Java interop: non-static generic inner class

[Dzmitry Lazerka]

I'm trying to override a Java method:

• with "override" keyword compiler says it overrides nothing.
  
• without "override" keyword compiler says "name clash between defined and inherited member ... have same type after erasure"

My Scala code:

import org.apache.hadoop.mapreduce.Mapper

class MyMapper extends Mapper[String, String, String, String] {

  override def setup(context: Context){

  }

}

Library Java code:

public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {

  public class Context extends MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {}

  protected void setup(Context context) throws IOException, InterruptedException {}

}

Error:  method setup overrides nothing

Without "override": name clash between defined and inherited member:

  method setup:(context: MyMapper.this.Context)Unit and

  method setup:(x$1: org.apache.hadoop.mapreduce.Mapper[String,String,String,String]#Context)Unit in class Mapper

  have same type after erasure: (context: org.apache.hadoop.mapreduce.Mapper#Context)Unit

    def setup(context: Context){

Any ideas?

[Dzmitry Lazerka]

I was unable to make work any StackOverflow answers on this question.

[Jason Zaugg]

If you're just trying to get things moving, you could write MyMapper in Java and delegate to an implementation in Scala.

-jason

[Kevin Wright]

This looks familiar...

I think I had to specify @throws[classOf[IOException]] in my Scala override when faced with a similar problem before, so that the checked exception would also be matched in the signature.

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[Moritz Uhlig]

On 07.10.2011, at 10:39, Dzmitry Lazerka wrote:

I was unable to make work any StackOverflow answers on this question.

Works for me. In your case it should be:

    import org.apache.hadoop.mapreduce._

   

    class MyMapper extends Mapper[String,String,String,String] {

      type Context = Mapper[String,String,String,String]#Context

      override def setup(context: Context) = {}

      override def map(key: String, value: String, context: Context) = sys.error("Not implemented")

    }

As Context is an inner class of Mapper you need to use a type projection here. Either override setup like this:

  override def setup(context: Mapper[String,String,String,String]#Context)

or use the type alias in the example above.

Moritz

[Dzmitry Lazerka]

Yes, it works, but actually I'm extending AppEngineMapper which in turn extends Mapper:

public abstract class AppEngineMapper<KEYIN,VALUEIN,KEYOUT,VALUEOUT>

    extends Mapper<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {}

And :

class MyMapper extends AppEngineMapper[String, String, String, String] {

  override def setup(context: Mapper[String,String,String,String]#Context){}

}

shows "name clash ... have same type after erasure: (context: org.apache.hadoop.mapreduce.Mapper#Context)Unit"

While

class MyMapper extends AppEngineMapper[String, String, String, String] {

  override def setup(context: AppEngineMapper[String,String,String,String]#Context){}

}

shows "method setup overrides nothing".

[Dzmitry Lazerka]

Thank you, already done. Just want to know Scala in details.

[Dzmitry Lazerka]

No, @throws(...) doesn't affect behavior in any case.

[Dzmitry Lazerka]

Bug: https://issues.scala-lang.org/browse/SI-1409
Looks like it gonna be fixed in Scala v3.