Can you provide scala view bounds from Java?

Question

Suppose I have

val a: Option[String] = None
someJavaFunction(a)

And then in the java file, I want to do something like this:

public someJavaFunction(Option<String> o) {
  o.orNull();
}

The signature of orNull, however, is this:

orNull [A1 >: A](implicit ev : <:<[Null, A1]) : A1

So from Java, I'd need to supply this evidence function that is usually magicked in by Scala (from I know not where). How might I get hold of the evidence value to pass in here?

This is clearly not sensible.

Is it possible?

http://stackoverflow.com/questions/7808821/java-calling-scala-case-class-w-implicit-parameter — user3707125, Jan 04 '16 at 16:31
A possibly could be to simply pass `null`, but that doesn't seem very clean. `o.orNull(null)`. — Clashsoft, Jan 04 '16 at 16:31
@user3707125 Thanks for the pointer. I know how to pass the parameter - the question is, what do I pass in? — MHarris, Jan 04 '16 at 17:01
@Clashsoft Good idea - unfortunately, that results in an NPE, looks like the implementation of orNull is using the evidence parameter. — MHarris, Jan 04 '16 at 17:06

Rex Kerr · Accepted Answer · 2016-01-04T18:15:37.317

The <:< class is defined in scala.Predef and normally it's scala.Predef.conforms() that gives you an instance in Scala. So you could do something like

public class Foo {
  public static String foo(scala.Option<String> o) {
    return o.orNull(
      (scala.Predef.$less$colon$less< scala.runtime.Null$ , String >)
      (Object)scala.Predef.conforms()
    );
  }
}

which gives an unchecked operation warning, but works.

You can also create an instance of scala.Predef.$less$colon$less that does the right thing (just returns its argument), and there the cast goes through without warning.

Note: you must leave a space after Null$ or javac gets confused.

Can you provide scala view bounds from Java?

1 Answers1