code: public synchronized boolean changePeriod(final Job job,
                                               final long newPeriod) {
          if (newPeriod <= 0) {
              throw new IllegalArgumentException(
                      "Period must be an integer langer than zero");
          }
  
          int i = findIndex(job);
          if (i == -1) {
              return false;
          } else {
              ScheduledJobEntry se = (ScheduledJobEntry) jobList.get(i);
              se.period = newPeriod;
              return true;
          }
      }
docs: Change the period of a job. The original job must exist for its period
  to be changed.
  <p/>
  The method returns true if the period could be changed, and false
  otherwise.
  @param job job.
  @param newPeriod new repeat period.
  @return true if period could be changed.