How to get java class from generic method param?

Question

I try to use the next method:

fun <T> put(value: T){
    val clazz = T::class.java
}

but has exception Kotlin: Only classes are allowed on the left hand side of a class literal

How obtain class from generic param?

What are other options except class can be passed as param?

Have a look at this answer: http://stackoverflow.com/questions/3437897/how-to-get-a-class-instance-of-generics-type-t — Shlomi Uziel, Mar 11 '16 at 15:06
why do you want the class from the generics instead of `value` which obviously knows its own class? — Jayson Minard, Mar 12 '16 at 00:17
@JaysonMinard I'm going to use a some sort of fabric and reflection for annotation processig — Alex Klimashevsky, Mar 14 '16 at 09:35

score 6 · Answer 1 · edited Mar 12 '16 at 00:19

6

To access generic types within a function you need to make the types reified. Since this is not natively supported by the JVM its only available in inlined functions:

inline fun <reified T : Any> put(value: T) {
    val clazz = T::class.java
}

The type bound of Any is required to prevent some complications with nullable types.

edited Mar 12 '16 at 00:19

Jayson Minard

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answered Mar 11 '16 at 15:02

Kiskae

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what are restriction for using this method from Java code? – Alex Klimashevsky Mar 11 '16 at 15:06
@AlexKlimashevsky you can search for the 1000 examples of TypeReference, TypeToken, TypeRef and so on that do this similar type of thing from a Java viewpoint, you'll need to follow those patterns as Kotlin can make Kotlin nicer, but not make Java nicer. – Jayson Minard Mar 12 '16 at 00:21

score 0 · Accepted Answer · answered Mar 11 '16 at 15:23

0

fixed by

fun <T: Any> put(value: T){
    val clazz = value.javaClass
}

answered Mar 11 '16 at 15:23

Alex Klimashevsky

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This may fail sometimes: consider `put(0 as Number)`, here `T` is `Number`, but `value.javaClass` will be `Integer`. If this is not what you want, use reified generics as stated in the other answer. – hotkey Mar 11 '16 at 15:26
@hotkey I think more about Java compatibility. so this is what I want now – Alex Klimashevsky Mar 11 '16 at 15:42
In that case you might as well make `clazz` the parameter. – Kirill Rakhman Mar 11 '16 at 16:32
@AlexKlimashevsky you should make it clear in your question that you want the class of `value` and not of the generic type parameter, because your comments are to the contrary of what you originally stated. And @hotkey is correct, you will get a wrong answer for the generic parameter, but a correct answer for the class of the parameter passed to the function. – Jayson Minard Mar 12 '16 at 00:18

How to get java class from generic method param?

2 Answers2