Scala vs Java method invocation

Question

I had to write my own JDK dynamic proxy to provide the ability of dynamic interface implementation. Since I work with akka it has to be written in Scala. But I faced with some pretty strange behavior differs from Java:

object Main extends App {
    println(classOf[Message].getDeclaredMethod("msg")
                            .invoke(Message("Test"), null)) 
                          //throws IllegalArgumentException
}

case class Message(message: String){
  def msg() = message
}

StackTrace:

Exception in thread "main" java.lang.IllegalArgumentException: wrong number of arguments
    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:497)
    at Main$.delayedEndpoint$Main$1(Main.scala:2)
    at Main$delayedInit$body.apply(Main.scala:1)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:34)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:76)
    at scala.App$$anonfun$main$1.apply(App.scala:76)
    at scala.collection.immutable.List.foreach(List.scala:381)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
    at scala.App$class.main(App.scala:76)
    at Main$.main(Main.scala:1)
    at Main.main(Main.scala)

DEMO

But I in Java it works fine:

public static void main (String[] args) throws java.lang.Exception
{
    System.out.println(Message.class.getDeclaredMethod("msg")
              .invoke(new Message(), null)); //prints Message
}

public static class Message{
    public String msg(){
        return "Message";
    }
}

DEMO

Why does Scala throw exception? And how to fix it?

I bet Scala wraps the null argument differently, into its own arguments instead of to mean no arguments. Just remove it from the invocation. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Aug 26, 2016 at 14:30
@SotiriosDelimanolis is right, without the null yields what you expect — James Tobin
– James Tobin, Commented Aug 26, 2016 at 14:36
@SotiriosDelimanolis Very interesting, but don't you know the exact Scala magic about null? — user3663882
– user3663882, Commented Aug 26, 2016 at 14:54
Akka has Java interface, you are not obliged to write in Scala. — Alexei Kaigorodov
– Alexei Kaigorodov, Commented Dec 6, 2016 at 13:05

pedrofurla · Accepted Answer · 2016-08-26 17:03:01Z

Hopefully I can demonstrate what is going on:

class TheClass { 
  def theMethod:Unit = { println("It's me the method"); } 
}

scala> classOf[TheClass].getDeclaredMethod("theMethod").invoke(new TheClass, null)
java.lang.IllegalArgumentException: wrong number of arguments
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

scala> classOf[TheClass].getDeclaredMethod("theMethod").invoke(new TheClass)
It's me the method
res1: Object = null

As expected, but now have a look at this:

scala> classOf[TheClass]
      .getDeclaredMethod("theMethod").invoke(new TheClass, Array.empty)
java.lang.IllegalArgumentException: wrong number of arguments

Versus this:

scala> classOf[TheClass]
       .getDeclaredMethod("theMethod").invoke(new TheClass, Array.empty : _*)
It's me the method
res4: Object = null

Notice the difference between the invokes: .invoke(new TheClass, Array.empty) vs .invoke(new TheClass, Array.empty : _*). What happens is that invokes second argument is a varargs, which should be turned into a array under the hood, when it's given a plain Array.empty or null scalac is replacing them with Array(Array.empty) and Array(null). When the argument is not provided it's actually Array.empty and when we use : _* it just telling the compiler that the argument we are proving is the varargs itself.

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