JavaFX :选择空白项后再次显示提示文本

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

public class JavaFXExample extends Application {

    @Override
    public void start(Stage primaryStage) {
        TextField textField = new TextField();
        textField.setPromptText("请输入内容");

        VBox root = new VBox(textField);
        Scene scene = new Scene(root, 300, 200);

        primaryStage.setTitle("JavaFX Example");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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