code: public static Hours hoursIn(ReadableInterval interval) {
          if (interval == null)   {
              return Hours.ZERO;
          }
          int amount = BaseSingleFieldPeriod.between(interval.getStart(), interval.getEnd(), DurationFieldType.hours());
          return Hours.hours(amount);
      }
docs: Creates a <code>Hours</code> representing the number of whole hours
  in the specified interval.
  
  @param interval  the interval to extract hours from, null returns zero
  @return the period in hours
  @throws IllegalArgumentException if the partials are null or invalid