将Scala单位转换为Java Void [英] Convert Scala Unit to Java Void

问题描述

我有一个采用Function[String, Void]的Java类，该类类似于Consumer功能接口.我正在从Scala代码中调用此Java函数.如下所示，我接受Scala上的function: => Unit，因为它等效于void.当我尝试应用此功能代替Void返回类型时，会发生类型不匹配的情况. .asInstanceOf[Void]的显式强制转换也引发了异常.如何将=> Unit转换为=> Void?

I have a Java class which takes Function[String, Void], which is similar to Consumer functional interface. I am calling this Java function from Scala code. As you can see below, I accept function: => Unit on Scala side as it is void equivalent. When I attempt to apply this function in place of Void return type type mismatch happens. Explicit casting of .asInstanceOf[Void] also threw an exception. How do we convert => Unit to => Void?

// Java 
class AcceptFunction {
   public void temp(Function<String, Void> f) {
      f.apply("Hello");
   }
}

// Scala
def temp(f: String => Unit): Unit = {
   new AcceptFunction().temp(new Function[String, Void]() {
   override def apply(t: String): Void = f(t) // <===== error expecting Void instead of Unit
   })
}

推荐答案

显然，

scala> new jfunc.Acceptor().f(s => println(s))
<console>:12: error: type mismatch;
 found   : Unit
 required: Void
       new jfunc.Acceptor().f(s => println(s))
                                          ^

scala> new jfunc.Acceptor().f { s => println(s) ; null }
hello, world.

您显示的Java所在的位置:

where the java is like you show:

package jfunc;

import java.util.function.*;

public class Acceptor {
  public void f(Function<String, Void> g) { g.apply("hello, world."); }
}

可以:

scala> :javap -c -
  public static final java.lang.Void $anonfun$res0$1(java.lang.String);
    Code:
       0: getstatic     #33                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
       3: aload_0
       4: invokevirtual #37                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
       7: aconst_null
       8: areturn

  public $line3.$read$$iw$$iw$();
    Code:
       0: aload_0
       1: invokespecial #39                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: putstatic     #41                 // Field MODULE$:L$line3/$read$$iw$$iw$;
       8: aload_0
       9: new           #43                 // class jfunc/Acceptor
      12: dup
      13: invokespecial #44                 // Method jfunc/Acceptor."<init>":()V
      16: invokedynamic #63,  0             // InvokeDynamic #0:apply:()Ljava/util/function/Function;
      21: invokevirtual #67                 // Method jfunc/Acceptor.f:(Ljava/util/function/Function;)V
      24: getstatic     #72                 // Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
      27: putfield      #74                 // Field res0:Lscala/runtime/BoxedUnit;
      30: return

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