Issues Converting String dates into Long values and performing calculations

Question

I have a map of string values which represent down times for different components.

dependencyMap.put ("sut", "14:26:12,14:27:19,00:01:07;15:01:54,15:02:54,00:01:00;15:44:30,15:46:30,00:02:00;16:10:30,16:11:30,00:01:00");
dependencyMap.put ("jms", "14:26:12,14:28:12,00:02:00;15:10:50,15:12:55,00:02:05;15:42:30,15:43:30,00:01:00;16:25:30,16:27:30,00:02:00");

The strings represent the start, end and duration of down times.

(start)14:26:12,(end)14:27:19,(duration)00:01:07

I read the values in, then add them to a list of DependencyDownTime objects which hold the Long values startTime, endTime and duration.

jArray.forEach (dependency ->{
            String downTimeValues = knownDowntimesMap.get(dependency);
            final String[] downtime = downTimeValues.split (";");
            for (final String str : downtime) {
                final DependencyDownTime depDownTime = new DependencyDownTime ();
                final String[] strings = str.split (",");
                if (strings.length == 3) {
                    final DateFormat dateFormat = new SimpleDateFormat ("HH:mm:ss");
                    try {
                        depDownTime.setStartTime(dateFormat.parse (strings[0]).getTime ());
                        depDownTime.setEndTime (dateFormat.parse (strings[1]).getTime ());
                        depDownTime.setDuration (dateFormat.parse (strings[2]).getTime ());
                        downTimes.add (depDownTime);
                    } catch (final ParseException e) {
                        //logger.warn (e.getMessage (), e);
                    }
                } else {
                    //logger.warn ("");
                }
            }

I then perform simple arithmetic on the values, which calculates the total down time for each component.

// sort the list by start time
        Collections.sort(downTimes, Comparator.comparing (DependencyDownTime::getStartTime));

        int i = 1;
        Long duration = 0L;
        for(DependencyDownTime dts: downTimes){
            Long curStart = dts.getStartTime ();
            Long curEnd = dts.getEndTime();

            Long nextStart = downTimes.get(i).getStartTime ();
            Long nextEnd = downTimes.get(i).getEndTime ();

            if(duration == 0){
                duration = dts.getDuration();
            }
            if(curStart.equals(nextStart) && curEnd < nextEnd){
                duration += (nextEnd - curEnd);
            }
            else if(nextStart > curEnd){
                duration += downTimes.get(i).getDuration();
            }
            else if( curStart < nextStart && curEnd > nextStart){
                duration += (nextEnd - curEnd);
            }
            else if(curEnd == nextStart){
                duration += downTimes.get(i).getDuration();
            }
            i++;
            if(i == downTimes.size ()){
                componentDTimeMap.put (application, duration);
                return;
            }

The expected values should be something like 1970-01-01T 00:14:35 .000+0100, a matter of minutes. The actual result is usually extremely high off by a matter of hours in the difference 1969-12-31T 15:13:35 .000+0100

I have 2 questions.

1. Am I parsing the values correctly?

2. If my calculations are a little off when adding and subtracting the long values. When I convert the values back to Date format will there be a drastic difference in the expected value?

Answer 1

As explained in your other question, don't mistake those 2 different concepts:

• a time of the day: it represents a specific point of a day, such as 10 AM or 14:45:50
• a duration: it represents an amount of time, such as "1 hour and 10 minutes" or "2 years, 3 months and 4 days". The duration doesn't tell you when it starts or ends ("1 hour and 10 minutes" relative to what?), it's not attached to a chronology, it doesn't correspond to a specific point in the timeline. It's just the amount of time, by itself.

In your input, you have:

(start)14:26:12,(end)14:27:19,(duration)00:01:07

The start and end represents times of the day, and the duration represents the amount of time. SimpleDateFormat is designed to work with dates and times of the day, but not with durations. Treating the duration as a time of the day might work, but it's a hack as explained in this answer.

Another problem is that when SimpleDateFormat parses only a time, it defaults the day to January 1^st 1970 at the JVM default timezone, leading to all the strange results you see. Unfortunately there's no way to avoid that, as java.util.Date works with full timestamps. A better alternative is to use the new date/time API.

As in your other question you're using Java 8, I'm assuming you can also use it here (but if you're using Java <= 7, you can use the ThreeTen Backport, a great backport for Java 8's new date/time classes. The only difference is the package names (in Java 8 is java.time and in ThreeTen Backport (or Android's ThreeTenABP) is org.threeten.bp), but the classes and methods names are the same).

As you're working only with times, there's no need to consider date fields (day/month/year), we can use a LocalTime instead. You can parse the strings directly, because they are in ISO861 compliant format:

LocalTime start = LocalTime.parse("14:26:12");
LocalTime end = LocalTime.parse("14:27:19");

Unfortunately there are no built-in parsers for a duration, so you'll have to parse it manually:

// parse the duration manually
String[] parts = "00:01:07".split(":");
Duration d = Duration
    // get hours
    .ofHours(Long.parseLong(parts[0]))
    // plus minutes
    .plusMinutes(Long.parseLong(parts[1]))
    // plus seconds
    .plusSeconds(Long.parseLong(parts[2]));

Another alternative is to remove the durations from your input (or ignore them) and calculate it using the start and end:

Duration d = Duration.between(start, end);

Both will give you a duration of 1 minute and 7 seconds.

My suggestion is to change the DependencyDownTime to store start and end as LocalTime objects, and the duration as a Duration object. With this, your algorithm would be like this:

Duration total = Duration.ZERO;
for (...) {
    LocalTime curStart = ...
    LocalTime curEnd = ...
    LocalTime nextStart = ...
    LocalTime nextEnd = ...

    if (total.toMillis() == 0) {
        duration = dts.getDuration();
    }
    if (curStart.equals(nextStart) && curEnd.isBefore(nextEnd)) {
        total = total.plus(Duration.between(curEnd, nextEnd));
    } else if (nextStart.isAfter(curEnd)) {
        total = total.plus(downTimes.get(i).getDuration());
    } else if (curStart.isBefore(nextStart) && curEnd.isAfter(nextStart)) {
        total = total.plus(Duration.between(curEnd, nextEnd));
    } else if (curEnd.equals(nextStart)) {
        total = total.plus(downTimes.get(i).getDuration());
    }
    i++;
    if (i == downTimes.size()) {
        // assuming you want the duration as a total of milliseconds
        componentDTimeMap.put(application, total.toMillis());
        return;
    }
}

You can either store the Duration object, or the respective value of milliseconds. Don't try to transform it to a Date, because a date is not designed nor supposed to work with durations. You can adapt this code to format a duration if you want (unfortunately there are no native formatters for durations).

---

Limitations

The code above assumes that all start and end times are in the same day. But if you have start at 23:50 and end at 00:10, should the duration be 20 minutes?

If that's the case, it's a little bit trickier, because LocalTime is not aware of the date (so it considers 23:50 > 00:10 and the duration between them is "minus 23 hours and 40 minutes").

In this case, you could do a trick and assume the dates are all at the current date, but when start is greater than end, it means that end time is in the next day:

LocalTime start = LocalTime.parse("23:50");
LocalTime end = LocalTime.parse("00:10");
// calculate duration
Duration d;
if (start.isAfter(end)) {
    // start is after end, it means end is in the next day

    // current date
    LocalDate now = LocalDate.now();
    // start is at the current day
    LocalDateTime startDt = now.atTime(start);
    // end is at the next day
    LocalDateTime endDt = now.plusDays(1).atTime(end);
    d = Duration.between(startDt, endDt);
} else {
    // both start and end are in the same day
    // just calculate the duration in the usual way
    d = Duration.between(start, end);
}

In the code above, the result will be a Duration of 20 minutes.

---

Don't format dates as durations

Here are some examples of why SimpleDateFormat and Date aren't good to handle durations of time.

Suppose I have a duration of 10 seconds. If I try to transform it to a java.util.Date using the value 10 to a date (AKA treating a duration as a date):

// a 10 second duration (10000 milliseconds), treated as a date
Date date = new Date(10 * 1000);
System.out.println(date);

This will get a date that corresponds to "10000 milliseconds after unix epoch (1970-01-01T00:00Z)", which is 1970-01-01T00:00:10Z. But when I print the date object, the toString() method is implicity called (as explained here). And this method converts this millis value to the JVM default timezone.

In the JVM I'm using, the default timezone is America/Sao_Paulo, so the code above outputs:

Wed Dec 31 21:00:10 BRT 1969

Which is not what is expected: the UTC instant 1970-01-01T00:00:10Z corresponds to December 31^st 1969 at 9 PM in São Paulo timezone.

This happens because I'm erroneously treating the duration as a date (and the output will be different, depending on the default timezone configured in the JVM).

A java.util.Date can't (must not) be used to work with durations. Actually, now that we have better API's, it should be avoided whenever possible. There are too many problems and design issues with this, just don't use it if you can.

---

SimpleDateFormat also won't work properly if you handle the durations as dates. In this code:

SimpleDateFormat dateFormat = new SimpleDateFormat("HH:mm:ss");
Date d = dateFormat.parse("10:00:00");

The input has only time fields (hour, minute and second), so SimpleDateFormat sets the date to January 1^st 1970 at the JVM default timezone. If I System.out.println this date, the result will be:

Thu Jan 01 10:00:00 BRT 1970

That's January 1^st 1970 at 10 AM in São Paulo timezone, which in UTC is equivalent to 1970-01-01T13:00:00Z - so d.getTime() returns 46800000.

If I change the JVM default timezone to Europe/London, it will create a date that corresponds to January 1^st 1970 at 10 AM in London (or UTC 1970-01-01T09:00:00Z) - and d.getTime() now returns 32400000 (because 10 AM in London and 10 AM in São Paulo happened at different instants).

SimpleDateFormat isn't the right tool to work with durations - it isn't even the best tool to work with dates, actually.