Formatting and Parsing Dates

Part:   1  2  

(Continued from previous part...)

Parsing Strings for Dates

DateFormat objects can also be used to convert string back to Date objects using the parse() method, see the following demonstration program:

import java.util.*;
import java.text.*;
class DateParsingTest {
   public static void main(String[] a) {
      parseDate();
      parseDateSpecial();
   }
   public static void parseDate() {
      SimpleDateFormat mf = new SimpleDateFormat("MMM-yyyy");
      SimpleDateFormat tf = new SimpleDateFormat("HH:mm:ss.SSS");
      SimpleDateFormat df = new SimpleDateFormat("dd-MMM-yyyy");
      SimpleDateFormat dtf 
         = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss.SSS");

      String ms = "Oct-2002";
      String ts = "22:13:19.123";
      String ds = "21-Oct-2002";
      String dts = "21-Oct-2002 22:13:19.123";
      try {
         System.out.println("Parsing " + ms + " with "+mf.toPattern());
         System.out.println("   " + mf.parse(ms));
         System.out.println("Parsing " + ts + " with "+tf.toPattern());
         System.out.println("   " + tf.parse(ts));
         System.out.println("Parsing " + ds + " with "+df.toPattern());
         System.out.println("   " + df.parse(ds));
         System.out.println("Parsing " + dts + " with "+
            dtf.toPattern());
         System.out.println("   " + dtf.parse(dts));
         System.out.println("Parsing " + dts + " with "+df.toPattern());
         System.out.println("   " + df.parse(dts));
         System.out.println("Parsing " + ds + " with "+dtf.toPattern());
         System.out.println("   " + dtf.parse(ds));
      } catch (ParseException e) {
         System.out.println("Exception: " + e.toString());
      }
   }
   public static void parseDateSpecial() {
      SimpleDateFormat dtf 
         = new SimpleDateFormat("EEE MMM dd HH:mm:ss zzz yyyy");
      String str0 = "Mon Oct 21 20:54:05 EDT 2002"; // correct
      String str1 = "Mon Oct 20 20:54:05 EDT 2002"; // wrong d.o.w.
      String str2 = "Mon Oct 21 25:54:05 EDT 2002"; // wrong hour
      String str3 = "Mon Oct 21 20:54:05 PDT 2002"; // time zone changed
      String str4 = "Mon Oct 1 20:54:05 EDT 2002"; // missing a 'd'
      try {
         System.out.println("Parsing " + str0 + " with " 
            + dtf.toPattern());
         System.out.println("   " + dtf.parse(str0));
         System.out.println("Parsing " + str1 + " with " 
            + dtf.toPattern());
         System.out.println("   " + dtf.parse(str1));
         System.out.println("Parsing " + str2 + " with " 
            + dtf.toPattern());
         System.out.println("   " + dtf.parse(str2));
         System.out.println("Parsing " + str3 + " with " 
            + dtf.toPattern());
         System.out.println("   " + dtf.parse(str3));
         System.out.println("Parsing " + str4 + " with " 
            + dtf.toPattern());
         System.out.println("   " + dtf.parse(str4));
      } catch (ParseException e) {
         System.out.println("Exception: " + e.toString());
      }
   }
}

Output:

Parsing Oct-2002 with MMM-yyyy
   Tue Oct 01 00:00:00 EDT 2002
Parsing 22:13:19.123 with HH:mm:ss.SSS
   Thu Jan 01 22:13:19 EST 1970
Parsing 21-Oct-2002 with dd-MMM-yyyy
   Mon Oct 21 00:00:00 EDT 2002
Parsing 21-Oct-2002 22:13:19.123 with dd-MMM-yyyy HH:mm:ss.SSS
   Mon Oct 21 22:13:19 EDT 2002
Parsing 21-Oct-2002 22:13:19.123 with dd-MMM-yyyy
   Mon Oct 21 00:00:00 EDT 2002
Parsing 21-Oct-2002 with dd-MMM-yyyy HH:mm:ss.SSS
Exception: java.text.ParseException: Unparseable date: "21-Oct-2002"
Parsing Mon Oct 21 20:54:05 EDT 2002 with EEE MMM dd HH:mm:ss zzz yyyy
   Mon Oct 21 20:54:05 EDT 2002
Parsing Mon Oct 20 20:54:05 EDT 2002 with EEE MMM dd HH:mm:ss zzz yyyy
   Sun Oct 20 20:54:05 EDT 2002
Parsing Mon Oct 21 25:54:05 EDT 2002 with EEE MMM dd HH:mm:ss zzz yyyy
   Tue Oct 22 01:54:05 EDT 2002
Parsing Mon Oct 21 20:54:05 PDT 2002 with EEE MMM dd HH:mm:ss zzz yyyy
   Mon Oct 21 23:54:05 EDT 2002
Parsing Mon Oct 1 20:54:05 EDT 2002 with EEE MMM dd HH:mm:ss zzz yyyy
   Tue Oct 01 20:54:05 EDT 2002

Examing the output carefully, you will see each parsing example shows a different feature of the parse() method:

• If a field is missing, it will be set to its lowermost value.
• If the year field is missing, it will be set to 1970.
• If time fields are missing, they will be set to 0.
• If time zone is missing, the local time zone will be used.
• If the input string is longer than the pattern, the remaining part of the string will be ignored.
• If the input string is not matching the pattern, you will get an exception.
• Long pattern is not a problem.
• Day of month overrides day of week.
• An overflow lower field will change the higher field. Hour 25 gives hour 1 plus 1 day.
• Specified time zone overrides local time zone.
• Single digit day of month matches double digits pattern.

Part:   1  2