I kind of understand what is happening here but how best to kotlinlang #getting-started

fun <T> Foo<T>.getValue(): T

val foo: Foo<Boolean> = ...
println("no problem: " + foo.getValue())
val fooAnyValue: Any = foo.getValue()
println("also no problem: " + fooAnyValue)
val fooValue = foo.getValue()
println("class cast exception (fooValue is not a Boolean): " + fooValue)

asdf asdf

03/19/2023, 5:18 AM

The type of

fooValue

is implicitly marked as a Boolean, as it is the expected return type of the

getValue

method

However it seems like the value of

foo

is not actually a boolean (possibly from an unchecked cast), so it is fine when the variable type is

Any

, but not

Boolean

Mark

03/19/2023, 8:23 AM

Yes, it’s interesting that we can get access to such an illegal value by assigning to a property of type

Any

(or by passing it straight into some place that accepts

Any

) and so avoid the

ClassCastException

. However with

val fooValue = foo.getValue()

there is an implicit type

Boolean

fooValue

and so the class cast cannot be avoided. A part of me thinks that when getting the value there should be a check/exception regardless.

For context, I’m playing around with Android Jetpack DataStore which stores different types in a big map. So I’m checking what would happen if, say, a random

String

ended up in a value for what would normally be a

Boolean

If I want to add my own check, is there a way (perhaps using an inline function with reified type) to ensure the value is the correct type? Indeed, we can do this:

Copy code

inline fun <reified T> Foo<T>.getValueSafely(): T? = getValue() as? T

Even though the compiler says “No cast needed” it does actually avoid the

ClassCastException

ilya.gorbunov

03/19/2023, 7:59 PM

This is tracked as https://youtrack.jetbrains.com/issue/KT-12451/checkcast-is-missing-when-passing-return-value-of-generic-function-into-another-generic-function However Kotlin exhibits here the same behavior as Java, as I recall.

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