import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextField;
import javafx.scene.layout.VBox;

public class App extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        TextField numInput = new TextField();
        Button doubleButton = new Button("Dupla");
        doubleButton.setOnAction(e -> {
            String numStr = numInput.getText();
            int num = Integer.parseInt(numStr);
            Integer result = num * 2;
            numInput.setText(result.toString());
        });

        VBox vbox = new VBox();
        vbox.setPadding(new Insets(10, 10, 10, 10));
        vbox.getChildren().addAll(numInput, doubleButton);

        Scene scene = new Scene(vbox, 300, 250);
        primaryStage.setScene(scene);

        primaryStage.show();
    }

}