我不熟悉javafx，但也许您应该试试eventhandler/actionevent：

button_roll.setOnAction(new EventHandler<ActionEvent>() {
      @Override
      public void handle(ActionEvent event) {
        mylab.setText("Hello");
      }
    });

你的问题是你从不添加 Label 到 Scene . 将根节点作为 StackPane 将堆叠 Label 以及 Button 在对方身上。你需要替换 StackPane 与 VBox , HBox ，或更合适的 Node . 代码中的注解。

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

public class App extends Application
{
    @Override
    public void start(Stage stage) throws Exception
    {
        String helloString = "Hello";//String use in the lable once the button is pressed.
        Label lblHello = new Label();//Label that will show helloString once the button is pressed.

        Button btnRoll = new Button("Roll");//The button that will trigger the action to change the label from empty string to helloString.
        btnRoll.setOnAction((t) -> {//Set the button's action
            lblHello.setText(helloString);//set the label's text to helloString.
        });

        VBox vbLayoutRoot = new VBox(lblHello, btnRoll);//The root layout is a VBox. Add the label and the btn to the root layout.
        Scene scene = new Scene(vbLayoutRoot, 600, 600);//Add the root layout to the scene.
        stage.setScene(scene);//Set the scene.
        stage.setTitle("Greetings");  //creates title
        stage.show();//Show the stage.
    }

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

你应该喜欢this:-

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

@Override
public void start(Stage stage) throws Exception {
    String dd = "Hello";
    stage.setTitle("Greetings");  //creates title

    Button button_roll = new Button("Roll");

    StackPane layout1 = new StackPane();
    layout1.getChildren().add(button_roll);

    Scene scene1 = new Scene(layout1, 600, 600);
    stage.setScene(scene1);

    Label mylab = new Label();
    mylab.setText(dd);

    button_roll.setOnAction(e -> {
        layout1.getChildren().clear();
        layout1.getChildren().add(mylab);
    });

    stage.show();

}