控制器文件中的空指针

Question

我在使用scenebuilder 的javafx 项目中不断出现Nullpointer 异常。我通过“opgaveLbl”、“feedbackLbl”和“groepsbewerkingListView”得到空指针。此外，当我尝试创建一个新对象 groepsbewerkingenListView = new ListView<String>(); 时，我的列表视图没有填满我的项目。

控制器

public class OefeningToevoegenSchermController extends AnchorPane {
private DomeinController controller;
private Opgave opgave;
private Feedback feedback;
private List<Groepsbewerking> groepsbewerkingen;

@FXML
private TextField OefeningNaam;
@FXML
private TextField Antwoord;
@FXML
private Button UploadOefening;
@FXML
private Button UploadFeedback;
@FXML
private AnchorPane AnchorPane;
@FXML
private TextField operator;
@FXML
private TextField operand;
@FXML
private ListView<String> groepsbewerkingenListView;
@FXML
private Label opgaveLbl;
@FXML
private Label feedbackLbl;
@FXML
private ListView<String> OefGroepsbewerkingenListView;

public OefeningToevoegenSchermController(DomeinController controller) {
    this.controller = controller;
    groepsbewerkingen = new ArrayList<>();

    opgaveLbl.setVisible(false);
    feedbackLbl.setVisible(false);        

    System.out.println(controller.geefGroepsBewerkingen());

    groepsbewerkingenListView.setItems(controller.geefGroepsBewerkingen());

    FXMLLoader loader = new FXMLLoader(getClass().getResource("OefeningToevoegenScherm.fxml"));        

    loader.setRoot(this);
    loader.setController(this);

    try {
        loader.load();
    } catch (IOException ex) {
        throw new RuntimeException(ex);
    }

}

FXML 文件 该文件与我的控制器文件在同一个包中。

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

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.ListView?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>

<fx:root id="AnchorPane" prefHeight="518.0" prefWidth="604.0" type="AnchorPane" xmlns="http://javafx.com/javafx/9.0.1" xmlns:fx="http://javafx.com/fxml/1">
   <children>
      <Label alignment="CENTER" layoutX="223.0" layoutY="25.0" text="Nieuwe Oefening">
         <font>
            <Font size="20.0" />
         </font>
      </Label>
      <TextField id="OefeningNaam" fx:id="OefeningNaam" layoutX="140.0" layoutY="104.0" prefWidth="196.0" />
      <Label layoutX="140.0" layoutY="87.0" text="Naam (duidelijke omschrijving)" />
      <TextField id="Antwoord" fx:id="Antwoord" layoutX="140.0" layoutY="272.0" prefWidth="196.0" />
      <Label layoutX="140.0" layoutY="255.0" text="Antwoord" />
      <Button id="UploadOefening" fx:id="UploadOefening" layoutX="141.0" layoutY="159.0" mnemonicParsing="false" onAction="#UploadOefening" text="upload" />
      <Label layoutX="141.0" layoutY="142.0" text="PDF toevoegen met opgave" />
      <Button id="UploadFeedback" fx:id="UploadFeedback" layoutX="141.0" layoutY="213.0" mnemonicParsing="false" onAction="#UploadFeedback" text="upload" />
      <Label layoutX="141.0" layoutY="196.0" text="opgave" />
      <Label layoutX="143.0" layoutY="307.0" text="Groepsbewerkingen" />
      <ListView fx:id="OefGroepsbewerkingenListView" layoutX="140.0" layoutY="330.0" prefHeight="109.0" prefWidth="164.0" />
      <Button layoutX="35.0" layoutY="427.0" mnemonicParsing="false" onAction="#GroepsbewerkingToevoegen" prefHeight="25.0" prefWidth="89.0" text="Add" />
      <TextField fx:id="operator" layoutX="30.0" layoutY="330.0" prefHeight="25.0" prefWidth="89.0" />
      <Label layoutX="35.0" layoutY="307.0" text="Operator" />
      <TextField fx:id="operand" layoutX="30.0" layoutY="390.0" prefHeight="25.0" prefWidth="89.0" />
      <Label layoutX="35.0" layoutY="367.0" text="Operand" />
      <Button layoutX="237.0" layoutY="460.0" mnemonicParsing="false" onAction="#MaakOefening" prefHeight="25.0" prefWidth="89.0" text="Opslaan" />
      <Label fx:id="opgaveLbl" layoutX="143.0" layoutY="163.0" prefHeight="17.0" prefWidth="196.0" />
      <Label fx:id="feedbackLbl" layoutX="141.0" layoutY="217.0" prefHeight="17.0" prefWidth="203.0" />
      <ListView fx:id="groepsbewerkingenListView" layoutX="377.0" layoutY="330.0" prefHeight="109.0" prefWidth="164.0" />
   </children>
</fx:root>

Answer 1

任何@FXML-injected 字段在调用load() 期间被初始化；因此在加载 FXML 之前它们为空。只需在构造函数中重新排序代码即可：

public OefeningToevoegenSchermController(DomeinController controller) {
    this.controller = controller;
    groepsbewerkingen = new ArrayList<>();


    FXMLLoader loader = new FXMLLoader(getClass().getResource("OefeningToevoegenScherm.fxml"));        

    loader.setRoot(this);
    loader.setController(this);

    try {
        loader.load();
        opgaveLbl.setVisible(false);
        feedbackLbl.setVisible(false);        

        System.out.println(controller.geefGroepsBewerkingen());

        groepsbewerkingenListView.setItems(controller.geefGroepsBewerkingen());
    } catch (IOException ex) {
        throw new RuntimeException(ex);
    }

}