Question

I have the following method to sum time:

public static String sumTime(String date1, String date2) throws ParseException {
    Calendar calendar = Calendar.getInstance();
    SimpleDateFormat formatter = new SimpleDateFormat("HH:mm:ss.SSS");

    Date d1 = formatter.parse(date1);
    Date d2 = formatter.parse(date2);
    calendar.setTime(d2);

    d1 = DateUtils.addHours(d1, calendar.get(Calendar.HOUR_OF_DAY));
    d1 = DateUtils.addMinutes(d1, calendar.get(Calendar.MINUTE));
    d1 = DateUtils.addSeconds(d1, calendar.get(Calendar.SECOND));
    d1 = DateUtils.addMilliseconds(d1, calendar.get(Calendar.MILLISECOND));

    return formatter.format(d1);
}

DateUtils is from Apache Commons Lang 3
It works quite well for what I want, unless the sum is bigger than 24 hours.

For example:

String time = "00:00:00.000";

try {
    for (int i = 0; i < 24; i++) {
            time = sumTime(time, "01:00:00.123");
    }

    System.out.println(time);
} catch (ParseException e) {
    e.printStackTrace();
}

The result is:

00:00:02.952

But this is what I’d like it to be:

24:00:02.952

Is there any (easy) way to accomplish that?
I don’t mind using different libraries/methods, as long as I get the correct result.

Keep in mind that time will always start in 00:00:00.000;

Answer 1

Date is not the right thing class to use. Date is a instant of time, not a “Date Difference”.

The right thing to do will be to use a library like Joda Time as someone has already suggested. If you don’t want to do so – here’s a possible alternative:

Parse the string into hours, minutes and seconds yourself, and then add it yourself.

I would encourage you to look into a “well accepted” library though. There may be things I’m not thinking of in my solution. Also, you have add all the error checking.

Here’s the starter code:

public class TimeInterval {
short milliSeconds;
short seconds;
short minutes;
int hours;

public TimeInterval (String dateString) {
    // HHHHHH:MI:SS.SSS
    Pattern pattern = Pattern.compile("(\\d+):(\\d\\d):(\\d\\d)\\.(\\d\\d\\d)");
    Matcher matcher = pattern.matcher(dateString);
    if ( matcher.find() ) {
        hours = Integer.parseInt(dateString.substring(matcher.start(1), matcher.end(1)));
        minutes = Short.parseShort(dateString.substring(matcher.start(2), matcher.end(2)));
        seconds = Short.parseShort(dateString.substring(matcher.start(3), matcher.end(3)));
        milliSeconds = Short.parseShort(dateString.substring(matcher.start(4), matcher.end(4)));
    }
}

private TimeInterval() {
}

public TimeInterval add(TimeInterval interval) {
    TimeInterval ret = new TimeInterval();
    ret.milliSeconds = (short) ((interval.milliSeconds + milliSeconds)%1000);
    int carry = (interval.milliSeconds + milliSeconds)/1000;
    ret.seconds = (short) ((interval.seconds + seconds)%60 + carry );
    carry =(interval.seconds + seconds)/60;
    ret.minutes = (short) ((interval.minutes + minutes)%60 + carry);
    carry = (interval.minutes + minutes)/60;
    ret.hours = (interval.hours + hours + carry);
    return ret;
}

@Override
public String toString() {
    return String.format("%d:%02d:%02d.%03d", hours, minutes, seconds, milliSeconds);
}
}

Using this class your program will be like :

TimeInterval time = new TimeInterval("00:00:00.000");

try {
    for (int i = 0; i < 24; i++) {
      time = time.add(new TimeInterval("01:00:00.123"));
    }

    System.out.println(time.toString());
} catch (ParseException e) {
    e.printStackTrace();
}