无法从 java class 扩展 scala class，它具有将内部 class 作为参数的抽象函数

Question

我有 scala - java 多语言项目，scala 版本 2.13

下面是标准的项目结构

    $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

下面是Collector.java

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

    void close();
}

下面是Function.java

public interface Function extends java.io.Serializable {}

下面是Window.java

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

下面是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);
    }
}

下面是扩展 JProcessAllWindowFunction

的 ScalaProcessAllWindowFunctionWrapper.scala

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 = ???
}

当我编译项目时出现以下错误 -

  > 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.

项目在 github here.

我该如何解决这个错误？

Answer 1

在ScalaProcessAllWindowFunctionWrapper中修改方法的签名

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

至

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

即将类型投影 JProcessAllWindowFunction[IN, OUT, W]#Context 替换为路径相关类型 Context 又名 this.Context。类型投影是一种典型的 Java 类型，可以在那里覆盖，但在 Scala 中这里的类型太粗糙了。

在 Java 中 Context 表示 JProcessAllWindowFunction<IN, OUT, W>.Context 即 Scala 中的 JProcessAllWindowFunction[IN, OUT, W]#Context。但在 Scala 中 Context 表示 this.Context.

this.Context 和 x.Context（对于特定的 x: JProcessAllWindowFunction[IN, OUT, W]）是 JProcessAllWindowFunction[IN, OUT, W]#Context.

的子类型

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?

同时将修饰符 override 添加到 ScalaProcessAllWindowFunctionWrapper 中的 process 并从 JProcessAllWindowFunction 中的 process 中删除修饰符 final。

Answer 2

您应该可以使用 this:

覆盖它

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 = ???
}

您还应该删除 process 方法上的 final 修饰符

无法从 java class 扩展 scala class，它具有将内部 class 作为参数的抽象函数

couldn't extend scala class from java class which has abstract function which takes inner class as parameter

java

scala

generics

scala-java-interop

type-projection