import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.VBox;
import javafx.scene.control.Button;
import javafx.event.EventHandler;
import javafx.event.ActionEvent;

public class HeiVerden extends Application {
    Button knapp = null;
    @Override
    public void start(Stage stage) {

        knapp = new Button("Si hei!");
        knapp.setPrefSize(100, 100);
        Knappebehandler behandler = new Knappebehandler();
        knapp.setOnAction(behandler);

        VBox box = new VBox(knapp);
        stage.setScene(new Scene(box));
        stage.show();
    }
    private class Knappebehandler implements EventHandler<ActionEvent> {
        @Override
        public void handle(ActionEvent e) {
            knapp.setText("Hei verden!");
        }
    }
}