I've scala - java polyglot project with scala version 2.13

Below is the standard project structure

    $scala-java-extend-demo on master
    ± tree .
    .
    ├── build.gradle.kts
    ├── gradlew
    ├── gradlew.bat
    ├── settings.gradle.kts
    └── src
        ├── main
             ├── java
             │   ├── Collector.java
             │   ├── Function.java
             │   ├── JProcessAllWindowFunction.java
             │   └── Window.java
             ├── resources
             └── scala
                 └── ScalaProcessAllWindowFunctionWrapper.scala


    24 directories, 15 files

Below is the Collector.java

public interface Collector<T> {
    void collect(T record);

    void close();
}

Below is the Function.java

public interface Function extends java.io.Serializable {}

Below is the Window.java

public abstract class Window {
    public abstract long maxTimestamp();
}

Below is the JProcessAllWindowFunction.java

public abstract class JProcessAllWindowFunction<IN, OUT, W extends Window> {

    private static final long serialVersionUID = 1L;

    public void process(Context context, Iterable<IN> elements, Collector<OUT> out){}

    public void clear(Context context) {}

    public abstract class Context {
        public abstract W window();
        public abstract <X> void output( X value);
    }
}

Below is the ScalaProcessAllWindowFunctionWrapper.scala which extends JProcessAllWindowFunction

import java.lang

final class ScalaProcessAllWindowFunctionWrapper[IN, OUT, W <: Window] extends JProcessAllWindowFunction[IN, OUT, W] {
  override def process(context: JProcessAllWindowFunction[IN, OUT, W]#Context, elements: lang.Iterable[IN], out: Collector[OUT]): Unit = ???
}

When I compile the project I get below error -

  > scala-java-extend-demo on master $
  ± gradle clean build
  
  > Task :compileScala FAILED
  [Error] /Users/myuser/Coding/misc/scala-java-extend-demo/src/main/scala/ScalaProcessAllWindowFunctionWrapper.scala:4: method process overrides nothing.
  Note: the super classes of class ScalaProcessAllWindowFunctionWrapper contain the following, non final members named process:
  def process: ((context: _1.Context, elements: Iterable[IN], out: Collector[OUT]): Unit) forSome { val _1: ScalaProcessAllWindowFunctionWrapper[IN,OUT,W] }
  one error found
  
  FAILURE: Build failed with an exception.

The project is on github here.

How can I fix this error?

CodePudding user response：

In ScalaProcessAllWindowFunctionWrapper modify the signature of method from

def process(context: JProcessAllWindowFunction[IN, OUT, W]#Context, elements: lang.Iterable[IN], out: Collector[OUT]): Unit = ???

to

def process(context: Context, elements: lang.Iterable[IN], out: Collector[OUT]): Unit = ???

i.e. replace type projection JProcessAllWindowFunction[IN, OUT, W]#Context with path-dependent type Context aka this.Context. Type projection is a typical Java type and would work for overriding there but too rough type here in Scala.

In Java Context means JProcessAllWindowFunction<IN, OUT, W>.Context i.e. JProcessAllWindowFunction[IN, OUT, W]#Context in Scala. But in Scala Context means this.Context.

this.Context is a subtype of JProcessAllWindowFunction[IN, OUT, W]#Context.

Why does one select Scala type members with a hash instead of a dot?

What are type projections useful for?

What does the `#` operator mean in Scala?

What is meant by Scala's path-dependent types?

What is the difference between path-dependent types and dependent types?

Are path-dependent types type projections?

Also add modifier override to process in ScalaProcessAllWindowFunctionWrapper and remove modifier final from process in JProcessAllWindowFunction.

CodePudding user response：

You should be able to override it by using this:

import java.lang

final class ScalaProcessAllWindowFunctionWrapper[IN, OUT, W <: Window] extends JProcessAllWindowFunction[IN, OUT, W] {
  override def process(context: this.Context, elements: lang.Iterable[IN], out: Collector[OUT]): Unit = ???
}

You should also remove the final modifier on the process method