java - ComboBox 占位符 NullPointerException JavaFX

Question

我使用的是 ComboBox，我添加了一些字符串，还向其中添加了一个占位符。 我的目标是让用户从 ComboBox 中选择一些东西，但是当我尝试获取 ComboBox 的值并且我没有选择任何东西时，它会给我一个 NullPointerException，因为所选的 Item 是一个占位符。

如何检测所选项目是 PlaceHolder？

Controller :

package sample;
public class Controller implements Initializable {
    public ComboBox Cb_tipusDocument;
    @Override
    public void initialize(URL location, ResourceBundle resources) {
        ObservableList<String> ol_str_llistatPerSelect = FXCollections.observableArrayList("A", "B", "C");
        Cb_tipusDocument.setItems(ol_str_llistatPerSelect);
    }
    public void executar(ActionEvent actionEvent) {
        if ( Cb_tipusDocument.getValue().equals("")) {        
            //...
        }else {
            //...
        }
    }

示例.fxml:

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.ComboBox?>
<AnchorPane fx:id="Ap_principal" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="500.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8.0.172-ea" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
   <children>
      <Pane prefHeight="500.0" prefWidth="600.0">
         <children>
 <ComboBox fx:id="Cb_tipusDocument" layoutX="94.0" layoutY="131.0" prefHeight="25.0" prefWidth="406.0" promptText="Seleccioneu una opció" />
 </children>
      </Pane>
      <ToolBar layoutY="440.0" prefHeight="60.0" prefWidth="600.0">
         <items>
            <Pane prefHeight="0.0" prefWidth="200.0" />
            <Button mnemonicParsing="false" onAction="#executar" prefHeight="33.0" prefWidth="180.0" text="Generar Config" />
         </items>
      </ToolBar>
   </children>
</AnchorPane>

主要

package sample;

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("Trying new things");
        primaryStage.setScene(new Scene(root, 600, 500));
        primaryStage.show();
    }


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

Answer 1

添加一个简单的空检查就可以解决:Cb_tipusDocument.getValue() == null

package sample;
public class Controller implements Initializable {
public ComboBox Cb_tipusDocument;
@Override
public void initialize(URL location, ResourceBundle resources) {
    ObservableList<String> ol_str_llistatPerSelect = 
    FXCollections.observableArrayList("A", "B", "C");
    Cb_tipusDocument.setItems(ol_str_llistatPerSelect);
}
public void executar(ActionEvent actionEvent) {
    if ( Cb_tipusDocument.getValue() == null) {        
        //...
    }else {
        //...
    }
}