Behavior when calling Java variadic methods from Scala / Kotlin / Java

What article?

Methods with the same name in the same class Java classes (in short, classes like ↓) that are a mixture of variable-length methods and fixed-length methods We investigated which one is called when called from Scala / Kotlin / Java with one argument.

public class CalledClazz {
    public static void method(Object... args){
        System.out.println("Variadic method" + Arrays.toString(args));
    }
    public static void method(Object args){
        System.out.println("Fixed-length argument method" + args);
    }
}

Conclusion

Behavior when one argument is passed

--Java => Basically, method (Object args) is executed, but in some cases it becomes [strange behavior](called from #java). --Kotlin => method (Object args) is executed --Scala => The variable length argument method (Object ... args) is called ... so [How to call the fixed length argument Java method from Scala](fixed length argument java method from #scala) The method of calling) was also examined.

Survey method (survey code)

Call from Java

public class JavaClazz {
    public static void main(String... args) throws IOException {
        CalledClazz.method("Java");
        CalledClazz.method(null);
        Object s = null;
        CalledClazz.method(s);
        CalledClazz.method((Object)null);
    }
}

Execution result

Fixed-length argument method Java
Variadic method null
Fixed-length argument method null
Fixed-length argument method null

If you pass null, will it come to variadic? Moreover, it is not an Object type array with 1 element, but null is passed ... What's more, if you declare a variable with some type with null and pass it, or cast it to Object, it will come to a fixed-length argument method ...

Call from Kotlin

fun main() {
    CalledClazz.method("Kotlin")
    CalledClazz.method(null)
}

Execution result

Fixed-length argument method Kotlin
Fixed-length argument method null

It looks like a natural behavior.

Call from Scala

object CallTestByScala extends App {
  CalledClazz.method("Scala")
  CalledClazz.method(null)
}

Execution result

Variadic method[Scala]
Variadic method[null]

Why is it an array and the variadic method is called?

How to call a fixed-length argument Java method from Scala

val f: Object => Unit = CalledClazz.method
f("Scala")

Fixed-length argument method Scala

It came to come out.

In this way, declaring the type of a function without relying on type inference seems to be called as intended. Is there any other way?

By the way,

val f: Object => Unit = CalledClazz.method
f("Scala", "aaaaa")

Then, the method with fixed-length argument is called, and one value of Tuple2 type is passed.

Fixed-length argument method(Scala,aaaaa)

Will come out.

Bonus

From Scala's point of view, method (Object ... args) seems to be treated as an iterative parameter. If you want to pass scala.List to a Java variable length method ** as intended **,

CalledClazz.method(List("a","b","b"): _*)

Must be. If you do not add it, it will be treated as an array with 1 element, with one object called List ("a", "b", "b") as an element.

Variadic method[List(a, b, b)]

Will come out.

Reference material

Scala: How to pass as a List to Repeated Parameters \ (Repeated Parameters ) -Qiita