code: public static BackoffStrategy fixedBackoff(long sleepTime, TimeUnit timeUnit) throws IllegalStateException {
          Preconditions.checkNotNull(timeUnit, "The time unit may not be null");
          return new FixedBackoffStrategy(timeUnit.toMillis(sleepTime));
      }
docs: Returns a backoff strategy that waits a fixed amount of time before retrying.
  
  @param sleepTime the time to wait
  @param timeUnit  the unit of the time to wait
  @return a backoff strategy that waits a fixed amount of time
  @throws IllegalStateException if the wait time is &lt; 0