JavaFX ChoiceBox NullPointerException无论我做什么

Question

我正在尝试向我用SceneBuilder创建的FXML文件中存在的ChoiceBox添加值，但当我尝试这样做时（在main类中），无论我尝试什么，我都会得到一个NullPointerException。请注意，当我注释掉initialize()方法中的ChoiceBox代码（参见下面）时，项目成功运行，因此NPE没有其他原因。

null

<ChoiceBox fx:id="itemKeuze" layoutX="132.0" layoutY="33.0" prefWidth="150.0" />

public class HoofdScherm extends Application {
   @FXML
   private ChoiceBox<String> itemKeuze;

   @Override
   public void start(Stage scherm) throws IOException
   {
      // The setup and loading of the FXML file...
   }

   @FXML
   private void initialize()
   {
      // Now I'm only trying to disable the combobox instead of setting the values but the NPE is also thrown
      itemKeuze.setDisable(true);
   }

private void toonItemScherm()
{
    Parent itemScherm = null;

    try {
        itemScherm = FXMLLoader.load(getClass().getResource("ItemScherm.fxml"));
    } catch (Exception e) {
        System.out.println("Kon itemScherm niet laden: " + e.getMessage());
    }

    //itemKeuze.setItems(FXCollections.observableArrayList("CD", "Film", "Spel"));
    //itemKeuze.setDisable(true);

    stage = new Stage();
  stage.setTitle("Voeg item toe");
  stage.setResizable(false);
  stage.setAlwaysOnTop(true);
  stage.setScene(new Scene(itemScherm));  
  stage.show();
}
}

堆栈跟踪

Exception in Application start method
Exception in thread "main" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$155(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: javafx.fxml.LoadException: 
/C:/Users/Isaak/Documents/workspace/JavaPracticumOpdracht3/bin/view/HoofdScherm.fxml

    at javafx.fxml.FXMLLoader.constructLoadException(Unknown Source)
    at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
    at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
    at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
    at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
    at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
    at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
    at javafx.fxml.FXMLLoader.loadImpl(Unknown Source)
    at javafx.fxml.FXMLLoader.load(Unknown Source)
    at view.HoofdScherm.start(HoofdScherm.java:38)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$null$173(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(Unknown Source)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(Unknown Source)
    ... 1 more
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.Trampoline.invoke(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.MethodUtil.invoke(Unknown Source)
    ... 18 more
Caused by: java.lang.NullPointerException
    at view.HoofdScherm.initialize(HoofdScherm.java:109)
    ... 27 more

Answer 1

我找到了答案，部分归功于James_D提出的观点。

我没有将控制器添加到第二个FXML文件fx:controller=“”中的AnchorPane。因为没有设置控制器，所以不能注入变量(fx:id->class variables）。我还为每个FXML文件创建了一个单独的控制器，因为所有内容都在扩展Apllication类的一个主类中。这也可能是问题的一部分。