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javainterop: Builder pattern from scala to java returns upperbound type e.g. Foo[Bar] in java instead of concrete type e.g. Bar #6243

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scabug opened this issue Aug 15, 2012 · 4 comments
Open

javainterop: Builder pattern from scala to java returns upperbound type e.g. Foo[Bar] in java instead of concrete type e.g. Bar #6243

scabug opened this issue Aug 15, 2012 · 4 comments
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java interop
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@scabug
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scabug commented Aug 15, 2012

Consider a recursive type definition on the scala side and Test on the java side and a Test2 on the scala side.

recurse.scala

package test
trait Foo[A <: Foo[A]] { 
      this: A =>
      def qux(): A = this
} 
class Bar extends Foo[Bar] 
class Duh extends Foo[Duh]

object Test2 extends App {
  val bar: Bar = new Bar().qux(); 
  val duh: Duh = new Duh().qux();
  
  println(bar)
  println(duh)
}

and

Test.java

package test;

public class Test {

  public static void main(String[] args) {
     Bar bar = new Bar().qux();
     Duh duh = new Duh().qux();
     System.out.println(bar); 
     System.out.println(duh); 
     
  }
}

Then the scala side Test2 will compile without problem and run while the java side will generate an error

C:\scala-2.10.0-M6\recurse>javac -d . -cp .;..\lib\scala-library.jar *.java
Test.java:6: incompatible types
found   : test.Foo
required: test.Bar
     Bar bar = new Bar().qux();
                            ^
Test.java:7: incompatible types
found   : test.Foo
required: test.Duh
     Duh duh = new Duh().qux();
                            ^
2 errors

A workaround is to either cast it on the java side to the proper type like Duh or override the method in the concrete class to return a concrete type like Bar.

recurse.scala

package test
trait Foo[A <: Foo[A]] { 
      this: A =>
      def qux(): A = this
} 
class Bar extends Foo[Bar] {
  override def qux(): Bar = super.qux()
}
class Duh extends Foo[Duh]

object Test2 extends App {
  val bar: Bar = new Bar().qux(); 
  val duh: Duh = new Duh().qux();
  
  println(bar)
  println(duh)
}

and

Test.java

package test;

public class Test {

  public static void main(String[] args) {
     Bar bar = new Bar().qux();
     Duh duh = (Duh) new Duh().qux();
     System.out.println(bar); 
     System.out.println(duh); 
     
  }
}

see thread
https://groups.google.com/forum/?hl=en&fromgroups#!topic/scala-user/PPDIPfeKGzo%5B1-25%5D
@scabug
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scabug commented Aug 15, 2012

Imported From: https://issues.scala-lang.org/browse/SI-6243?orig=1
Reporter: DaveScala (davescala)
Affected Versions: 2.10.0-M6
See #5289, #6169, #6168

@scabug
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scabug commented Oct 25, 2012

@adriaanm said:
probably can't do this in 2.10 since it would affect binary compatibility

@scabug
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scabug commented Jan 26, 2016

@szeiger said:
Possibly a duplicate of #5289

@scabug
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scabug commented Jul 6, 2016

@lrytz said:
I think it's not related to #5289. That one is about static forwarders generated for scala modules. The one here is about types, possibly signatures, but the Java code does not use static forwarders.

@scabug scabug added this to the Backlog milestone Apr 7, 2017
@scala scala deleted a comment from scabug May 1, 2023
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