;
          }
      }
docs: Gets the cluster after the given cluster
  
  @param cluster
  @return long The next cluster number or -1 which means eof.