How should I make this (Spring) formatter implemen...

# announcements

[Jukka Siivonen]

How should I make this (Spring) formatter implementation support LocalDate? (nullable)

fun localDateFormatter(): Formatter<LocalDate> {
        return object : Formatter<LocalDate> {
            override fun parse(text: String, locale: Locale): LocalDate {
                return LocalDate.parse(text, DateTimeFormatter.ISO_DATE)
            }
            override fun print(obj: LocalDate, locale: Locale): String {
                return DateTimeFormatter.ISO_DATE.format(obj)
            }
        }
    }

[Jukka Siivonen]

I can't make parse return LocalDate? no matter what

[Jukka Siivonen]

If I put

override fun parse(text: String, locale: Locale): LocalDate? {

it gives me error Return type is LocalDate? which is not a subtype of overridden

[Nikky]

LocalDate is a subtype of LocalDate?

[Nikky]

so you need to make the type it formats nullable or handle nullability

?: defaultDate

[Nikky]

or throw a exception

[Jukka Siivonen]

Using

[Jukka Siivonen]

override fun parse(text: String, locale: Locale): LocalDate? {

does not compile

[Jukka Siivonen]

whatever the implementation is

[Jukka Siivonen]

because changing return type LocalDate -> LocalDate? then gives compiler error

Return type is LocalDate? which is not a subtype of overridden

[Nikky]

because its for Formatter<LocalDate> with that signature you would need Formatter<LocalDate?>

[Jukka Siivonen]

same problem

[Nikky]

thats why you should isntead handle null on the parser call

[Nikky]

LocalData.parse returns LocalDate? because the input could be junk

[Jukka Siivonen]

I don't understand

[Nikky]

so adding elvis operator to get a default or to throw would fix that

[Jukka Siivonen]

because the problem is not the actual implementation but defining the nullable type

[Nikky]

to me it seems like the actual problem is that the return type is non nullable but your function return null, maybe i am confused because i do not have the code in a IDE

[Jukka Siivonen]

@Bean
    fun localDateFormatter(): Formatter<LocalDate?> {
        return object : Formatter<LocalDate?> {
            override fun parse(text: String, locale: Locale): LocalDate? {
                return null
            }
            override fun print(obj: LocalDate, locale: Locale): String {
                return DateTimeFormatter.ISO_DATE.format(obj)
            }
        }
    }

[Jukka Siivonen]

does not compile

[Jukka Siivonen]

"Return type is LocalDate? which is not a subtype of overridden"

[Nikky]

because the generic argument is LocalDate and not LocalDate?

[Jukka Siivonen]

fixed, same problem

[Nikky]

it could also be that formatter only takes T : Any and nullability was not considered

[Nikky]

now the print function should take a nullable though ?

[Jukka Siivonen]

Formatter is Java

[Jukka Siivonen]

public interface Formatter<T> extends Printer<T>, Parser<T> {

}

[Jukka Siivonen]

@FunctionalInterface
public interface Printer<T> {

	/**
	 * Print the object of type T for display.
	 * @param object the instance to print
	 * @param locale the current user locale
	 * @return the printed text string
	 */
	String print(T object, Locale locale);

}

[Jukka Siivonen]

@FunctionalInterface
public interface Parser<T> {

	/**
	 * Parse a text String to produce a T.
	 * @param text the text string
	 * @param locale the current user locale
	 * @return an instance of T
	 * @throws ParseException when a parse exception occurs in a java.text parsing library
	 * @throws IllegalArgumentException when a parse exception occurs
	 */
	T parse(String text, Locale locale) throws ParseException;

}

[Nikky]

it looks like it should work, yeah

[Jukka Siivonen]

changing print LocalDate? give me "print overrides nothing"

[Nikky]

seems broken, do you want nullable Datetime though ?

[Nikky]

i'd just hadle it in the parser and throw exception when something is invalid