How am I able to find the elapsed time between 2 strings?

Question

Having some trouble working this one out. I'm making a workclock simply for myself to help calculate how many hours I worked that day. I have two pickers, one for the start time and one for the finish time. Both pickers format the selected time into a string. Now I just need to find the hours that have passed between the start string and the finish string. I don't work over a date change so no need to work out the days/date which previously confused me here -> Calculate date/time difference in java

These are my picker listeners.

private TimePickerDialog.OnTimeSetListener startPickerListener = new TimePickerDialog.OnTimeSetListener() {

    public void onTimeSet(TimePicker view, int selectedHour, int selectedMinute) {
        startHour = selectedHour;
        startMinute = selectedMinute;

        startTime.setText(new StringBuilder().append(padding_str(startHour)).append(":").append(padding_str(startMinute)));

    }
};
private TimePickerDialog.OnTimeSetListener finishPickerListener = new TimePickerDialog.OnTimeSetListener() {

    public void onTimeSet(TimePicker view, int selectedHour, int selectedMinute) {
        finishHour = selectedHour;
        finishMinute = selectedMinute;

        finishTime.setText(new StringBuilder().append(padding_str(finishHour)).append(":").append(padding_str(finishMinute)));

    }
};

Any help would be GREATLY appreciated! Cheers! (If you need any more info just lemme know.)

Answer 1

If you import the strings into a Date using a SimpleDateFormat you can calculate the difference in time easily. Something like this:

Date startDate = new SimpleDateFormat("HH:mm", Locale.ENGLISH).parse(startHour + ":" + startMinute);
Date endDate = new SimpleDateFormat("HH:mm", Locale.ENGLISH).parse(endHour + ":" + endMinute);
long milliDiff = endDate - startDate;
long hoursDiff = milliDiff / (60 * 60 * 1000); //Convert millisecond of difference into hours

This should work even if you do happen to work over a date change, as the milliseconds are calculated from Jan. 1, 1970 and not from the beginning of the day.

Answer 2

A Day ≠ 24 Hours

You may need dates. Because of Daylight Saving Time (DST) and other anomalies, a day may not always be 24 hours long. In the United States for example, it may be 23, 24, or 25 hours.

But if you are sure you want to work in 24-hour periods, then use the LocalTime class in Joda-Time or the new java.time package in Java 8. The java.util.Date and .Calendar classes bundled with Java are notoriously troublesome and should be avoided. (Hence the need for java.time in Java 8)

java.time

The java.time framework includes a LocalTime class similar to Joda-Time.

LocalTime start = LocalTime.of( 9 , 0 );
LocalTime stop = LocalTime.of( 13 , 0 );

The Duration class tracks a span of time in terms of seconds and nanoseconds. See Oracle Tutorial.

Duration duration = Duration.between( start , stop );

Oddly, in Java 8 the Duration class lacks methods to obtain the number of hours part, number of minutes part, etc. You can get the span as a total number of hours and as a total number of minutes, and do the math yourself. Java 9 rectifies this lacking with new “get part” methods.

Joda-Time

You should search StackOverflow.com for "joda parse time" and "joda elapsed time". But to get you going, here is some Joda-Time 2.3 code off the top of my head (not tested, just typed).

LocalTime start = new LocalTime( 13, 10 );
LocalTime stop = new LocalTime( 13, 14 );
int minutesBetween = Minutes.minutesBetween( start, stop ).getMinutes();
Period period = new Period( start, stop );
Duration duration = period.toDuration();

The Period object can be queried for a number of hours if that is your goal.

Try calling the toString method on those objects. Search StackOverflow to learn more.