How to access children of javafx elements?

Question

I came across some documents on internet that in javafx you can apply controller to only parent element.

I have a javafx application (built with scenebuilder, eclipse) which has basic structure like this :

SplitPane
{
     AnchorPane
     {

     }
     AnchorPane
     {
          GridPane
          {
               Pane
               {
                     Label
                     {

                     }
               }
               Pane
               {
                     Label
                     {

                     }
               }
          }
     }
}

I want to change the values of those labels at runtime. But it is throwing java.lang.NullPointerException. My assumption why this is hapening is I'm applying controller to split pane and the labels are not direct children of it so I can't access them.

So questions are : 1) Is my assumption correct ? If not where I'm wrong or missing something ?

2) How to access the labels ?

3) Can I use controller for inner elements (not parent) ?

Thanks in advance, and sorry if the question doesn't make any sense, I'm very very new to javafx.

UPDATE : HERE'S THE CODE i'M USING AND EXCEPTION STACK TRACE :

 public class Controller implements Initializable 
    {
        @FXML
        private SplitPane splitPane;

        @FXML
        private AnchorPane anchorPane1;

        @FXML
        private AnchorPane anchorPane2;

        @FXML
        private GridPane gridPane;

        @FXML
        private static Label z1;
        @FXML
        private static Label z2;
        private HashMap<Integer,Label> zoneLabelNames = new HashMap<Integer,Label>();
        public Controller()
        {
        // I have around 20 such labels which's value i'm setting using a loop. For now I've writter two only.
        zoneLabelNames.put(0, z1);
        zoneLabelNames.put(1, z2);
         new Thread(() -> {
                while(true)
                {
                    try
                    {
                        //some logic to generate new values which will update labels
                        Platform.runLater(() -> {
                            //here I'm actually setting values in loop, for now I wrote only two labels.
                            for(int i =0;i<2;i++)
                            {
                                zoneLabelNames.get(i).setText("newly generated value"); // nullPointerException at this line
                            }
                        });
                    }
                    catch(Exception e)
                    {
                        e.printStackTrace();
                    }
                }
            }).start();
        }

Answer 1

To answer your questions:

1) You can access them if you're creating an FXML file. See point 2.

2) If you are wanting to access the Labels at runtime, you can put an attribute on them in the FXML file you're loading like so: fx:id="_label1". Then in your controller class, you are able to create a field for that Label like so: @FXML private Label _label1;

3) You cannot add a controller to another element other than the root element of the FXML file. If you would like to have a controller on a specific element of the FXML, you have to use an <fx:include source="..."/> tag. See documentation

I highly recommend that you read through a tutorial on FXML and JavaFX such as the one from Oracle: https://docs.oracle.com/javafx/2/get_started/fxml_tutorial.htm