How to calculate sum from 2 period with negative sign on Joda Time

Question

I'm building an android app and I would do a sum of two periods, using jodatime library. Actually, I've a problem to manage the sign because I could have only one period with negative sign. I tried a code to do this, but the result is wrong for me.

example:

case 1: 
time1 = 4:55   
time2 = -7:10  
time1 + time2 = -01:-55  --> this is wrong... I'm expected -3:45

case 2:
time1 = -4:55 
time2 = 7:10
time1 + time2 = 4:05  --> this is correct

This is my sample code:

public static void main(String[] args) 
    {
        String output;
        String time1 = "-4:55";
        String time2 = "7:10";

        Duration durationSum = Duration.ZERO;
        PeriodFormatter formatter = getFormatterBuilder();
        Period period1 = formatter.parsePeriod(time1);
        Duration duration1 = period1.toStandardDuration();
        Period period2 = formatter.parsePeriod(time2);
        Duration duration2 = period2.toStandardDuration();

        output = formatter.print(durationSum.plus(duration1).plus(duration2).toPeriod());

        System.out.println(output);

    }

    private static PeriodFormatter getFormatterBuilder()
    {
        return new PeriodFormatterBuilder()
                .minimumPrintedDigits(2)
                .printZeroAlways()
                .appendHours()
                .appendLiteral(":")
                .appendMinutes()
                .toFormatter();
    }

What am I wrong?

Thanks!

Meno Hochschild · Accepted Answer · 2014-10-13T08:49:20.940

Maybe the exotic sign handling of Joda-Time has confused you (like me on first try). In detail:

case 1:  
time1 = 4:55
time2 = -7:10
time1 + time2 = -01:-55  --> this is wrong... I'm expected -3:45

The Joda-Time strategy for interpreting the negative sign means the sign to be applied on every single time component, NOT on the whole duration itself. So we don't have 55 minutes - 10 minutes but 55 minutes + 10 minutes:

time1 + time2 = (4:55) + (-7:10) = (+4 hours + 55 minutes) + (-7 hours + 10 minutes) = (4 * 60 minutes + 55 minutes) + (-7 * 60 minutes + 10 minutes) = 295 minutes - 410 minutes = -115 minutes = -1 hour - 55 minutes

We see that Joda-Time is consequently doing its sign strategy.

The same sign strategy confirms your second expectation however:

case 2:
time1 = -4:55 
time2 = 7:10
time1 + time2 = 4:05  --> this is correct

time1 + time2 = (-4:55) + (7:10) = (-4 hours + 55 minutes) + (7 hours + 10 minutes) = (-4 * 60 minutes + 55 minutes) + (7 * 60 minutes + 10 minutes) = -185 minutes + 430 minutes = 245 minutes = 4 hours + 5 minutes

Important side note about the strategy of Joda-Time: As we can see in the examples given by you, Joda-Time violates the symmetry rule

-(time1 - time2) != (time2 - time1)

In math, this symmetry rule represents a specialized form of common distributive law where the minus sign can be interpreted as multiplication by factor -1. If Joda-Time had choosen the much more appropriate sign strategy to apply the sign on the whole duration instead then the calculation would yield:

case 1: (4:55) + (-7:10) = (4 hours + 55 minutes) + (-7 hours - 10 minutes) = 295 minutes - 430 minutes = -135 minutes = -2:15 (meaning -2 hours - 15 minutes)

case 2: (-4:55) + (7:10) = (-4 hours - 55 minutes) + (7 hours + 10 minutes) = -295 minutes + 430 minutes = 135 minutes = 2:15

This intuitive strategy would have ensured the mentioned symmetry rule above. But Jodas strategy (and now overtaken by Period-class in JSR-310 aka java.time library in Java-8, too) is set in stone (unfortunately) and must be seen as "feature" if we like it or not.

Thanks man for exhaustive reply! Now is more clear for me and I understood some steps that I can't understand before...thanks! :) — user3449772, Oct 14 '14 at 14:22

How to calculate sum from 2 period with negative sign on Joda Time

1 Answers1