getting the buttons in a gridPane and change their text using javaFx

Question

Im trying to get all members of a gridPane (they are button) in my controller class , but i'm getting exception that you can see it below:

Exception in Application start methodjava.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767) 
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$152(LauncherImpl.java:182)
    at com.sun.javafx.application.LauncherImpl$$Lambda$51/1263764.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:745)
Caused by: javafx.fxml.LoadException: 
file:/C:/Users/Family/Documents/NetBeansProjects/FarsiCallender/dist/run1583526066/FarsiCallender.jar!/farsicallender/FXMLDocument.fxml
at javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2605)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2583)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2445)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3218)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3179)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3152)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3128)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3108)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:3101)
at farsicallender.FarsiCallender.start(FarsiCallender.java:29)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$159(LauncherImpl.java:863)
at com.sun.javafx.application.LauncherImpl$$Lambda$54/4239053.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$172(PlatformImpl.java:326)
at com.sun.javafx.application.PlatformImpl$$Lambda$47/20085625.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$null$170(PlatformImpl.java:295)
at com.sun.javafx.application.PlatformImpl$$Lambda$49/3796086.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$171(PlatformImpl.java:294)
at com.sun.javafx.application.PlatformImpl$$Lambda$48/2900468.run(Unknown Source)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$145(WinApplication.java:101)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/17230114.run(Unknown Source)
... 1 more
Caused by: java.lang.NullPointerException
    at farsicallender.FXMLDocumentController.setDays(FXMLDocumentController.java:249)
    at farsicallender.FXMLDocumentController.initialize(FXMLDocumentController.java:181)
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2552)
    ... 22 more
Exception running application farsicallender.FarsiCallender
Java Result: 1

this exception comes from this part of my code , before checking my code you have to know that i have a gridPane (6*7) and this is my code:

for (int i = 0; i < 6; i++) {
     for (int j = 0; j < 7; j++) {
         for (Node node : grid.getChildren()) {
             if (grid.getRowIndex(node) == i && grid.getColumnIndex(node) == j) {
                  Button b = (Button)node;
                  b.setText(ar[i][j]+"");
                  System.out.println("z:" + i+" "+j);
                  break;
             }
         }
     }
}

line 249 is :

if (grid.getRowIndex(node) == i && grid.getColumnIndex(node) == j) {

Answer 1

As an aside, you should not call static methods from a non-static context. Your IDE should warn you about this. But you should use

GridPane.getRowIndex(node)

not

grid.getRowIndex(node)

etc.

---

The method GridPane.getRowIndex(Node) and GridPane.getColumnIndex(node) return a wrapper Integer object, not a primitive int. Consequently when you compare by identity using == you are implicitly using unboxing. In other words, your code is equivalent to

if (GridPane.getRowIndex(node).intValue() == i 
  && GridPane.getColumnIndex(node).intValue() == j) { ... }

If there are any nodes added to your grid pane without a row index and column index specified, then GridPane.getRowIndex(...) or GridPane.getColumnIndex(...) will return null, so you would implicitly attempt to call intValue() on a null reference, resulting in a null pointer exception. Note that if you have called grid.setGridLinesVisible(true);, this causes the GridPane to add a node that has no row index or column index set.

So the quick fix for this is to do a null check:

Integer rowIndex = GridPane.getRowIndex(node);
Integer columnIndex = GridPane.getColumnIndex(node);

if (rowIndex != null && rowIndex.intValue() == i
  && columnIndex != null && columnIndex.intValue() == j) {

    // ...
}

I would probably prefer to organize the code so the buttons are kept in an array, so you can access them without all that iteration, but I guess that is just a matter of style.