Change Date Format Java

Question

I am using this code :

final String OLD_FORMAT = "dd-MMM-yy";
final String NEW_FORMAT = "dd/MM/yyyy";

String oldDateString = "16-OCT-19";
String newDateString;
SimpleDateFormat sdf = new SimpleDateFormat(OLD_FORMAT);
Date d = sdf.parse(oldDateString);
sdf.applyPattern(NEW_FORMAT);
newDateString = sdf.format(d);
System.out.println(newDateString);

Is it posible to convert without a conversion to Date first so we don't see the format before (OLD_FORMAT)?

I have a String that can be any format date. example "dd-MM-yyyy", "dd-MMM-yyyy", "dd-MMMM-yyyy", or anything. and I want to format into specific format date "dd/MM/yyyy".

Answer 1

You can definitely use the approach that utilizes a list of possible or expected patterns and give each one a try, but you should use the most optimized library for it instead of one which is only still alive due to a mass of legacy code referencing it.

tl;dr
Your friends for this task (since Java 8) are

• java.time.LocalDate
• java.time.format.DateTimeFormatter
• java.time.format.DateTimeFormatterBuilder
• java.time.format.DateTimeParseException

Here's an example:

public static void main(String[] args) {
    // start with an example String
    String oldDateString = "16-OCT-19";
    
    // build up a list of possible / possibly expected patterns
    List<String> patterns = List.of(
            "dd-MM-uu", "dd-MMM-uu", "dd-MMMM-uu",
            "dd-MM-uuuu", "dd-MMM-uuuu", "dd-MMMM-uuuu"
    );
    
    // then build a formatter for the desired output
    DateTimeFormatter outFmt = DateTimeFormatter.ofPattern("dd/MM/uuuu");
    
    // for each pattern in your list
    for (String pattern : patterns) {
        try {
            // build a formatter that
            DateTimeFormatter dtf = new DateTimeFormatterBuilder()
                                            // doesn't care about case
                                            .parseCaseInsensitive()
                                            // uses the current pattern and
                                            .appendPattern(pattern)
                                            // considers the language/locale
                                            .toFormatter(Locale.ENGLISH);
            // then try to parse the String with that formatter and
            LocalDate localDate = LocalDate.parse(oldDateString, dtf);
            // print it using the desired output formatter
            System.out.println(localDate.format(outFmt));
            // finally stop the iteration in case of success
            break;
        } catch (DateTimeParseException dtpEx) {
            // build some meaningful statements
            String msg = String.format(
                            "%s !\n  ——> you cannot parse '%s' using pattern %s",
                            dtpEx.getMessage(), oldDateString, pattern);
            // and print it for each parsing fail
            System.err.println(msg);
        }
    }
}

Try it with different inputs and maybe extend the pattern list.

However, this code example fails for the first pattern in the list but the second one is a match, so this prints

Text '16-OCT-19' could not be parsed at index 3 !
  ——> you cannot parse '16-OCT-19' using pattern dd-MM-uu
16/10/2019

The remaining patterns are skipped.