convert string to datetime and compare for the 2 date diff - eclipse

Question

Question 1: I have 2 fields to let user enter start date and end date, but in string format
- DateStart (string: yyyy/mm/dd hh:mm)
- DateEnd (string: yyyy/mm/dd hh:mm)
May I how to compare both datetime? I want to know total how many hours is difference between the both date.
Question 2: user will enter 1 returnDate (string: yyyy/mm/dd hh:mm) also in string format, may I know how to update the returnDate if I will need to add 55hours on the returnDate?
Thanks

Answer 1

Start by taking a look at SimpleDateFormat, which will allow you to convert the String value to a Date object.
For example...
try {
// Note hh is Hour in am/pm (1-12), based on you example, it's not possible
// now the day part (ie am or pm), you could supply aa as the am/pm marker
// or use HH which is Hour in day (0-23)
SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd hh:mm");
Date date = sdf.parse("2014/04/11 4:46");
System.out.println(date);
} catch (ParseException exp) {
exp.printStackTrace();
}
Then you can use JodaTime to calculate the difference between the two dates, see How to find difference between two Joda-Time DateTimes in minutes for an example
It should be noted that you could skip the use of SimpleDateFormat and JodaTime all the way, check out String to joda LocalDate in format of "dd-MMM-yy" for an example of converting a String to a LocalDate using JodaTime
To add time to an existing Date, you can use either Calendar or JodaTime, see how to add days to java simple date format for an example of both
I would recommend that if you are using some kind of GUI, you might consider using one of the available date pickers as it will save you a lot of hassel

Answer 2

I suggest using Java8's java.util.time package
Example:
public static void main(String[] args) {
// example input
String dateString1 = "2014/04/10 00:00";
String dateString2 = "2014/04/11 23:59";
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm");
LocalDateTime date1 = LocalDateTime.parse(dateString1, dtf);
LocalDateTime date2 = LocalDateTime.parse(dateString2, dtf);
// do your stuff with the dates...
}

Answer 3

Here is my solution:
String date1 = "2014/04/10 15:30";
String date2 = "2014/04/11 09:00";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm");
try {
Date parsedDate1 = sdf.parse(date1);
Date parsedDate2 = sdf.parse(date2);
double secs = (parsedDate2.getTime() - parsedDate1.getTime()) / 1000;
double hours = secs / 3600;
System.out.println(hours);
} catch (ParseException e) {
e.printStackTrace();
}
For adding hours to a date:
Date date = new Date(someDateObject.getTime() + 55 * 3600 * 1000);