SimpleDateFormat doesn't work as expected

Question

I try to use this function but it doesn't work with this case '12/05/201a' somebody knows why happen this?

In my test I use this System.out.println(isThisDateValid("12/05/201a", "dd/MM/yyyy")); and the answer was true but I'm expected the result would be false because year contains letters.

 public static 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;
    }

its takes `201` for `year`. If you put all letters in place of year it will not parse. Since it expects `1` digit in place of `year`. — SatyaTNV, Jul 27 '15 at 16:09
@Reimeus: `SimpleDateFormat` doesn't *ignore* letters, it just doesn't consume the entire string. Here's an example of it not ignoring letters: http://ideone.com/gnbTwy — T.J. Crowder, Jul 27 '15 at 16:17
@Danielson: The OP is free to format their code how they like. — T.J. Crowder, Jul 27 '15 at 16:23
@Reimeus: "simply ignores letters" is simply incorrect, however. :-) — T.J. Crowder, Jul 27 '15 at 16:24

score 12 · Accepted Answer · edited Jul 27 '15 at 16:19

DateFormat#parse doesn't necessarily use the entire string:

Parses text from the beginning of the given string to produce a date. The method may not use the entire text of the given string.

(my emphasis)

SimpleDateFormat's docs tell us that yyyy doesn't necessarily mean it will require four digits for a year:

Year:

...

For parsing, if the number of pattern letters is more than 2, the year is interpreted literally, regardless of the number of digits. So using the pattern "MM/dd/yyyy", "01/11/12" parses to Jan 11, 12 A.D.

So it's correct (if possibly surprising) that it parses that string in the year 201.

You can use parse(String,ParsePosition) to figure out whether the entire string has been consumed, or validate it with a regular expression before parsing. Here's a version that will check that the whole string has been parsed, and not just the first characters:

public static boolean isThisDateValid(String dateToValidate, String dateFormat) {
    if (dateToValidate == null) {
        return false;
    }

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

    ParsePosition position = new ParsePosition(0);
    Date date = sdf.parse(dateToValidate, position);
    return date != null && position.getIndex() == dateToValidate.length();
}

SimpleDateFormat doesn't work as expected

1 Answers1