Using Option[Boolean] from Java

Question

I've got a Scala Trait with a method that returns Option[Boolean]. I'd like to write a Java class that implements this trait. Unfortunately the compiler complains about the following code:

trait WithBoolean {
  def doSth(): Option[Boolean]
}

public class MyClass implements WithBoolean {
  @Override
  public Option<Boolean> doSth() {
    return null;
  }
}

The compile error is:

doSth() in MyClass cannot implement doSth() in WithBoolean
public Option<Boolean> doSth() {
                       ^
return type Option<Boolean> is not compatible with Option<Object>

It does compile if i change the code slightly:

public class MyClass implements WithBoolean {
  @Override
  public Option<Object> doSth() {  //return type has been changed to Object
    return null;
  }
}

But this is obviously not a nice solution. What do I need to change in order to be able to use the correct return type?

score 2 · Answer 1 · answered Sep 01 '14 at 19:30

2

I can't test this right now, but my best guess is that you have some imports mixed up, and thus try to override Option<scala.Boolean> with Option<java.lang.Boolean>.

answered Sep 01 '14 at 19:30

Jörg W Mittag

363,080
75
446
653

There are no imports except for `import scala.Option;` – user3346601 Sep 02 '14 at 06:27
In that case, unqualified `Boolean` in Java code will refer to `java.lang.Boolean`, which is completely unrelated to the `scala.Boolean` in your Scala interface. – Jörg W Mittag Sep 02 '14 at 07:39
And scala.Boolean can't be used from Java, so the type Java sees _is_ Option – Alexey Romanov Sep 03 '14 at 18:57
@AlexeyRomanov: Ah, right, `scala.Boolean` doesn't actually exist, because it is a [fictional class invented by the compiler](https://github.com/scala/scala/blob/2.11.x/src/library/scala/Boolean.scala) to give the illusion that primitive `bool` is an object. – Jörg W Mittag Sep 03 '14 at 19:03

score 1 · Answer 2 · answered Sep 01 '14 at 15:32

1

Could you try

public class MyClass implements WithBoolean {
  @Override
  public Option<? super Boolean> doSth() {
    return null;
  }
}

answered Sep 01 '14 at 15:32

Zoltán

21,321
14
93
134

@user3346601 Sorry to hear that :( – Zoltán Sep 01 '14 at 16:08

score 0 · Answer 3 · edited Aug 30 '18 at 14:16

0

You can try this :

(value != null)?new Some<T>(value):scala.Option.apply((T) null);

(value != null)?new Some<Boolean>(value):scala.Option.apply((Boolean) null);

edited Aug 30 '18 at 14:16

Rabbid76

202,892
27
131
174

answered Aug 30 '18 at 13:14

Ruchita Surve

1

score 0 · Answer 4 · answered Aug 30 '18 at 14:24

From the class description:

Boolean (equivalent to Java's boolean primitive type) is a subtype of [[scala.AnyVal]]. Instances of Boolean are not represented by an object in the underlying runtime system.

Java Boolean extends AnyRef (more about this here).
AnyRef and AnyVal both extends Any which is equivalent to java.lang.Object.
This is why only Option<Object> works.

If you want type safety you could do this:

trait WithBoolean {
  def doSth(): Option[Boolean]
}

abstract class WithBooleanForJava extends WithBoolean {

  override def doSth(): Option[Boolean] = doSthJava().map(Boolean.box(_))

  def doSthJava(): Option[java.lang.Boolean]
}

So you java class will look like you wanted:

public class MyClass extends WithBooleanForJava {

    @Override
    public Option<Boolean> doSthJava() {
        return null;
    }
}

Using Option[Boolean] from Java

4 Answers4