"Hallo Welt" mit Fenster und Knopf

import javafx.application.*;
   import javafx.scene.Scene;
   import javafx.scene.control.Label;
   import javafx.scene.control.Button;
   import javafx.scene.layout.*;
   import javafx.stage.Stage;
 
   public class HelloFX extends Application {
 
       @Override
       public void start(Stage stage) {
           // Button
           Button btn = new Button();
           btn.setText("Hallo Welt!");
           btn.setOnAction( (event) -> Platform.exit() );
 
           // Stackpane "root" 
           Pane root = new StackPane();
 >>        // Knopf anfügen
           root.getChildren().add(btn);
 
           stage.setScene(new Scene(root, 300,150) );
           stage.show();
       }
 
       public static void main(String[] args) {
           launch();
       }
 
   }