Date format changing

Question

If input is 01-01-2015 it should change to 2015-01-01.
If input is 2015-01-01 it should change to 01-01-2015.
I used SimpleDateFormat but didn't get the correct output:

//Class to change date dd-MM-yyyy to yyyy-MM-dd and vice versa
public class ChangeDate {
  static SimpleDateFormat formatY = new SimpleDateFormat("yyyy-MM-dd");
  static SimpleDateFormat formatD = new SimpleDateFormat("dd-MM-yyyy");

  //This function change dd-MM-yyyy to yyyy-MM-dd
  public static String changeDtoY(String date) {
    try {
      return formatY.format(formatD.parse(date));
    }
    catch(Exception e) {
      return null;
    }
  }

  //This function change yyyy-MM-dd to dd-MM-yyyy
  public static String changeYtoD(String date) {
    try {
      return formatD.format(formatY.parse(date));
    }
    catch(Exception e) {
      return null;
    }
  }
}

I want some condition that automatically detects the date's pattern and change to the other format.

Answer 1

There are 2 options:

1. Try to check with regular expression sth. like:

   if (dateString.matches("\\d{4}-\\d{2}-\\d{2}")) {
       ...
   }
   

2. Try to convert to first pattern, if it throws exception, try to convert to another pattern (but it is bad practice to do so)

Answer 2

Regex Is Overkill

For date-time work, no need to bother with regex.

Simply attempt a parse with one format, trapping for the expected exception. If the exception is indeed thrown, attempt a parse with the other format. If an exception is thrown, then you know the input is unexpectedly in neither format.

java.time

You are using old troublesome date-time classes now supplanted by the java.time framework built into Java 8 and later. The new classes are inspired by the highly successful Joda-Time framework, intended as its successor, similar in concept but re-architected. Defined by JSR 310. Extended by the ThreeTen-Extra project. See the Oracle Tutorial.

LocalDate

The new classes include one, LocalDate, for date-only values without time-of-day. Just what you need.

Formatters

Your first format may be the standard ISO 8601 format, YYYY-MM-DD. This format is used by default in java.time.

If this first parse attempt fails because the input does not match ISO 8601 format, a DateTimeParseException is thrown.

LocalDate localDate = null;  
try {
    localDate = LocalDate.parse( input );  // ISO 8601 formatter used implicitly.
} catch ( DateTimeParseException e ) {
    // Exception means the input is not in ISO 8601 format.
}

The other format must be specified by a coded pattern similar to what you are doing with SimpleDateFormat. So if we catch the exception from the first attempt, make a second parse attempt.

DateTimeFormatter formatter = DateTimeFormatter.ofPattern( "MM-dd-yyyy" );
LocalDate localDate = null;  
try {
    localDate = LocalDate.parse( input );
} catch ( DateTimeParseException e ) {
    // Exception means the input is not in ISO 8601 format.
    // Try the other expected format.
    try {
        localDate = LocalDate.parse( input , formatter );
    } catch ( DateTimeParseException e ) {
        // FIXME: Unexpected input fit neither of our expected patterns.
    }
}