Calling java generic function from scala

Question

I am new to scala and am having an issue in calling a "generic" function in the java NIO library (from scala 2.10.x). Reducing the code to a simple test:

import java.net._
import java.nio.channels.{MembershipKey, DatagramChannel}

object Test {
  val channel = DatagramChannel.open(StandardProtocolFamily.INET)
    .setOption(StandardSocketOptions.SO_REUSEADDR, true)
    ...
}

This results in:

Error:(40, 48) type mismatch;
 found   : java.net.SocketOption[Boolean]
 required: java.net.SocketOption[Any]
Note: Boolean <: Any, but Java-defined trait SocketOption is invariant in type T.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
  channel.setOption(StandardSocketOptions.SO_REUSEADDR, true)
                                           ^

I assume there is some way to resolve this without resorting to writing a wrapper in java. Have tried recasting in a variety of ways without success.

Question: how do I resolve the above?

4e6 · Accepted Answer · 2014-09-04 13:05:34Z

7

Method setOption is polymorphic

setOption[T](name: SocketOption[T], value: T): DatagramChannel

so when calling

setOption(StandardSocketOptions.SO_REUSEADDR, true)

scala see that types of arguments are slightly different

StandardSocketOptions.SO_REUSEADDR: SocketOption[java.lang.Boolean]
true: scala.Boolean

and compiler tries to narrow T to the most common type between java.lang.Boolean <: AnyRef and scala.Boolean <: AnyVal which is Any

to fix this issue, you need to either provide explicit type for setOption

setOption[java.lang.Boolean](StandardSocketOptions.SO_REUSEADDR, true)

or use type ascription (then T will be inferred correctly)

setOption(StandardSocketOptions.SO_REUSEADDR, true: java.lang.Boolean)

answered Sep 4, 2014 at 13:05

4e6

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