Fastest way to tell if a string is a valid date

Question

I am supporting a common library at work that performs many checks of a given string to see if it is a valid date. The Java API, commons-lang library, and JodaTime all have

Answer 1

If following line throws exception then it is invalid date else this will return valid date. Please make sure you use appropriate DateTimeFormatter in the following statement.

LocalDate.parse(uncheckedStringDate, DateTimeFormatter.BASIC_ISO_DATE)

Answer 2

You can revert your thinking - try to fail as quickly as possible when the String definitely is no date:

• it's null
• its length is not 8 (based on your example date format!)
• it contains anything other that a number (if your date format is only for numerical dates)

If none of those apply, then try to parse it - preferably with a pre-made static Format object, don't create one on every method run.

---

EDIT after comments

Based on this neat trick, I wrote a fast validation method. It looks ugly, but is significantly faster than the usual library methods (which should be used in any standard situation!), because it relies on your specific date format and does not create a Date object. It handles the date as an int and goes on from that.

I tested the daysInMonth() method just a little bit (the leap year condition taken from Peter Lawrey), so I hope there's no apparent bug.

A quick (estimated!) microbenchmark indicated a speedup by a factor of 30.

public static boolean isValidDate(String dateString) {
    if (dateString == null || dateString.length() != "yyyyMMdd".length()) {
        return false;
    }

    int date;
    try {
        date = Integer.parseInt(dateString);
    } catch (NumberFormatException e) {
        return false;
    }

    int year = date / 10000;
    int month = (date % 10000) / 100;
    int day = date % 100;

    // leap years calculation not valid before 1581
    boolean yearOk = (year >= 1581) && (year <= 2500);
    boolean monthOk = (month >= 1) && (month <= 12);
    boolean dayOk = (day >= 1) && (day <= daysInMonth(year, month));

    return (yearOk && monthOk && dayOk);
}

private static int daysInMonth(int year, int month) {
    int daysInMonth;
    switch (month) {
        case 1: // fall through
        case 3: // fall through
        case 5: // fall through
        case 7: // fall through
        case 8: // fall through
        case 10: // fall through
        case 12:
            daysInMonth = 31;
            break;
        case 2:
            if (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0)) {
                daysInMonth = 29;
            } else {
                daysInMonth = 28;
            }
            break;
        default:
            // returns 30 even for nonexistant months 
            daysInMonth = 30;
    }
    return daysInMonth;
}

P.S. Your example method above will return true for "99999999". Mine will only return true for existent dates :).

Answer 3

This is my way to check if the date is in correct format and is actually a valid date. Presume we do not need SimpleDateFormat to convert incorrect to date to a correct one but instead a method just returns false. Output to console is used only to check how the method works on each step.

public class DateFormat {

public static boolean validateDateFormat(String stringToValidate){
    String sdf = "yyyy-MM-dd HH:mm:ss";
    SimpleDateFormat format=new SimpleDateFormat(sdf);   
    String dateFormat = "[12]{1,1}[0-9]{3,3}-(([0]{0,1}[1-9]{1,1})|([1]{0,1}[0-2]{1,1}))-(([0-2]{0,1}[1-9]{1,1})|([3]{0,1}[01]{1,1}))[ ](([01]{0,1}[0-9]{1,1})|([2]{0,1}[0-3]{1,1}))((([:][0-5]{0,1}[0-9]{0,1})|([:][0-5]{0,1}[0-9]{0,1}))){0,2}";
    boolean isPassed = false;

    isPassed = (stringToValidate.matches(dateFormat)) ? true : false;


    if (isPassed){
        // digits are correct. Now, check that the date itself is correct
        // correct the date format to the full date format
        String correctDate = correctDateFormat(stringToValidate);
        try
        {
            Date d = format.parse(correctDate);
            isPassed = (correctDate.equals(new SimpleDateFormat(sdf).format(d))) ? true : false;
            System.out.println("In = " + correctDate + "; Out = " 
                    + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(d) + " equals = " 
                    + (correctDate.equals(new SimpleDateFormat(sdf).format(d))));
            // check that are date is less than current
            if (!isPassed || d.after(new Date())) {
                System.out.println(new SimpleDateFormat(sdf).format(d) + " is after current day " 
                        + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date()));
                isPassed = false;
            } else {
                isPassed = true;
            }
        } catch (ParseException e) {
            System.out.println(correctDate + " Exception! " + e.getMessage());
            isPassed = false;
        }
    } else {
        return false;
    }
    return isPassed;
}

/**
 *  method to fill up the values that are not full, like 2 hours -> 02 hours
 *  to avoid undesirable difference when we will compare original date with parsed date with SimpleDateFormat
 */
private static String correctDateFormat(String stringToValidate) {
    String correctDate = "";
    StringTokenizer stringTokens = new StringTokenizer(stringToValidate, "-" + " " + ":", false);
    List<String> tokens = new ArrayList<>();
    System.out.println("Inside of recognizer");
    while (stringTokens.hasMoreTokens()) {
        String token = stringTokens.nextToken();
        tokens.add(token);
        // for debug
        System.out.print(token + "|");
    }
    for (int i=0; i<tokens.size(); i++){
        if (tokens.get(i).length() % 2 != 0){
            String element = tokens.get(i);
            element = "0" + element;
            tokens.set(i, element);
        }
    }
    // build a correct final string
    // 6 elements in the date: yyyy-MM-dd hh:mm:ss
    // come through and add mandatory 2 elements
    for (int i=0; i<2; i++){
        correctDate = correctDate + tokens.get(i) + "-";
    }
    // add mandatory 3rd (dd) and 4th elements (hh)
    correctDate = correctDate + tokens.get(2) + " " + tokens.get(3);
    if (tokens.size() == 4){
        correctDate = correctDate + ":00:00";
    } else if (tokens.size() == 5){
        correctDate = correctDate + ":" + tokens.get(4) + ":00";
    } else if (tokens.size() == 6){
        correctDate = correctDate + ":" + tokens.get(4) + ":" + tokens.get(5);
    }  

    System.out.println("The full correct date format is " + correctDate);
    return correctDate;
}

}

A JUnit test for that:

import static org.junit.Assert.*;
import junitparams.JUnitParamsRunner;
import junitparams.Parameters;
import org.junit.Test;
import org.junit.runner.RunWith;

@RunWith(JUnitParamsRunner.class)
public class DateFormatTest {

    @Parameters
    private static final Object[] getCorrectDate() {
        return new Object[] {
                new Object[]{"2014-12-13 12:12:12"},
                new Object[]{"2014-12-13 12:12:1"},
                new Object[]{"2014-12-13 12:12:01"},
                new Object[]{"2014-12-13 12:1"},
                new Object[]{"2014-12-13 12:01"},
                new Object[]{"2014-12-13 12"},
                new Object[]{"2014-12-13 1"},
                new Object[]{"2014-12-31 12:12:01"},
                new Object[]{"2014-12-30 23:59:59"},
        };
    }
    @Parameters
    private static final Object[] getWrongDate() {
        return new Object[] {
                new Object[]{"201-12-13 12:12:12"},
                new Object[]{"2014-12- 12:12:12"},
                new Object[]{"2014- 12:12:12"},
                new Object[]{"3014-12-12 12:12:12"},
                new Object[]{"2014-22-12 12:12:12"},
                new Object[]{"2014-12-42 12:12:12"},
                new Object[]{"2014-12-32 12:12:12"},
                new Object[]{"2014-13-31 12:12:12"},
                new Object[]{"2014-12-31 32:12:12"},
                new Object[]{"2014-12-31 24:12:12"},
                new Object[]{"2014-12-31 23:60:12"},
                new Object[]{"2014-12-31 23:59:60"},
                new Object[]{"2014-12-31 23:59:50."},
                new Object[]{"2014-12-31 "},
                new Object[]{"2014-12 23:59:50"},
                new Object[]{"2014 23:59:50"}
        };
    }

    @Test
    @Parameters(method="getCorrectDate")
    public void testMethodHasReturnTrueForCorrectDate(String dateToValidate) {
        assertTrue(DateFormat.validateDateFormatSimple(dateToValidate));
    }

    @Test
    @Parameters(method="getWrongDate")
    public void testMethodHasReturnFalseForWrongDate(String dateToValidate) {
        assertFalse(DateFormat.validateDateFormat(dateToValidate));
    }

}

Answer 4

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

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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.

Answer 5

I think that the better way to know if a certain date is valid is defining a method like:

public static boolean isValidDate(String input, String format) {
    boolean valid = false;

    try {
        SimpleDateFormat dateFormat = new SimpleDateFormat(format);
        String output = dateFormat.parse(input).format(format);
        valid = input.equals(output); 
    } catch (Exception ignore) {}

    return valid;
}

On one hand the method checks the date has the correct format , and on the other hand checks the date corresponds to a valid date . For example, the date "2015/02/29" will be parsed to "2015/03/01", so the input and output will be different, and the method will return false.

Answer 6

One could use a combination of regex and manual leap year checking. Thus:

if (matches ^\d\d\d\d((01|03|05|07|08|10|12)(30|31|[012]\d)|(04|06|09|11)(30|[012]\d)|02[012]\d)$)
    if (endsWith "0229")
         return true or false depending on the year being a leap year
    return true
return false