I am having trouble implementing a method from an ...

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[Matheus]

I am having trouble implementing a method from an interface that was defined in Java.
It has a Class<T> return type, but I can't seem to make it work with nullable types without triggering a compiler warning

override fun toType(): Class<Boolean?> = 
        Boolean::class.java as Class<Boolean?>

This works, but the cast triggers the warning: Unchecked cast Class<Boolean> to Class<Boolean?>

I tried to qualify it as <out Boolean>, but I can't seem to make it work?

[Zachary Grafton]

are you looking for Class<Boolean>? instead?

[Matheus]

no. the return type needs to be a Class<T>

[Matheus]

I managed to make this work without any warnings:

override fun toType(): Class<Boolean?> =
            toTypeReified()

    private inline fun <reified T>toTypeReified(): Class<T> = T::class.java

but I'm not sure it's the best solution

[Ruckus]

There is no Class<Boolean?>, so you may need to look at your generics.

[Zachary Grafton]

@Ruckus is correct, there is no Class<Boolean?> What is the actual return type of the method from the Java interface? Assuming that Java interface isn't marked with @Nullable the return type should be Class<Boolean>! So, in this case, Class<Boolean> or Class<Boolean>? should work.

[Matheus]

Oh, I hadn't tried with Boolean before, since I always use it with Boolean? types 🤪

[Matheus]

Can you help me understand why Class<Boolean?> doesn't exist , I didn't get any compiler errors?

[Zachary Grafton]

Class<Boolean?> implies that Class<Boolean> and Class<null> are valid. Class<null> isn't possible. Class<Boolean>? implies that null is a valid return value.

[Zachary Grafton]

Class<null> doesn't work because null is a primitive type and you can't pass primitive types as type parameters to any generic.

[Matheus]

Got everything else, but not sure I could wrap my head around the implies that [...] Class<null> is valid bit.

Isn't the T in generics supposed to be the "placeholder of a type"? And isn't Boolean? a type?

[Zachary Grafton]

Boolean? is a type, but there are restrictions on what you can pass to generics as parameters.

[Zachary Grafton]

And Class is even more special

[Matheus]

Ah ok! Do you know where i can read more on that?
i couldn't find these specifics in the documentation

[Zachary Grafton]

https://kotlinlang.org/docs/reference/generics.html

[Matheus]

Thanks for your help 🙂

[diesieben07]

You are running into a platform type. If Java says Boolean, Kotlin cannot know if it's nullable or not, so it sees it as a platform type, which adjusts to being nullable or not nullable depending on context.
If you are overriding a method, IntelliJ will choose the conservative way and choose nullable. but in this case that's not correct, so if you change the return type to Class<Boolean> your code still compiles and makes sense.

[diesieben07]

See here: https://kotlinlang.org/docs/reference/java-interop.html#null-safety-and-platform-types

[Matheus]

The interface is Class<T> toType();

It is applied to my Kotlin class, tho, which has a Boolean? type

[Matheus]

Well, I tried to present a minimal example but I think I have to explain the whole thing lol

I'm using JOOQ's Converter interface to convert a BigDecimal (from the database) to a Boolean (domain type) (it's complicated, but I can't change the database type, it's basically a 0|1 column)

The class is

class EnabledIndicatorConverter: Converter<BigDecimal, Boolean?> {

    override fun to(isEnabled: Boolean?): BigDecimal? {
        return isEnabled?.let { BigDecimal.ONE } 
                ?: BigDecimal.ZERO
    }

    override fun fromType(): Class<BigDecimal> =
            BigDecimal::class.java

    override fun toType(): Class<Boolean?> =
            Boolean::class.java as Class<Boolean?>

}

[diesieben07]

You can probably do something like this:

class EnabledIndicatorConverter: Converter<BigDecimal, Boolean> {

    override fun to(isEnabled: Boolean?): BigDecimal? {
        return isEnabled?.let { BigDecimal.ONE } 
                ?: BigDecimal.ZERO
    }

    override fun fromType(): Class<BigDecimal> =
            BigDecimal::class.java

    override fun toType(): Class<Boolean> =
            Boolean::class.java

}

[Matheus]

Yeah, that worked!