java: summing LocalTimes difference

Question

I have multiple working hours in LocalTime and I want to check if the time exceeds 24 hours. The time is in 24h format.

For example:

02:00 - 08:00
10:00 - 12:00
23:00 - 03:00

In the example above, it exceeds 24h since it starts at 02:00 and goes till 03.00. It is only allowed to go till 02:00.

I implemented a loop and tried to calculate the time difference and sum it, for example:

02:00 - 08:00 --> 6 h
08:00 - 10:00 --> 2 h
10:00 - 12:00 --> 2 h
12:00 - 23:00 --> 11h
23:00 - 03:00 --> 4 h

In total it would be 25h, so it is bigger than 24h. However my problem is that I cannot calculate the time difference between 23:00 - 03:00 because LocalTime only goes till 23:59:59. So I currently I cannot calculate durations beyond a 24 hour period.

So using ChronoUnit won't work:

ChronoUnit.HOURS.between(23:00, 03:00);

I am not sure what kind of approach I should use to solve this problem

Does [this](https://stackoverflow.com/questions/49791495/difference-in-time-from-before-midnight-to-after-midnight-without-date/49792561#49792561) answer your question? — Sweeper, Mar 26 '20 at 11:04
Just to clarify: your input is a list of chronologically ordered LocalTime ranges? — VGR, Mar 26 '20 at 14:44
If the calculated difference comes out negative, you must have crossed midnight. In this case just add 24 hours to get the correct difference. — Ole V.V., Mar 26 '20 at 20:35

deHaar · Accepted Answer · 2020-03-26T21:22:47.570

If you are using LocalTime, then you have to use LocalTime.MIN and LocalTime.MAX for an intermediate calculation of the minutes between critical time slots. You can do it like done in this method:

public static long getHoursBetween(LocalTime from, LocalTime to) {
    // if start time is before end time...
    if (from.isBefore(to)) {
        // ... just return the hours between them,
        return Duration.between(from, to).toHours();
    } else {
        /*
         * otherwise take the MINUTES between the start time and max LocalTime
         * AND ADD 1 MINUTE due to LocalTime.MAX being 23:59:59
         */
        return ((Duration.between(from, LocalTime.MAX).toMinutes()) + 1
                /*
                 * and add the the MINUTES between LocalTime.MIN (0:00:00)
                 * and the end time
                 */
                + Duration.between(LocalTime.MIN, to).toMinutes())
                // and finally divide them by sixty to get the hours value
                / 60;
    }
}

and you can use that in a main method like this:

public static void main(String[] args) {
    // provide a map with your example data that should sum up to 24 hours
    Map<LocalTime, LocalTime> fromToTimes = new HashMap<>();
    fromToTimes.put(LocalTime.of(2, 0), LocalTime.of(8, 0));
    fromToTimes.put(LocalTime.of(8, 0), LocalTime.of(10, 0));
    fromToTimes.put(LocalTime.of(10, 0), LocalTime.of(12, 0));
    fromToTimes.put(LocalTime.of(12, 0), LocalTime.of(23, 0));
    fromToTimes.put(LocalTime.of(23, 0), LocalTime.of(3, 0));

    // print the hours for each time slot
    fromToTimes.forEach((k, v) -> System.out.println("from " + k + " to " + v 
            + "\t==>\t" + getHoursBetween(k, v) + " hours"));

    // sum up all the hours between key and value of the map of time slots
    long totalHours = fromToTimes.entrySet().stream()
            .collect(Collectors.summingLong(e -> getHoursBetween(e.getKey(), e.getValue())));

    System.out.println("\ttotal\t\t==>\t" + totalHours + " hours");
}

which produces the output

from 08:00 to 10:00 ==> 2 hours
from 23:00 to 03:00 ==> 4 hours
from 10:00 to 12:00 ==> 2 hours
from 02:00 to 08:00 ==> 6 hours
from 12:00 to 23:00 ==> 11 hours
        total       ==> 25 hours

score 3 · Answer 2 · answered Mar 26 '20 at 11:28

ChronoUnit.HOURS.between(LocalTime.of(23, 0), LocalTime.of(03,0)

this will give you -20, then you can do some math if the result is less then zero, means 24 -20 = 4, so using ChronoUnit can work, but better use LocalDateTime

java: summing LocalTimes difference

2 Answers2