Question

I’m doing like that now:

......

DateFormat dateFormat = new SimpleDateFormat("dd.MM.yyyy");
try{
    dateFormat.parse(criteria.getPeriodFrom());
    dateFormat.parse(criteria.getPeriodTo());
}
catch{
    errors.reject("Incorrect format");
}

......

But what if I need to validate against few acceptable patterns (ex. “dd.MM.yyyy”, “ddMMyyyy” ….). And I don’t want to do any copy&paste or iterate through collection of DateFormats 🙂 Are there cool libraries for that?

Answer 1

Just put the loop outside the try/catch block:

boolean success = false;
for (DateFormat candidate : formats) {
    try {
        candidate.parse(criteria.getPeriodFrom());
        candidate.parse(criteria.getPeriodTo());
        success = true;
        break;
    }
    catch (ParseException e) {
        // Expected... move on
    }
}
if (!success) {
    errors.reject("Incorrect format");
}

Unforunately neither the Java built-in libraries nor the normally-excellent Joda Time have anything like .NET’s DateTime.TryParseExact which lets you test whether a parse operation works, without the ugly exception 🙁 Mind you, at least Joda Time’s formatters are thread-safe and immutable.

EDIT: I may be wrong… apparently DateFormat.parse(String, ParsePosition) just returns null on failure, so you could use:

for (DateFormat candidate : formats) {
    if (isValid(candidate, criteria)) {
        // whatever
    }
}

...
private static boolean isValid(DateFormat format, Criteria criteria) {
    return format.parse(criteria.getPeriodFrom(), new ParsePosition(0)) != null &&
           format.parse(criteria.getPeriodTo(), new ParsePosition(0)) != null))
}