Parsing Ambiguous Dates in Java

Question

Is there any way in Java to guess the date format when it is not explicitly defined?

For example a user types in 11Mar09 or 11-09-2009 or 11/09/2009 or 11-09 what is

Answer 1

My recommendation is don't. Use a date picker or an explicitely noted format. Guessing will lead to all kinds of problems, easily including, if the date is a critical one, litigation.

If you have to guess, provide some form of feedback that is non-ambiguous, something like a confirmation page that says "Your flight will be booked on the 9th of November, 2009. Is this correct?".

Answer 2

I don't think you want to do this, especially based on your examples, but if you must, I think your best bet will be to use something like Apache's DateUtils in commons-lang:

String[] datePatterns = new String[] {
  "ddMMMyy",    // ex. 11Mar09
  "dd-MM-yyyy", // ex. 11-09-2009
  "dd/MM/yyyy", // ex. 11/09/2009
  "dd-MM"       // ex. 11-09 
}

Date date = DateUtils.parseDate(stringDate, datePatterns);

Unfortunately dates like the fourth one above will be problematic - is "11/09" September 11th, November 9th, September 2011, November 2009, or something else?

Answer 3

You could have a bunch of regular expressions and cycle through until you find a match. I think you could also have a bunch of DateFormats and cycle through them, catching exceptions on ones that failed.

The first avoids using exceptions for non-exceptional cases, the second is maybe better from a design point of view in that you're using a date parsing framework for what it was designed for. But overall, I don't think either approach is necessarily "best" or "worst" per se-- more a matter of personal preferences/beliefs.

Answer 4

As the domain of possible date-strings are infinite, I don't see how it could be possible to recognize them all. You can however pick a subset for pattern-matching.

You give no clues about your user-interface, but the best approach here would be to help the user input the date. For example with a pop-up calendar or just forcing a predefined format.