Scala映射值扩展接口

Question

This method is in a Java library we are calling

   public void setNumbers(Map<Integer, ? extends COSObjectable> numbers) {
...
    }

I have a mixed Java/Scala project, I moved working code from a Java class into a Scala class and now I can't get this call to work from Scala. The best I've been able to get is

  val numbers = new util.HashMap[Integer, _ <: COSObjectable ]
  node.setNumbers(numbers)

which is failing to compile with

class type required but java.util.HashMap[Integer, _ <: org.apache.pdfbox.pdmodel.common.COSObjectable] found

The COSObjectable parameter of the map is a java interface

public interface COSObjectable {
    COSBase getCOSObject();
}

I'm not sure Scala likes this syntax when used with an interface. It seems to be saying I can only do this with a class.

Answer 1

我想也许我只是太努力了，脱掉_ <：似乎有效

val numbers = new util.HashMap[Integer, COSObjectable ]

Answer 2

I was able to make it work with:

object Tes {
  case class RandomStuff()
  val numbers = new java.util.HashMap[Integer, _ <: RandomStuff ]
}

or

  trait COSObjectable {
    def getCOSObject: Nothing
  }

  val numbers: Map[Integer, _ <: COSObjectable ] = Map.empty

Answer 3

When you instantiate a class you need to use a specific type, not a wildcard. Just like in Java you cannot do new util.HashMap<Integer, ? extends COSObjectable> new util.HashMap<Integer, ? extends COSObjectable> you can do the equivalent new util.HashMap[Integer, _ <: COSObjectable] .

You need to specify the acutal type like new util.HashMap[Integer, COSObjectable] (or whatever subtype of COSObjectable you require).