B4J Question Questions about dynamically create Nodes

Sub Process_Globals
   Private fx As JFX
   Private MainForm As Form
End Sub

Sub AppStart (Form1 As Form, Args() As String)
   MainForm = Form1
   MainForm.Show
   CreateCheckboxes(MainForm.RootPane, 5, 6)
End Sub

Sub CreateCheckboxes(Parent As AnchorPane, NumX As Int, NumY As Int) As CheckBox(,)
   Dim width As Int = Parent.width / NumX
   Dim height As Int = Parent.height / NumY
   Dim cb(NumX, NumY) As CheckBox
   For x = 0 To NumX - 1
     For y = 0 To NumY - 2
       Dim chk As CheckBox
       chk.Initialize("chk")
       Parent.AddNode(chk, 20 + x * width, y * height, width - 5, height - 5)
       chk.Text = "Checkbox (" & x & "," & y & ")"
       cb(x, y) = chk
     Next
   Next
   Return cb
End Sub

Sub chk_CheckedChange(Checked As Boolean)
   Dim chk As CheckBox = Sender
   Log(chk.Text & ": " & Checked)
End Sub