什么时候kotlin lambda有效地转换成java SAM？

Question

considering this bit of code

java side :

public class Executor {
    public void execute (Runnable runner) { /* do something with runner */ }
}

kotlin side:

val executor = Executor() 
var runner: () -> Unit = { } // expected Runnable
executor.execute(runner)

Is SAM transformation done at the variable declaration line var runner: () -> Unit = {} or at the execute() function ?

Is runner reference inside execute scope is different from runner declaration ?

Answer 1

The conversion takes place at call site, ie when the Runnable is actually needed. Have a look at what is being generated by the compiler (represented in Java code):

final class FileKt$sam$Runnable$9c7e667b implements Runnable {

   private final Function0 function;

   FileKt$sam$Runnable$9c7e667b(Function0 var1) {
      this.function = var1;
   }

   public final void run() {
      Intrinsics.checkExpressionValueIsNotNull(this.function.invoke(), "invoke(...)");
   }
}

The above code shows that an implementation of Runnable is generated by the compiler, which takes a Function as its construction parameter. in the run implementation that function is simply being invoked.

//the call

Executor executor = new Executor();
Function0 runner = (Function0)null.INSTANCE;
Object var10001 = runner;
if (runner != null) {
   var10001 = new FileKt$sam$Runnable$9c7e667b(runner);
}

executor.execute((Runnable)var10001);

The call side shows how the lambda, compiled to a Function object gets converted into the proper Runnable instance.