[FIXED] Kotlin. Basic JavaFX application

Issue

Trying out Kotlin lang and I had an impression that it is compatible with Java and therefore with JavaFX and i tried following:

public object TestKt: Application() {

    public override fun start(stage: Stage){
        val pane= Pane()
        val scene=Scene(pane,200.0,200.0)
        stage.scene = scene
        stage.show()

    }
    @JvmStatic public fun main(args: Array<String>){
        launch()
    }
}

this is basically same as Java's

public class Test extends Application {
    @Override
    public void start(Stage stage)  {
        Pane pane=new Pane();
        Scene scene=new Scene(pane, 200,200);
        stage.setScene(scene);
        stage.show();
    }
    public static  void  main(String[] args){
        launch();
    }
}

but Kotlin gives an error: Exception in thread "main" java.lang.RuntimeException: Unable to construct Application instance: class Test

Solution

The code samples you provided are not equivalent: an object declaration in Kotlin is a singleton, so it only has one instance constructed by calling the private constructor when the class is initialized. JavaFX is trying to call the constructor of the class reflectively but fails because the constructor is private as it should be.

What you may be looking instead is a simple class declaration, with the main in its companion object. If no explicit constructors are declared, Kotlin, like Java, will generate a default one, allowing JavaFX to instantiate the application:

class Test : Application() {
    override fun start(stage: Stage) {
        ...
    }

    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            launch(Test::class.java)
        }
    }
}

Answered By - Alexander Udalov