Using GregorianCalendar with SimpleDateFormat

Question

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

User · Answer

You are putting there a two-digits year  The first century  And the Gregorian calendar started in the 16th century  I think you should add 2000 to the year  Month in the function new GregorianCalendar year  month  days  is 0-based  Subtract 1 from the month there  Change the body of the second function as follows       String dateFormatted   null      SimpleDateFormat fmt   new SimpleDateFormat  dd-MMM-yyyy        try           dateFormatted   fmt format date             catch   IllegalArgumentException e           System out println e getMessage               return dateFormatted     After debugging  you ll see that simply GregorianCalendar can t be an argument of the fmt format      Really  nobody needs GregorianCalendar as output  even you are told to return  a string    Change the header of your format function to   public static String format final Date date     and make the appropriate changes  fmt format   will take the Date object gladly    Always after an unexpected exception arises  catch it yourself  don t allow the Java machine to do it  This way  you ll understand the problem

User · Answer

A SimpleDateFormat  as its name indicates  formats Dates  Not a 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

User · Answer

Why such complications   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

User · Answer

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

User · Answer

tl dr  LocalDate parse       23-Mar-2017        DateTimeFormatter ofPattern   dd-MMM-uuuu    Locale US        Avoid legacy date-time classes  The Question and other Answers are now outdated  using troublesome old date-time classes that are now legacy  supplanted by the java time classes   Using java time  You seem to be dealing with date-only values  So do not use a date-time class  Instead use LocalDate  The LocalDate class represents a date-only value without time-of-day and without time zone   Specify a Locale to determine  a  the human language for translation of name of day  name of month  and such  and  b  the cultural norms deciding issues of abbreviation  capitalization  punctuation  separators  and such   Parse a string   String input    23-Mar-2017    DateTimeFormatter f   DateTimeFormatter ofPattern   dd-MMM-uuuu    Locale US     LocalDate ld   LocalDate parse  input   f      Generate a string   String output   ld format  f      If you were given numbers rather than text for the year  month  and day-of-month  use LocalDate of   LocalDate ld   LocalDate of  2017   3   23          year   month 1-12   day-of-month     See this code run live at IdeOne com      input  23-Mar-2017      ld toString    2017-03-23      output  23-Mar-2017     About java time  The java time framework is built into Java 8 and later  These classes supplant the troublesome old legacy date-time classes such as java util Date  Calendar   amp  SimpleDateFormat   The Joda-Time project  now in maintenance mode  advises migration to the java time classes   To learn more  see the Oracle Tutorial  And search Stack Overflow for many examples and explanations  Specification is JSR 310   Using a JDBC driver compliant with JDBC 4 2 or later  you may exchange java time objects directly with your database  No need for strings nor java sql   classes   Where to obtain the java time classes     Java SE 8  Java SE 9  and later   Built-in   Part of the standard Java API with a bundled implementation  Java 9 adds some minor features and fixes   Java SE 6 and Java SE 7   Much of the java time functionality is back-ported to Java 6  amp  7 in ThreeTen-Backport   Android   Later versions of Android bundle implementations of the java time classes  For earlier Android  the ThreeTenABP project adapts ThreeTen-Backport  mentioned above   See How to use ThreeTenABP        The ThreeTen-Extra project extends java time with additional classes  This project is a proving ground for possible future additions to java time  You may find some useful classes here such as Interval  YearWeek  YearQuarter  and more

[java] Using GregorianCalendar with SimpleDateFormat

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