你应该只有一个scene，在你的例子中，你可以把你的hbox放到一个vbox中，而把vbox放到scene中。
下面是代码的修改版本：

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

public class SimpleCalculaterFX extends Application {

    @Override
    public void start(Stage stage) {

        stage.setTitle("simple Calculater");

        Label lb1 = new Label("Nuber1");
        Label lb2 = new Label("Nuber2");
        Label lb3 = new Label("Result");
        TextField tx1 = new TextField();
        TextField tx2 = new TextField();
        TextField tx3 = new TextField();

        HBox hbox = new HBox();
        hbox.getChildren().addAll(lb1, tx1, lb2, tx2, lb3, tx3);
        hbox.setAlignment(Pos.TOP_RIGHT);

        Button b1 = new Button("ADD");
        Button b2 = new Button("SUBTRACT");
        Button b3 = new Button("DIVIDE");
        Button b4 = new Button("MULTIPLY");

        HBox hbox2 = new HBox();
        hbox2.getChildren().addAll(b1, b2, b3, b4);
        hbox2.setAlignment(Pos.BOTTOM_CENTER);

        VBox vBox = new VBox();
        vBox.getChildren().addAll( hbox, hbox2);

        Scene scene = new Scene(vBox);
        stage.setScene(scene);
        stage.show();

        // Integer num1 = Integer.parseInt(tx1.getText());
        // nteger num2 = Integer.parseInt(tx2.getText());
        b1.setOnAction(e -> {

            Integer num1 = Integer.parseInt(tx1.getText());
            Integer num2 = Integer.parseInt(tx2.getText());
            Integer res;
            res = num1 + num2;
            tx3.setText(String.format("%d", res));
        });
        b2.setOnAction(e -> {

            Integer num1 = Integer.parseInt(tx1.getText());
            Integer num2 = Integer.parseInt(tx2.getText());
            Integer res;
            res = num1 - num2;
            tx3.setText(String.format("%d", res));

        });
        b3.setOnAction(e -> {

            Integer num1 = Integer.parseInt(tx1.getText());
            Integer num2 = Integer.parseInt(tx2.getText());
            Integer res;
            res = num1 / num2;
            tx3.setText(String.format("%d", res));

        });
        b4.setOnAction(e -> {

            Integer num1 = Integer.parseInt(tx1.getText());
            Integer num2 = Integer.parseInt(tx2.getText());
            Integer res;
            res = num1 * num2;
            tx3.setText(String.format("%d", res));

        });

    }

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

}

首先，你需要创建一个控制器类，并在其中声明你的按钮：

主控制器.java

public class MainController implements Initializable {
    
    @FXML
    private Button button1;
    
    @Override
    public void initialize(URL location, ResourceBundle resources) {
        System.out.println("The document loaded!!!");
        
        button1.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                System.out.println("button1 clicked!!");
            }
        });
        
    }

}

将控制器链接到FXML文件（fx:controller="ma.desktop.MainController"）：

主布局.fxml

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

<?import javafx.scene.control.Button?>
<?import javafx.scene.layout.Pane?>

<Pane fx:controller="ma.desktop.MainController" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="472.0" prefWidth="692.0" xmlns="http://javafx.com/javafx/15.0.1" xmlns:fx="http://javafx.com/fxml/1">
   <children>
      <Button fx:id="button1" layoutX="65.0" layoutY="14.0" mnemonicParsing="false" text="Button" textFill="#19d713" />
   </children>
</Pane>

最后，在应用程序类上加载FXML文件：

Java应用程序

public class App extends Application {

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

    @Override
    public void start(Stage primaryStage) throws Exception {

        Pane pane = FXMLLoader.load(getClass().getResource("MainLayout.fxml"));

        Scene scene = new Scene(pane);
        primaryStage.setScene(scene);
        primaryStage.show();

    }

}

注意：我使用场景生成器作为FXML文件的编辑器

快乐编码