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(); }

Edit your question and include the entire stack trace of the NullPointerException, and include the code which generated that exception. — VGR
– VGR, Commented Jun 27, 2018 at 14:37
You can first get the root SplitPane. Then get its children. Then get the AnchorPane with with children.size > 0. Then get the Gridpane. Then get all of its Pane. Finally get the Pane's child. I would not recommend this approach. You should assign each node a fx:id. — SedJ601
– SedJ601, Commented Jun 27, 2018 at 14:57
@Sedrick I thought of this approach of getting childrens, but couldn't find getChildren() method for SplitPane. Also I've assigned fx:id to each element, to each SplitPane, AnchorPane, GridPane, Pane and Label. Still getting the nullPointerException. — Amol.Shaligram
– Amol.Shaligram, Commented Jun 27, 2018 at 15:16

Blake Ordway · Accepted Answer · 2018-06-27 16:45:17Z

4

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

edited Jun 27, 2018 at 16:45

answered Jun 27, 2018 at 14:52

Blake Ordway

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9 Comments

Amol.Shaligram Over a year ago

Yes, I'm creating an FXML file. Also I've your second point implemented.but still it's throwing nullPointerException. Can you please check the code I've updated and response back ? Thanks !!

Blake Ordway Over a year ago

I'm not sure how you're initializing your class, but if you've linked the class to the FXML file, you probably don't need to have a constructor--you may need one if you have to pass params, but that's a different discussion. Try swapping your constructor to another method: @FXML public void initialize(){ ... }

Amol.Shaligram Over a year ago

@Override public void initialize(URL arg0, ResourceBundle arg1) { } What should i pass here for arg0 and arg1 ? and from where ? this is auto generated method.

Blake Ordway Over a year ago

Use the one I just commented, not that one. The initialize() function is called by FXML after the FXML file is loaded and all the variables are created. You should also post how you're generating this controller class in your code, because that is probably one reason why you get a null pointer.

Amol.Shaligram Over a year ago

I'll surely try that sometime later. Thanks for that. But for now i fugured out thta exception occured at the line inside the for loop where I'm fetching the label from hashmap and then calling the setText() method. So when I do z1.setText() it works, but when I do ` zoneLabelNames.get(i).setText();`, throws a nullPointerException. Can you please tell proper way to access it with hashmap ? I can't access them individually because there are too many labels (around 20-25).

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