简单的Javafx TreeView引发Nullpointer异常

Question

I've got the following classes:

Main:

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{

        BorderPane root = FXMLLoader.load(getClass().getResource("../view/PersonOverview.fxml"));

        AnchorPane view2 = FXMLLoader.load(getClass().getResource("../view/view2.fxml"));
        root.setLeft(view2);

        primaryStage.setScene(new Scene(root, 1000, 600));
        primaryStage.show();
    }

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

TreeController:

public class TreeController implements Initializable {

     //Set icon for folder
     Node folderIcon = new ImageView(new Image(this.getClass().getResourceAsStream("../icon/icon.jpg")));

     //Set root
     TreeItem<String> root; 

     @FXML TreeView<String> tree;

     //Set other Items
     private TreeItem<String> item1 = new TreeItem<String>("item1", folderIcon);
     private TreeItem<String> item2 = new TreeItem<String>("item2", folderIcon);
     private TreeItem<String> item3 = new TreeItem<String>("item3", folderIcon);
     private TreeItem<String> item4 = new TreeItem<String>("item4", folderIcon);
     private TreeItem<String> item5 = new TreeItem<String>("item5", folderIcon);

     //Add Children to root
     private void makeChildren() {
         root.getChildren().add(item1);
         root.getChildren().add(item2);
         root.getChildren().add(item3);
         root.getChildren().add(item4);
         root.getChildren().add(item5);
     }

    @Override
    public void initialize(URL location, ResourceBundle resources) {
        root = new TreeItem<String>("root", folderIcon);
        makeChildren();
        root.setExpanded(true);
        tree.setRoot(root);
    } 
}

And of course my view2 fxml file:

    <AnchorPane
        maxHeight="-Infinity" maxWidth="-Infinity"
        minHeight="-Infinity" minWidth="-Infinity"
        prefHeight="400.0" prefWidth="354.0"
        xmlns="http://javafx.com/javafx/8.0.40"
        xmlns:fx="http://javafx.com/fxml/1"
        fx:controller="control.TreeController">
      <children>
      <TreeView
        layoutX="69.0" layoutY="118.0" 
        prefHeight="400.0" prefWidth="354.0" 
        AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" 
        AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0" />
      </children>
   </AnchorPane>

Now the problem I have is that it will throw a Nullpointer Exception at tree.setRoot(root);

And a ConstructLoad Exception in my Main at:

AnchorPane view2 = FXMLLoader.load(getClass().getResource("../view/view2.fxml"));

I'm still learning this stuff but I was told that when using FXML, you don't need to initialize TreeViews using "new" as the @FXML annotation will already take care of this with tree.setRoot(root).

Sorry for such a noobish question but I've been googling for the past 2 hours and haven't gotten any wiser.

Answer 1

I'm still learning this stuff but I was told that when using FXML, you don't need to initialize TreeViews using "new" as the @FXML annotation will already take care of this with tree.setRoot(root).

You guessed right, but in order for JavaFX to inject your Treeview (= make the "new" for you) you need to declare something like:

<Treeview fx:id="tree" />

in view2.fxml.

With the fx:id attribute setted to the same name as your Treeview variable in Java code.