Unable to construct Application instance

Question

I am trying to launch in my public class an application defined in another class, earning the "Unable to construct Application instance"-Exception.The example (below) is quite minimalistic. What am I missing?---Clues would be very much appreciated.

This question is different from the question

Unable to construct javafx.application.Application instance

as I would like to have the definition of an application and its launch in separate classes.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.stage.Stage;

public class Main {
    public static void main(String[] args) {
        Application.launch(OKButton.class, args);
    }
}

class OKButton extends Application {
    @Override
    public void start(Stage stage) {
        Button btn = new Button("OK");
        Scene scene = new Scene(btn, 200, 250);
        stage.setScene(scene);
        stage.show();
    }
}

Maybe the same Exception thrown, but quite a different setting and question. — Clyde Frog, Jul 30 '18 at 07:53

score 5 · Accepted Answer · answered Jul 30 '18 at 07:37

5

The problem is, that the constructor of your OKButton class is not accessible for JavaFX. You have to either put your OKButton class in a separate file or use it this way:

public class Main {
    public static void main(String[] args) {
        Application.launch(OKButton.class, args);
    }

    public static class OKButton extends Application {

        @Override
        public void start(Stage stage) {
            Button btn = new Button("OK");
            Scene scene = new Scene(btn, 200, 250);
            stage.setScene(scene);
            stage.show();
        }
    }
}

answered Jul 30 '18 at 07:37

Halko Karr-Sajtarevic

2,248
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14

Thanks, Halko. This is an option. However, I wanted to have my main-class small and tidy and therefore keep the OKButton-class somewhere else. – Clyde Frog Jul 30 '18 at 07:50
then put your OKButton-class in a OKButton.java file in the same directory – Halko Karr-Sajtarevic Jul 30 '18 at 07:55
Same Exception, nothing changes. – Clyde Frog Jul 30 '18 at 08:05
did you add `public` before your class name? e.g. `public class OKButton extends Application {`? – Halko Karr-Sajtarevic Jul 30 '18 at 08:08
1

Here we go.... Thank you, indeed. – Clyde Frog Jul 30 '18 at 08:14
you're welcome! – Halko Karr-Sajtarevic Jul 30 '18 at 08:15

Unable to construct Application instance

1 Answers1