Invoke Scala call-by-name method from Java

[刘斌]

This my scala code:

class A {

  def ensure(f: => Unit): Unit

}

How can I invoke this method from Java?

I tried many ways but failed...

[Jasper-M]

Let's have a look.

scala> abstract class A {

     |   def ensure(f: => Unit): Unit
     | }

defined class A


scala> :javap -p A
Compiled from "<console>"
public abstract class A {
  public abstract void ensure(scala.Function0<scala.runtime.BoxedUnit>);
  public A();
}

So I guess you have to give it a Function0 object whose apply method returns a BoxedUnit.

a.ensure(new scala.Function0<scala.runtime.BoxedUnit>{
    public scala.runtime.BoxedUnit apply() {
        //do some stuff
        return scala.runtime.BoxedUnit.UNIT
    }
})

Op donderdag 25 februari 2016 14:36:48 UTC+1 schreef 刘斌:

[Seth Tisue]

see also http://stackoverflow.com/a/8380694/86485 — making a scala.FunctionN from Java is much easier if you use AbstractFunctionN

[Jasper-M]

Okay, apparently because of Function0 being specialized for primitive types this is a lot more complex (maybe even not possible to write in Java, I'm not sure) :/

You're probably better off writing some Java-compatible wrapper in Scala first. Or wait until someone who knows more about the internals comes along.

Op donderdag 25 februari 2016 15:03:36 UTC+1 schreef Jasper-M:

[Oliver Ruebenacker]

     Hello,

  Not sure how specialization is a problem, unless you want to write generic functions and specialize them.

     Best, Oliver

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Oliver Ruebenacker

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[Jasper-M]

If you extend from Function0 you have to implement a whole bunch of methods which are needed for specialization.

But I'm pretty sure Seth's answer solves that problem. I didn't see it in time.

Op donderdag 25 februari 2016 16:17:41 UTC+1 schreef Oliver Ruebenacker:

[Oliver Ruebenacker]

     Hello,

  Looks like in Scala, you only have to implement the apply method - do you have to implement more when extending it in Java?

scala> val f = new Function0[Int] { def apply() : Int = 42 }
f: () => Int = <function0>

scala> println(f())
42

     Best, Oliver

[Jasper-M]

Yup, you need to implement at least all these methods:

scala> :javap -p scala.Function0
Compiled from "Function0.scala"
public interface scala.Function0<R> {
  public abstract R apply();
  public abstract java.lang.String toString();
  public abstract boolean apply$mcZ$sp();
  public abstract byte apply$mcB$sp();
  public abstract char apply$mcC$sp();
  public abstract double apply$mcD$sp();
  public abstract float apply$mcF$sp();
  public abstract int apply$mcI$sp();
  public abstract long apply$mcJ$sp();
  public abstract short apply$mcS$sp();
  public abstract void apply$mcV$sp();
}

Op donderdag 25 februari 2016 16:29:44 UTC+1 schreef Oliver Ruebenacker:

[Oliver Ruebenacker]

     Hello,

  Why you don't have to implement these in Scala? Thanks!

     Best, Oliver

[Seth Tisue]

because Scala understands that traits can include default implementations for some methods. from Java, the trait just shows up as an interface, with all methods abstract.

[Oliver Ruebenacker]

     Hello,

  Where are these default methods defined? I don't see them in Function0, and there are no super-types besides AnyRef.

     Best, Oliver

On Thu, Feb 25, 2016 at 11:58 AM, Seth Tisue <se...@tisue.net> wrote:

because Scala understands that traits can include default implementations for some methods. from Java, the trait just shows up as an interface, with all methods abstract.

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[Clint Gilbert]

They're added by the compiler due to the @specialized annotation on
Function0's type param. From Scala code they're invoked when/if
possible by the compiler; see the semi-mangled names.

On 02/25/2016 02:17 PM, Oliver Ruebenacker wrote:
>
> Hello,
>
> Where are these default methods defined? I don't see them in
> Function0, and there are no super-types besides AnyRef.
>
> Best, Oliver
>
> On Thu, Feb 25, 2016 at 11:58 AM, Seth Tisue <se...@tisue.net
> <mailto:se...@tisue.net>> wrote:
>
> because Scala understands that traits can include default
> implementations for some methods. from Java, the trait just shows up
> as an interface, with all methods abstract.
>
> --
> You received this message because you are subscribed to the Google
> Groups "scala-user" group.
> To unsubscribe from this group and stop receiving emails from it,
> send an email to scala-user+...@googlegroups.com

> <mailto:scala-user+...@googlegroups.com>.

> For more options, visit https://groups.google.com/d/optout.
>
>
>
>
> --
> Oliver Ruebenacker
> Senior Software Engineer, Diabetes Portal

> <http://www.type2diabetesgenetics.org/>, Broad Institute
> <http://www.broadinstitute.org/>

>
> --
> You received this message because you are subscribed to the Google
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[sunnykaka]

Thanks Jasper-M.

I tried your solution, this is my Test class:

class A {
  def ensure(f: => Unit): Unit = println("ensure")

}

public class B {

    public static void main(String[] args) {
        A a = new A();
        a.ensure(new scala.Function0<scala.runtime.BoxedUnit>() {
            @Override
            public scala.runtime.BoxedUnit apply() {

                return scala.runtime.BoxedUnit.UNIT;
            }
        });
    }

}

The IDEA error is: 

ensure (scala.Function0<void>) in A cannot be applied to (anonymous scala.Function0<scala.runtime.BoxedUnit>)

If I ignore the IDEA error and compile the code, I get this error:

Error:(12, 65) java: <匿名com.akkafun.B$1>不是抽象的, 并且未覆盖scala.Function0中的抽象方法apply$mcV$sp()

which means anonymous com.akkafun.B$1 is not abstract, and not override the abstract method apply$mcV$sp() in scala.Function0.

[sunnykaka]

Hi Seth, I visited the link you provided.

This is not the problem I want to solve.

I want to invoke a call-by-name scala method from Java.

e.g.

def ensure1(f: => Unit): Unit

not this: 

def ensure2(f: Unit => Unit): Unit

I can invoke the latter method from Java easily:

class A {
  def ensure1(f: => Unit): Unit = println("ensure1")

  def ensure2(f: Unit => Unit): Unit = {
    println("before invoke f in ensure2")
    f()
  }

}

public class B {

    public static void main(String[] args) {
        A a = new A();

//        a.ensure1(new scala.Function0<scala.runtime.BoxedUnit>() {
//            @Override
//            public scala.runtime.BoxedUnit apply() {
//                return scala.runtime.BoxedUnit.UNIT;
//            }
//        });

        a.ensure2(new AbstractFunction1<BoxedUnit, BoxedUnit>() {
            @Override
            public BoxedUnit apply(BoxedUnit v1) {
                System.out.println("invoke ensure2 success");
                return BoxedUnit.UNIT;
            }
        });
    }

}

Run class B will print:

before invoke f in ensure2

invoke ensure2 success

[Seth Tisue]

javap will show you that this:

  def ensure1(f: => Unit): Unit = ...

compiles to the same bytecode, from Java's point of view, as:

  def ensure2(f: () => Unit): Unit = ...

note `() => Unit`, not `Unit => Unit`.

so calling it from Java using AbstractFunction0 works fine:

  A a = new A();

  a.ensure1(new scala.runtime.AbstractFunction0<scala.runtime.BoxedUnit>() {
  @Override public scala.runtime.BoxedUnit apply() { 

    return BoxedUnit.UNIT;
  }));

[sunnykaka]

Yes, it works, the code is good to compile and run.

although IDEA still report the same error.

Thanks a lot.