Fastest way to tell if a string is a valid date

If you’re really concerned about performance and your date format is really that simple, just pre-compute all the valid strings and hash them in memory. The format you have above only has ~ 8 million valid combinations up to 2050

EDIT by Slanec – reference implementation

This implementation depends on your specific dateformat. It could be adapted to any specific dateformat out there (just like my first answer, but a bit better).

It makes a set of all dates from 1900 to 2050 (stored as Strings – there are 54787 of them) and then compares the given dates with those stored.

Once the dates set is created, it’s fast as hell. A quick microbenchmark showed an improvement by a factor of 10 over my first solution.

private static Set<String> dates = new HashSet<String>();
static {
    for (int year = 1900; year < 2050; year++) {
        for (int month = 1; month <= 12; month++) {
            for (int day = 1; day <= daysInMonth(year, month); day++) {
                StringBuilder date = new StringBuilder();
                date.append(String.format("%04d", year));
                date.append(String.format("%02d", month));
                date.append(String.format("%02d", day));
                dates.add(date.toString());
            }
        }
    }
}

public static boolean isValidDate2(String dateString) {
    return dates.contains(dateString);
}

P.S. It can be modified to use Set<Integer> or even Trove’s TIntHashSet which reduces memory usage a lot (and therefore allows to use a much larger timespan), the performance then drops to a level just below my original solution.