如何为 Java 调用者声明返回类型为“void”的 Kotlin 函数？

Question

I have a library that is completely written in Kotlin including its public API. Now a user of the library uses Java, the problem here is that Kotlin functions with return type Unit are not compiled to return type void . The effect is that the Java side has always to return Unit.INSTANCE for methods that are effectivly void. Can this be avoided somehow?

Example:

Kotlin interface

interface Foo{
  fun bar()
}

Java implementation

class FooImpl implements Foo{
   // should be public void bar()
   public Unit bar(){  
      return Unit.INSTANCE 
      // ^^ implementations should not be forced to return anything 
   }
}

Is it possible to declare the Kotlin function differently so the compiler generates a void or Void method?

Answer 1

Both Void and void work, you just need to skip that Unit ...

Kotlin interface:

interface Demo {
  fun demoingVoid() : Void?
  fun demoingvoid()
}

Java class implementing that interface:

class DemoClass implements Demo {

    @Override
    public Void demoingVoid() {
        return null; // but if I got you correctly you rather want to omit such return values... so lookup the next instead...
    }

    @Override
    public void demoingvoid() { // no Unit required...

    }
}

Note that while Kotlins reference guide 'Calling Kotlin from Java' does not really mention it, the Unit documentation does:

This type corresponds to the void type in Java.

And as we know, the following two are equivalent:

fun demo() : Unit { }
fun demo() { }

Answer 2

I implemented a simple variant on this using a java interface.

--- In ACallback.java --

public interface ACallback<T> {
     void invoke( T data );
}

-- In a Kotlin file

fun dostuff( arg: String , callback: ACallback<String> ) { 
   stuff();
   callback(arg) ; // yes 'invoke' from java 
}

--- In java ----

  doStuff( "stuff" , ( String arg) -> voidReturningFunction( arg ) ) ;

Answer 3

example:

override fun doInBackground(vararg params: Void?):Void{
    for (i in 0 until 10) {

        Thread.sleep(1000);
    }

    return null as Void;

}