Java SAM interface created from Kotlin gives ClassCastException - kotlin

Question

I have a java method:
addHandler(HttpServiceHandler handler)
HttpServiceHandler is
interface HttpServiceHandler extends Consumer<HttpHandlerContext>
The point is to avoid Consumer<HttpHandlerContext> copy-paste across the project, so it's kind of a type alias.
In Java code, this works all right:
addHandler({ context -> context.blah(); })
Now, in Kotlin, I have this method that generates handlers:
private companion object {
fun newHandler(notimportant: Long): HttpServiceHandler {
return HttpServiceHandler { context -> context.blah() }
}
}
HttpServiceHandler {} is important, it doesn't compile if I don't specify HttpServiceHandler for the lambda.
And this compiles:
addHandler(newHandler(1L))
But at runtime, throws:
java.lang.ClassCastException: blah.BlahTest$Companion$newHandler$1 cannot be cast to kotlin.jvm.functions.Function1
at blah.BlahTest.test(BlahTest.kt:42)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
And I can't figure out why. Help please?
Update: The class cast exception is gone if I write it like this:
addHandler(
object : HttpServiceHandler {
override fun accept(c: HttpHandlerContext) {
c.complete()
}
}
)
But still throws the exception when written like this:
fun newHandler(blah: Long): HttpServiceHandler {
return object : HttpServiceHandler {
override fun accept(c: HttpHandlerContext) {
c.complete()
}
}
}
addHandler(newHandler(1L))
And I have no idea why.
Update2: Test code at https://github.com/wilem82/testcases/tree/master/kotlinsam1 . Does not reproduce the problem, sadly.

Answer 1

the error has already said exactly:
java.lang.ClassCastException: blah.BlahTest$Companion$newHandler$1 cannot be cast to kotlin.jvm.functions.Function1
you try to cast a Consumer<HttpHandlerContext> to a Function1 in addHandler method or somewhere, for example:
fun addHandler(handler: Consumer<HttpHandlerContext>) {
val it: Function1<*, *> = handler as Function1<*, *>
// ^
// ClassCastException was thrown since it is not a Function1
}
you should using method reference expression in java / function reference expression in kotlin to convert a SAM interface to another SAM interface, for example:
fun addHandler(handler: Consumer<HttpHandlerContext>) {
val it: Function1<HttpHandlerContext, Unit> = handler::accept
// using function reference expression here ---^
//...
}
IF you call java method addHandler in kotlin, you needn't to create such a bridge method newHandler, you can just call it with lambda in kotlin, for example:
addHandler{context->
context.blah()
// additional works ...
}