Wrong DateFormat isValid

Question

I do some Date test

String INCORRECT_DATE_1 = "101.10.2010";
String dtFormatString_104 = "dd.MM.yyyy";

@Test
public void isValidDate() {

    FastDateFormat formatter = FastDateFormat.getInstance(dtFormatString_104);

    try {
        final Date parse = formatter.parse(INCORRECT_DATE_1);
    } catch (java.text.ParseException e) {
        return false;
    }
    return true;
}

I get here true because parse is 9 January of 2011. How to validate it correctly? I want to get false here because of the incorrect date.

I use Java 8/17. Before FastDateFormatter we use SimpleDateFormatter, but it looks that library not thread safe, so I have task to change one to another.

Answer 1

Try:

import org.apache.commons.lang3.time.FastDateFormat;
import java.text.ParseException;
import java.util.Date;

public class DateTest {

    static final String INCORRECT_DATE_1 = "101.10.2010";
    static final String dtFormatString_104 = "dd.MM.yyyy";

    public static boolean isValidDate() {
        FastDateFormat formatter = FastDateFormat.getInstance(dtFormatString_104);

        try {
            Date parse = formatter.parse(INCORRECT_DATE_1);

            // Check if the input matches the correctly formatted output
            String reformatted = formatter.format(parse);
            return INCORRECT_DATE_1.equals(reformatted);

        } catch (ParseException e) {
            return false;
        }
    }

    public static void main(String[] args) {
        System.out.println(isValidDate()); // Should print false
    }
}

After parsing, it reformats the date and checks if it matches the original input. If 101.10.2010 is parsed as 09.01.2011, the round-trip check (INCORRECT_DATE_1.equals(reformatted)) fails, returning false.