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

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

    @Override
    public void start(Stage stage) {
        Button button = new Button("Mehet");
        button.setOnAction(e -> onButtonClick());
        
        StackPane root = new StackPane();
        root.getChildren().add(button);
        Scene scene = new Scene(root, 300, 250);
        stage.setScene(scene);
        stage.setTitle("Valami");
        stage.show();
    }

    private void onButtonClick() {
        System.out.println("Kattintás volt");
    }

}