error when I use ArrayList in fxml - javafx-8

Question

<children>
<fx:define>
<ArrayList fx:id="buttons">
<Button fx:id="btn1" layoutX="93.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn2" layoutX="187.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn3" layoutX="255.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn6" layoutX="505.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn4" layoutX="351.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn5" layoutX="436.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
</ArrayList>
</fx:define>
I get error in each button and I even can't open fxml in sceneBuilder. What is wrong with thisenter image description here
Full code of fxml below
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.geometry.Insets?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import java.util.ArrayList?>
<AnchorPane fx:controller="sample.Controller" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
<children>
<fx:define>
<ArrayList fx:id="buttons">
<Button fx:id="btn1" layoutX="93.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn2" layoutX="187.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn3" layoutX="255.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn6" layoutX="505.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn4" layoutX="351.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
<Button fx:id="btn5" layoutX="436.0" layoutY="31.0" mnemonicParsing="false" text="Button" />
</ArrayList>
</fx:define>
</children>
</AnchorPane>
I want to store all buttons in one array in controller

Answer 1

I don't think it's possible to add children in a List, especially standard ArrayList. FX components use ObservableList to store children components.
I suggest you add your buttons to a group, like this:
<AnchorPane>
<Group fx:id="group">
<Button fx:id="btn1" layoutX="93.0" layoutY="31.0" mnemonicParsing="false" text="Button"/>
<Button fx:id="btn2" layoutX="187.0" layoutY="31.0" mnemonicParsing="false" text="Button"/>
<Button fx:id="btn3" layoutX="255.0" layoutY="31.0" mnemonicParsing="false" text="Button"/>
<Button fx:id="btn6" layoutX="505.0" layoutY="31.0" mnemonicParsing="false" text="Button"/>
<Button fx:id="btn4" layoutX="351.0" layoutY="31.0" mnemonicParsing="false" text="Button"/>
<Button fx:id="btn5" layoutX="436.0" layoutY="31.0" mnemonicParsing="false" text="Button"/>
</Group>
</AnchorPane>
Your buttons will be visible in your app and you can do operations on group children almost like it was a standard list.