JavaFX 버튼 및 레이블

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root, 300, 275));
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

package sample;

import javafx.scene.control.Button;
import javafx.scene.control.Label;

public class Controller {
    public Button btn01;
    public Label lb01;

    public void onButtonClicked() {
        String string;
        string = "Button clicked";

        lb01.setText(string);

    }
}

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.GridPane?>

<GridPane alignment="center" hgap="10" vgap="10" xmlns="http://javafx.com/javafx/8.0.172-ea" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
    <Button fx:id="btn01" mnemonicParsing="false" onAction="#onButtonClicked" text="Button" GridPane.columnIndex="1" GridPane.rowIndex="1" />
   <Label fx:id="lb01" prefHeight="17.0" prefWidth="86.0" text="Label" />

</GridPane>