How to convert a Java Stream to Scala Array?

Question

Given

new Scanner(is)
    .tokens()
    .map(_.toInt)
    .toArray[Int]((_: Int) => Array.ofDim[Int](100000))

I get a compile error

Error:(40, 12) no type parameters for method map: (x$1: java.util.function.Function[_ >: String, _ <: R])java.util.stream.Stream[R] exist so that it can be applied to arguments (java.util.function.Function[String,Int])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : java.util.function.Function[String,Int]
 required: java.util.function.Function[_ >: String, _ <: ?R]
Note: String <: Any, but Java-defined trait Function is invariant in type T.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
          .map(_.toInt)
Error:(40, 18) type mismatch;
 found   : java.util.function.Function[String,Int]
 required: java.util.function.Function[_ >: String, _ <: R]
          .map(_.toInt)

What is wrong here?

Try mapToInt instead of map: while Scala tries to treat Int etc. as if they were normal classes, it has some problems when interoperating with Java. — Alexey Romanov
– Alexey Romanov, Commented Nov 9, 2018 at 8:28
mapToInt(_.toInt).toArray does work. Would you care to offer any explanations about the errors, it seems interesting. — Abhijit Sarkar
– Abhijit Sarkar, Commented Nov 9, 2018 at 8:32

Abhijit Sarkar · Accepted Answer · 2018-11-09 21:32:47Z

1

The compiler is trying to find the R type parameter for Stream#map. Because the argument of map is a Function[String,Int], and it must be compatible with Function[_ >: String, _ <: R] (from the signature of map), there is a constraint Int <: R. But because it's a Java method, there's also a constraint R <: Object (at least I think that's what happens). So there's no suitable R.

The error message is not very good in this case, because it doesn't give the second constraint.

To fix, as I mentioned in the comment, you can just use mapToInt, but actually there's an even worse error message which definitely should be fixed (I reported an issue): specifying map[Int] results in

type mismatch;
 found   : Array[Int]
 required: Array[Int]
Note: Int >: Int, but class Array is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: Int`. (SLS 3.2.10)

edited Nov 9, 2018 at 21:32

Abhijit Sarkar

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answered Nov 9, 2018 at 9:50

Alexey Romanov

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2 Comments

Abhijit Sarkar Over a year ago

Your last error is what I started with, if I didn’t have the map. Please include the link to the issue you reported, and I’ll be happy to accept your answer. Thank you.

Alexey Romanov Over a year ago

github.com/scala/bug/issues/11248 (and there are useful comments there). Note that the bug is just that the error message should be improved, not that the code should work.

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