Scala - mapping a list of integers to a method that receives java.lang.Object

Question

Working in Scala-IDE, I have a Java library, in which one of the methods receives java.lang.Object. And I want to map a list of Int values to it. The only solution that works is:

val listOfInts = groupOfObjects.map(_.getNeededInt)

for(int <- listOfInts) libraryObject.libraryMethod(int)

while the following one:

groupOfObjects.map(_.getNeededInt).map(libraryMethod(_)

and even

val listOfInts = groupOfObjects.map(_.getNeededInt)

val result = listOfInts.map(libraryObject.libraryMethod(_))

say

type mismatch; found : Int required: java.lang.Object Note: an implicit exists from scala.Int => java.lang.Integer, but methods inherited from Object are rendered ambiguous. This is to avoid a blanket implicit which would convert any scala.Int to any AnyRef. You may wish to use a type ascription: x: java.lang.Integer.

and something like

val result = listOfInts.map(libraryObject.libraryMethod(x => x.toInt))

or

val result = listOfInts.map(libraryObject.libraryMethod(_.toInt))

does not work also.

1) Why is it happening? As far as I know, the for and map routines do not differ that much!

2) Also: what means You may wish to use a type ascription: x: java.lang.Integer? How would I do that? I tried designating the type explicitly, like x: Int => x.toInt, but that is too erroneus. So what is the "type ascription"?

UPDATE:

The solution proposed by T.Grottker, adds to it. The error that I am getting with it is this:

missing parameter type for expanded function ((x$3) => x$3.asInstanceOf[java.lang.Object])

missing parameter type for expanded function ((x$3) => x$3.asInstanceOf{#null#}[java.lang.Object]{#null#}) {#null#}

and I'm like, OMG, it just grows! Who can explain what all these <null> things mean here? I just want to know the truth. (NOTE: I had to replace <> brakets with # because the SO engine cut out the whole thing then, so use your imagination to replace them back).

Did you try an explicit cast in groupOfObjects.map(_.getNeededInt).map(libraryMethod( _.asInstanceOf[java.lang.Object])? — tgr
listOfInts.map(x => libraryObject.libraryMethod(x.toInt)) should work — elbowich
Just a guess. Could this be because your try to call the java method with the primitve type int and not an object? In java land you can't do autounboxing and "widening" at the same time, not sure if the same applies to Scala thou. — Emil H
@Grottker nope, it takes us deeper into the mystery, see the update. Does it work in your environment? — noncom

Rex Kerr Rex Kerr · Accepted Answer · 2012-05-08T20:41:26

The type mismatch tells you exactly the problem: you can convert to java.lang.Integer but not to java.lang.Object. So tell it you want to ask for an Integer somewhere along the way. For example:

groupOfObjects.map(_.getNeededInt: java.lang.Integer).map(libraryObject.libraryMethod(_))

(The notation value: Type--when used outside of the declaration of a val or var or parameter method--means to view value as that type, if possible; value either needs to be a subclass of Type, or there needs to be an implicit conversion that can convert value into something of the appropriate type.)

Scala - mapping a list of integers to a method that receives java.lang.Object

1 Answers