How to sanity check a date in Java

Key is df.setLenient(false);. This is more than enough for simple cases. If you are looking for a more robust (I doubt) and/or alternate libraries like joda-time then look at the answer by the user "tardate"

final static String DATE_FORMAT = "dd-MM-yyyy";

public static boolean isDateValid(String date) 
{
        try {
            DateFormat df = new SimpleDateFormat(DATE_FORMAT);
            df.setLenient(false);
            df.parse(date);
            return true;
        } catch (ParseException e) {
            return false;
        }
}

As shown by @Maglob, the basic approach is to test the conversion from string to date using SimpleDateFormat.parse. That will catch invalid day/month combinations like 2008-02-31.

However, in practice that is rarely enough since SimpleDateFormat.parse is exceedingly liberal. There are two behaviours you might be concerned with:

Invalid characters in the date string Surprisingly, 2008-02-2x will "pass" as a valid date with locale format = "yyyy-MM-dd" for example. Even when isLenient==false.

Years: 2, 3 or 4 digits? You may also want to enforce 4-digit years rather than allowing the default SimpleDateFormat behaviour (which will interpret "12-02-31" differently depending on whether your format was "yyyy-MM-dd" or "yy-MM-dd")

A Strict Solution with the Standard Library

So a complete string to date test could look like this: a combination of regex match, and then a forced date conversion. The trick with the regex is to make it locale-friendly.

  Date parseDate(String maybeDate, String format, boolean lenient) {
    Date date = null;

    // test date string matches format structure using regex
    // - weed out illegal characters and enforce 4-digit year
    // - create the regex based on the local format string
    String reFormat = Pattern.compile("d+|M+").matcher(Matcher.quoteReplacement(format)).replaceAll("\\\\d{1,2}");
    reFormat = Pattern.compile("y+").matcher(reFormat).replaceAll("\\\\d{4}");
    if ( Pattern.compile(reFormat).matcher(maybeDate).matches() ) {

      // date string matches format structure, 
      // - now test it can be converted to a valid date
      SimpleDateFormat sdf = (SimpleDateFormat)DateFormat.getDateInstance();
      sdf.applyPattern(format);
      sdf.setLenient(lenient);
      try { date = sdf.parse(maybeDate); } catch (ParseException e) { }
    } 
    return date;
  } 

  // used like this:
  Date date = parseDate( "21/5/2009", "d/M/yyyy", false);

Note that the regex assumes the format string contains only day, month, year, and separator characters. Aside from that, format can be in any locale format: "d/MM/yy", "yyyy-MM-dd", and so on. The format string for the current locale could be obtained like this:

Locale locale = Locale.getDefault();
SimpleDateFormat sdf = (SimpleDateFormat)DateFormat.getDateInstance(DateFormat.SHORT, locale );
String format = sdf.toPattern();

Joda Time - Better Alternative?

I've been hearing about joda time recently and thought I'd compare. Two points:

1. Seems better at being strict about invalid characters in the date string, unlike SimpleDateFormat
2. Can't see a way to enforce 4-digit years with it yet (but I guess you could create your own DateTimeFormatter for this purpose)

It's quite simple to use:

import org.joda.time.format.*;
import org.joda.time.DateTime;

org.joda.time.DateTime parseDate(String maybeDate, String format) {
  org.joda.time.DateTime date = null;
  try {
    DateTimeFormatter fmt = DateTimeFormat.forPattern(format);
    date =  fmt.parseDateTime(maybeDate);
  } catch (Exception e) { }
  return date;
}

You can use SimpleDateFormat

For example something like:

boolean isLegalDate(String s) {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    sdf.setLenient(false);
    return sdf.parse(s, new ParsePosition(0)) != null;
}

The current way is to use the calendar class. It has the setLenient method that will validate the date and throw and exception if it is out of range as in your example.

Forgot to add: If you get a calendar instance and set the time using your date, this is how you get the validation.

Calendar cal = Calendar.getInstance();
cal.setLenient(false);
cal.setTime(yourDate);
try {
    cal.getTime();
}
catch (Exception e) {
  System.out.println("Invalid date");
}

Assuming that both of those are Strings (otherwise they'd already be valid Dates), here's one way:

package cruft;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class DateValidator
{
    private static final DateFormat DEFAULT_FORMATTER;

    static
    {
        DEFAULT_FORMATTER = new SimpleDateFormat("dd-MM-yyyy");
        DEFAULT_FORMATTER.setLenient(false);
    }

    public static void main(String[] args)
    {
        for (String dateString : args)
        {
            try
            {
                System.out.println("arg: " + dateString + " date: " + convertDateString(dateString));
            }
            catch (ParseException e)
            {
                System.out.println("could not parse " + dateString);
            }
        }
    }

    public static Date convertDateString(String dateString) throws ParseException
    {
        return DEFAULT_FORMATTER.parse(dateString);
    }
}

Here's the output I get:

java cruft.DateValidator 32-11-2010 31-02-2010 04-01-2011
could not parse 32-11-2010
could not parse 31-02-2010
arg: 04-01-2011 date: Tue Jan 04 00:00:00 EST 2011

Process finished with exit code 0

As you can see, it does handle both of your cases nicely.

looks like SimpleDateFormat is not checking the pattern strictly even after setLenient(false); method is applied on it, so i have used below method to validate if the date inputted is valid date or not as per supplied pattern.

import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;
public boolean isValidFormat(String dateString, String pattern) {
    boolean valid = true;
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern(pattern);
    try {
        formatter.parse(dateString);
    } catch (DateTimeParseException e) {
        valid = false;
    }
    return valid;
}

Two comments on the use of SimpleDateFormat.

it should be declared as a static instance if declared as static access should be synchronized as it is not thread safe

IME that is better that instantiating an instance for each parse of a date.

Here is I would check the date format:

 public static boolean checkFormat(String dateTimeString) {
    return dateTimeString.matches("^\\d{4}-\\d{2}-\\d{2}") || dateTimeString.matches("^\\d{4}-\\d{2}-\\d{2}\\s\\d{2}:\\d{2}:\\d{2}")
            || dateTimeString.matches("^\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}") || dateTimeString
            .matches("^\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}Z") ||
            dateTimeString.matches("^\\d{4}-\\d{2}-\\d{2}\\s\\d{2}:\\d{2}:\\d{2}Z");
}

        public static String detectDateFormat(String inputDate, String requiredFormat) {
        String tempDate = inputDate.replace("/", "").replace("-", "").replace(" ", "");
        String dateFormat;

        if (tempDate.matches("([0-12]{2})([0-31]{2})([0-9]{4})")) {
            dateFormat = "MMddyyyy";
        } else if (tempDate.matches("([0-31]{2})([0-12]{2})([0-9]{4})")) {
            dateFormat = "ddMMyyyy";
        } else if (tempDate.matches("([0-9]{4})([0-12]{2})([0-31]{2})")) {
            dateFormat = "yyyyMMdd";
        } else if (tempDate.matches("([0-9]{4})([0-31]{2})([0-12]{2})")) {
            dateFormat = "yyyyddMM";
        } else if (tempDate.matches("([0-31]{2})([a-z]{3})([0-9]{4})")) {
            dateFormat = "ddMMMyyyy";
        } else if (tempDate.matches("([a-z]{3})([0-31]{2})([0-9]{4})")) {
            dateFormat = "MMMddyyyy";
        } else if (tempDate.matches("([0-9]{4})([a-z]{3})([0-31]{2})")) {
            dateFormat = "yyyyMMMdd";
        } else if (tempDate.matches("([0-9]{4})([0-31]{2})([a-z]{3})")) {
            dateFormat = "yyyyddMMM";
        } else {
            return "Pattern Not Added";
//add your required regex
        }
        try {
            String formattedDate = new SimpleDateFormat(requiredFormat, Locale.ENGLISH).format(new SimpleDateFormat(dateFormat).parse(tempDate));

            return formattedDate;
        } catch (Exception e) {
            //
            return "";
        }

    }

setLenient to false if you like a strict validation

public boolean isThisDateValid(String dateToValidate, String dateFromat){

    if(dateToValidate == null){
        return false;
    }

    SimpleDateFormat sdf = new SimpleDateFormat(dateFromat);
    sdf.setLenient(false);

    try {

        //if not valid, it will throw ParseException
        Date date = sdf.parse(dateToValidate);
        System.out.println(date);

    } catch (ParseException e) {

        e.printStackTrace();
        return false;
    }

    return true;
}

With 'legacy' date format, we can format the result and compare it back to the source.

    public boolean isValidFormat(String source, String pattern) {
    SimpleDateFormat sd = new SimpleDateFormat(pattern);
    sd.setLenient(false);
    try {
        Date date = sd.parse(source);
        return date != null && sd.format(date).equals(source);
    } catch (Exception e) {
        return false;
    }
}

This execerpt says 'false' to source=01.01.04 with pattern '01.01.2004'

We can use the org.apache.commons.validator.GenericValidator's method directly without adding the whole library:

public static boolean isValidDate(String value, String datePattern, boolean strict) {

    if (value == null
            || datePattern == null
            || datePattern.length() <= 0) {

        return false;
    }

    SimpleDateFormat formatter = new SimpleDateFormat(datePattern, Locale.ENGLISH);
    formatter.setLenient(false);

    try {
        formatter.parse(value);
    } catch(ParseException e) {
        return false;
    }

    if (strict && (datePattern.length() != value.length())) {
        return false;
    }

    return true;
}

A simple and elegant way for Android developers (Java 8 not required):

// month value is 1-based. e.g., 1 for January.
public static boolean isDateValid(int year, int month, int day) {
    Calendar calendar = Calendar.getInstance();
    try {
        calendar.setLenient(false);
        calendar.set(year, month-1, day);
        calendar.getTime();
        return true;
    } catch (Exception e) {
        return false;
    }
}

Below code works with dd/MM/yyyy format and can be used to check NotNull,NotEmpty as well.

public static boolean validateJavaDate(String strDate) {

    if (strDate != null && !strDate.isEmpty() && !strDate.equalsIgnoreCase(" ")) {
        {

            SimpleDateFormat date = new SimpleDateFormat("dd/MM/yyyy");
            date.setLenient(false);

            try {
                Date javaDate = date.parse(strDate);
                System.out.println(strDate + " Valid Date format");
            }

            catch (ParseException e) {
                System.out.println(strDate + " Invalid Date format");
                return false;
            }
            return true;
        }

    } else {
        System.out.println(strDate + "----> Date is Null/Empty");
        return false;
    }

}