To work with a Java API I need to create a method whose retu

Question

To work with a Java API I need to create a method whose return type is `Void` and has the following behavior ``` override fun getResult Void if success throw RuntimeException ``` This doesn t compile because a function with a block body requires a return statement

Ellen Spertus · Answer

How can I fix this code

Ruckus · Answer

You probably actually want to return `Void ` remember Java doesn t have null safety ``` override fun getResult Void if success return null else throw RuntimeException ```

James · Answer

``` override fun getResult Unit if success throw RuntimeException ```

James · Answer

do you need to return Void explicitly

karelpeeters · Answer

Doesn t Java also require a return statement

James · Answer

that is kotlin

Ellen Spertus · Answer

I believe so because I m extending `Task lt Void gt ` which requires implementing `getResult Void`

Ellen Spertus · Answer

This compiles but it s not exactly what I want ``` override fun getResult Void throw RuntimeException ```

Ruckus · Answer

< Ellen Spertus> Did my code not do what you expected

Ellen Spertus · Answer

Sorry < Ruckus> your code scrolled off my screen and I missed it at first

Ruckus · Answer

Basically you can t actually return a `Void` value just like you can t return a `Nothing` value in Kotlin there is no instance of `Void` In order to get this to work you either need to diverge not return like your always fail example or change your return type you can use `Void ` to fit better into the Java world or if it makes more sense you can change to extend `Task lt Unit gt ` and use < James> s code

Casey Brooks · Answer

In Java `Void` has a private constructor so it is impossible to actually get an instance of `Void` Any method contract that requires you to return `Void` is implicitly requesting you return `null` instead I think having your Kotlin method have a return type of `Void ` and returning null on the `success` branch is the way to go

Ellen Spertus · Answer

Yes < Ruckus> your solution works I didn t think I d be able to change the return type Thank you very much

Ruckus · Answer

Yeah Kotlin has to make some assumptions when overriding Java so if there s no annotations it chooses the safest by default arguments are nullable and return types aren t but they re actually platform types so you can choose your nullability as you see fit

Stephan Schroeder · Answer

I m with James I assume Kotlin s Unit to be mapped to Java s void So ```override fun getResult Unit if success throw RuntimeException ``` should work and is more idiomatic than Void This playground code overrides Java s Object s finalize method which returns void in Java by using Unit and it works fine ```class KotlinObject java lang Object override fun finalize Unit ``` <https pl kotl in n umKF6c8>

karelpeeters · Answer

But none of that works if Java code is forcing you to implement `Task lt Void gt `

Stephan Schroeder · Answer

It s interesting that the interop doesn t automatically translate `Task lt Unit gt ` but it does `Task lt Void gt ` because technically `Void ` is as far from `Void` as is `Unit` What about Nothing ` which is the same set as Void ` I guess that the instance of `Task lt Void gt ` is passed back to Java code Because otherwise using Task lt Unit gt is probably the answer Something similar happens when I want to use the `ExecutorService` which only handles `Callable`s and not `Runable`s In that case `Callable lt Unit gt ` is the way to go

Ruckus · Answer

gt technically `Void ` is as far from `Void` as is `Unit` What How on Earth did you come to that conclusion `Void` has nothing to do with `Unit`

Casey Brooks · Answer

The difference is that `Task lt Unit gt ` must return something namely `Unit INSTANCE` `Task lt Void gt ` does not make sense since you cannot get a non null instance of `Void` to return and thus `Task lt Void gt ` must return null

Casey Brooks · Answer

`Unit` is analogous to `void` in the sense that it is the default return type but it is not the same thing as `void` `void` has no instance `unit` has an instance and is non null and has a different meaning from `void` in Kotlin s null safe world