Implement Java interface in Scala, with generics, and return null

Question

I have a Java interface that looks roughly like this:

public interface Foo {
  public <T> T bar();
}

I want to implement this interface in Scala, where all of my code uses Option. However, since the interface will be used by Java users, I want to return null instead of None. I tried the following:

class FooImpl extends Foo {
  def bar[T](): T = {
    val barOpt: Option[T] = getBar()
    barOpt.orNull
  }
}

This results in the following compile error:

Expression of type Null does not conform to expected type T

This makes sense, type T is unrestricted, it could be an Int or some other type that can't be null. No problem, just add T >: Null and you're done, right?

class FooImpl extends Foo {
  def bar[T >: Null](): T = {
    val barOpt: Option[T] = getBar()
    barOpt.orNull
  }
}

Nope, still no dice. Now you get a new compile error:

[error]  method bar has incompatible type

It seems that you can't apply any restrictions to T and still implement that interface.

Next, I tried using asInstanceOf:

class FooImpl extends Foo {
  def bar[](): T = {
    val barOpt: Option[T] = getBar()
    barOpt.orNull.asInstanceOf[T]
  }
}

But that just gets one more error:

Cannot prove that Null <:< T.

Is there any way to make this work?

@BevynQ: As I wrote in the description, the first thing I tried was T >: Null, but that leads to the second error. — Yevgeniy Brikman
– Yevgeniy Brikman, Commented Jun 20, 2013 at 4:23
I don't know scala, but does it support this kind of construct? barOpt.<T>orNull? — fge
– fge, Commented Jun 20, 2013 at 4:37

senia · Accepted Answer · 2013-06-20 05:03:47Z

6

You could use getOrElse and cast to T:

scala> def test[T](t: T): T = { Option(t).getOrElse(null).asInstanceOf[T] }
test: [T](t: T)T

You could move ugly stuff to helper class:

implicit class Defaults[T](val o: Option[T]) extends AnyVal {
  def orDefault(): T = o.getOrElse(null).asInstanceOf[T]
}

def test[T](t: T): T = { Option(t).orDefault }

edited Jun 20, 2013 at 5:03

answered Jun 20, 2013 at 4:45

senia

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1 Comment

senia Over a year ago

I know, but T could be AnyVal. You could hide casting see update.

Poko · Accepted Answer · 2013-06-22 17:41:40Z

2

Just an fyi, if you are using play, the same helper already exists in the libs package: https://github.com/playframework/Play20/blob/master/framework/src/play/src/main/java/play/libs/Scala.java#L12

answered Jun 22, 2013 at 17:41

Poko

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Comments

Yevgeniy Brikman · Accepted Answer · 2013-06-20 05:31:25Z

0

I suspect @senia's answer will prove to be the best option available in pure Scala. If nothing new comes in, I'll mark it as the "official" answer.

For completeness, here is a workaround using a Java class to encapsulate this dirty business, give it a clear name, and, just as importantly, document why it exists:

public class JavaOptionHelper {
  // http://stackoverflow.com/questions/17205142/implement-java-interface-in-scala-with-generics-and-return-null
  public static <T> T toValueOrNull(Option<T> option) {
    if (option.isDefined()) {
      return option.get();
    } else {
      return null;
    }
  }
}

It can be used as follows:

class FooImpl extends Foo {
  def bar[T](): T = {
    val barOpt: Option[T] = getBar()
    JavaOptionHelper.toValueOrNull(barOpt)
  }
}

edited Jun 20, 2013 at 5:31

answered Jun 20, 2013 at 5:16

Yevgeniy Brikman

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Comments

Community · Accepted Answer · 2017-05-23 12:28:14Z

0

The following worked for me:

scala> def test[T >: Null] = null
test: [T >: Null]=> Null

scala> test[String]
res18: Null = null

scala> def test2[T](x : T)(implicit ev: Null <:< T) = x
test2: [T](x: T)(implicit ev: <:<[Null,T])T

scala> test2("Hallo")
res19: java.lang.String = Hallo

scala> test2(null)
res20: Null = null

There is even a discussion on StackOverflow which covers the topic quite good. For details see here.

edited May 23, 2017 at 12:28

CommunityBot

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answered Jun 20, 2013 at 7:04

stefan.schwetschke

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