Using GregorianCalendar with SimpleDateFormat
So I've been racking my brain over this (should-be) simple exercise to make the program turn a date string into a GregorianCalendar object, format it, and return it again as a string when it's done.
This is the last little bit of a program that takes in a chuck of text from a file, breaks it down into individual records, then breaks the records into individual pieces of data and assigns them to a person object.
I've checked the code in multiple places and the code does exactly what it's supposed to be doing up until I call the format function, which throws an IllegalArgumentException. The GergorianCalendar object is assigned the values it is supposed to be assigned (though printing it is, again, a whole other story as seen below...), yet the format will not accept the object for formatting.
Unfortunately, the instructor wasn't too sure of how to use the GregorianCalendar and SimpleDateFormat (yet assigned us to work with them) and said "Just Google it"... I tried, and nothing I've found has helped.
The code I have so far is:
public class DateUtil {
public static GregorianCalendar convertFromDMY(String dd_mm_yy) throws ParseException{
// this actually works, got rid of the original code idea
String[] splitDate = dd_mm_yy.split("-");
int days = Integer.parseInt(splitDate[0]);
int month = Integer.parseInt(splitDate[1]);
int year = Integer.parseInt(splitDate[2]);
// dates are going in right, checked in print statement,
// but the object is not getting formatted...
GregorianCalendar dateConverted = new GregorianCalendar(year, month, days);
format(dateConverted);
return dateConverted;
}
public static String format(GregorianCalendar date){
SimpleDateFormat fmt = new SimpleDateFormat("dd-MMM-yyyy");
String dateFormatted = fmt.format(date);
return dateFormatted;
}
}
The error I get is:
Exception in thread "main" java.lang.IllegalArgumentException: Cannot format given Object >as a Date
at java.text.DateFormat.format(DateFormat.java:281)
at java.text.Format.format(Format.java:140)
at lab2.DateUtil.format(DateUtil.java:26)
at lab2.DateUtil.convertFromDMY(DateUtil.java:19)
at lab2.Lab2.createStudent(Lab2.java:75)
at lab2.Lab2.main(Lab2.java:34)
And another thing, am I even using the GregorianCalendar right?? When I print out that object's value (should be getting a date right?) I get this:
java.util.GregorianCalendar[time=?,areFieldsSet=false,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="America/Vancouver",offset=-28800000,dstSavings=3600000,useDaylight=true,transitions=189,lastRule=java.util.SimpleTimeZone[id=America/Vancouver,offset=-28800000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=8,startDayOfWeek=1,startTime=7200000,startTimeMode=0,endMode=3,endMonth=10,endDay=1,endDayOfWeek=1,endTime=7200000,endTimeMode=0]],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=?,YEAR=1985,MONTH=4,WEEK_OF_YEAR=?,WEEK_OF_MONTH=?,DAY_OF_MONTH=22,DAY_OF_YEAR=?,DAY_OF_WEEK=?,DAY_OF_WEEK_IN_MONTH=?,AM_PM=0,HOUR=0,HOUR_OF_DAY=0,MINUTE=0,SECOND=0,MILLISECOND=?,ZONE_OFFSET=?,DST_OFFSET=?]
The year, month and day_of_month values are all correct as they are the numbers I passed into the creation of it.
Thoughts, suggestions, am I even close?
edit
original issues cleared up (thank you assylias!) but I still can't print properly because the two functions aren't linked and the requirements are to have a GregorianCalendar date value printed out from the person object (as birthdate is a GregorianCalendar).
Updated code:
public class DateUtil {
static SimpleDateFormat fmt = new SimpleDateFormat("dd-MMM-yyyy");
public static GregorianCalendar convertFromDMY(String dd_mm_yy) throws ParseException{
// this actually works, got rid of the original code idea
String[] splitDate = dd_mm_yy.split("-");
int days = Integer.parseInt(splitDate[0]);
int month = (Integer.parseInt(splitDate[1]) - 1);
int year = Integer.parseInt(splitDate[2]);
// dates go in properly
GregorianCalendar dateConverted = new GregorianCalendar(year, month, days);
String finalDate = format(dateConverted);
return ;
}
public static String format(GregorianCalendar date) throws ParseException{
fmt.setCalendar(date);
String dateFormatted = fmt.format(date.getTime());
System.out.println(dateFormatted);
return dateFormatted;
}
}
last edit
Ok so it seems that I'm an idiot and DIDN'T need to link the two DateUtil functions together, but use them in tandem. First, convert the birthdate to a GregorianCalendar and store it in the person object. Then in the print statement just simply tell the program to format that date as it's being printed. Problem was solved. All works according to specifications now, and I feel that much stupider because I was flailing like a fish out of water for the last day or so with the DateUtil class, trying to get them to work at the same time.
Thanks for the help all on getting the date go in properly!
SimpleDateFormat#format method takes a Date as a parameter. You can get a Date from a Calendar by calling its getTime method:
public static String format(GregorianCalendar calendar){
SimpleDateFormat fmt = new SimpleDateFormat("dd-MMM-yyyy");
fmt.setCalendar(calendar);
String dateFormatted = fmt.format(calendar.getTime());
return dateFormatted;
}
Also note that the months start at 0 so you probably meant:
int month = Integer.parseInt(splitDate[1]) - 1;
为什么会出现这种并发症
public static GregorianCalendar convertFromDMY(String dd_mm_yy) throws ParseException
{
SimpleDateFormat fmt = new SimpleDateFormat("dd-MMM-yyyy");
Date date = fmt.parse(dd_mm_yy);
GregorianCalendar cal = GregorianCalendar.getInstance();
cal.setTime(date);
return cal;
}
A SimpleDateFormat, as its name indicates, formats Dates. Not Calendar. So if you want to format a GregorianCalendar using a SimpleDateFormat, you must convert the calendar to a date first:
dateFormat.format(calendar.getTime());
And what you see printed is the toString() representation of the calendar. It's intended usage is debugging. It's not intended to be used to display a date in a GUI. For that, use a (Simple)DateFormat.
Finally, to convert from a String to a Date, you should also use a (Simple)DateFormat (its parse() method), rather than splitting the String as you're doing. This will give you a Date object, and you can create a calendar from the Date by instanciating it ( Calendar.getInstance() ) and setting its time ( calendar.setTime() ).
My advice would be: Googling is not the solution here. Reading the API documentation is what you need to do.
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