Setting current date from ISO 8601 in object Calendar format using Java

Question

Im trying to set the Calendar date object in the following way, but I don't know how.

public void setTradeDate(String tradeDate) {
    Calendar tradeDate = ?
}

The String is in the format:

String tradeDate = "2017-06-01T15:49:18Z";

Answer 1

Are you able to use java-8?

ZonedDateTime dateTime = ZonedDateTime.now();
System.out.println(DateTimeFormatter.ISO_INSTANT.format(dateTime));

Answer 2

To create a Calendar from a String, you can use a SimpleDateFormat (as already suggested by the other answers). The formatter will parse the String and create a Date object, which will be set to the Calendar:

String tradeDate = "2017-06-01T15:49:18Z";

// create Calendar
Calendar cal = Calendar.getInstance();
// create formatter
SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssX");
// parse string and set the resulting date to Calendar
cal.setTime(df.parse(tradeDate));

The Calendar will contain the date equivalent to 2017-06-01T15:49:18Z. Note that this date/time is in UTC - the Z in the end is the UTC designator.

But if you try to print this date (using System.out.println(cal.getTime()), a logger, or even checking its value in a debugger), it'll implicity use the toString() method, and this converts the fields (day, month, year, hour, minute, seconds, etc) to the system's default timezone (if your default timezone is in India, for example, the date will be printed as Thu Jun 01 21:19:18 IST 2017, although the internal value won't be changed). But the value will still be equivalent to the UTC input.

Don't be misleaded by the output of toString() method from Calendar and Date classes. What matters is the value of the timestamp: the number of milliseconds since 1970-01-01T00:00Z, which can be checked with cal.getTimeInMillis(). Check this article for more information.

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As you're using Java 7, there's another (better) alternative: you can use the ThreeTen Backport, a great backport for Java 8's new date/time classes.

You can parse the input to a org.threeten.bp.ZonedDateTime and use the org.threeten.bp.DateTimeUtils class to convert it to a java.util.Calendar:

String tradeDate = "2017-06-01T15:49:18Z";
// parse input
ZonedDateTime zdt = ZonedDateTime.parse(tradeDate);
// convert to calendar
Calendar cal = DateTimeUtils.toGregorianCalendar(zdt);

Using this backport eliminates lots of problems and design issues of the old Calendar API. And makes a future migration to Java 8 much easier, as in new Java 8 API the classes and methods names are the same, just the packages are different (in Java 8 is java.time and in ThreeTen Backport is org.threeten.bp).

Answer 3

Read the javadoc of SimpleDateFormat.
'T' will yield a literal T. X will yield a ISO 8601 time zone (which is Z in case of UTC time zone).
So you can achieve it by:

DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssX");
df.setTimeZone(TimeZone.getTimeZone("UTC"));
Calendar calobj = Calendar.getInstance();
System.out.println(df.format(calobj.getTime()));

The X is not supported before Java 7. You can use 'Z' instead to get a literal Z.

Answer 4

A possible solution is

public void setTradeDate(String tradeDate) {
    Calendar cal = Calendar.getInstance();
    DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm'Z'");
    try {
        cal.setTime(df.parse(tradeDate));
    } catch(ParseException e) {
        //Handle exception
    }
    System.out.println(cal); //Change to setting the field instead.
}

use it by callling

setTradeDate("2017-06-01T15:49:18Z");