Why is the below SimpleDateFormat showing incorrect date?

Question

I am trying to validate incoming date string and then convert it to a LocalDate. Here is my code:

public class DateTester {

    public static void main(String[] args) {
        System.out.println("converted date is" +stringToLD("2023-01-23")); 
        System.out.println("converted date is" +stringToLD("2023-13-21")); 
        System.out.println("converted date is" +stringToLD("2023-24-31")); 
        System.out.println("converted date is" +stringToLD("2023-36-34"));

//        converted date is 2023-01-23
//        converted date is2024-01-21
//        converted date is2024-12-31
//        converted date is2026-01-03 
    }

    public static LocalDate stringToLD(String inputDate) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Date date;
        try {
            date = sdf.parse(inputDate);
        } catch (ParseException e) {
            throw new IllegalArgumentException("Unable to parse date, " + inputDate);
        }
        return LocalDate.parse(sdf.format(date));
    }
}

but however when I send invalid date like 2023-13-21 I get back converted date as 2024-01-21 which is invalid and unwanted result. So I wanted to understand why this is happening and also looking for an alternate solution in java 8

Answer 1

Try it like this. Don't use Date or SimpleDateFormat. As stated by Ole V.V the default, strict, format is DateTimeFormatter.ISO_LOCAL_DATE is used by LocalDate.parse(). That also matches your requirements.

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;

public static LocalDate stringToLD(String inputDate) {
    
    try {
        return LocalDate.parse(inputDate);
       
    } catch (DateTimeParseException e) {
        throw new IllegalArgumentException("Unable to parse date, " + inputDate);
    }
}