NullpointerException error while working with choiceBox and arrays

Question

I am trying to get two different choice box to work at the same time However I am getting a nullpointer error in the initialize method. which is kind of weird cuz I am trying to initialize what is in the choice box but the IDE is giving me a nullpointer exception.

this is my code

package application;
import java.net.URL;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.ResourceBundle;

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ChoiceBox;
import javafx.scene.control.Label;
import javafx.scene.control.ListView;
import javafx.scene.control.TextField;
import javafx.scene.layout.VBox;
import javafx.scene.Parent;
import javafx.stage.Stage;

public class SampleController implements Initializable{
    
    public static piece PieceClicked = new piece();
    ArrayList <Integer> possibleMoves = new ArrayList <Integer>();
    ListView<Integer> listView = new ListView<Integer>();
    
    private int enemyLocation;
    private boolean valid = true;
    
    @FXML 
    ChoiceBox<String> inputPieceType, inputPieceAllience;
    
    String[] pieces = {"CANNON","CHARIOT","ELAPHANT","GENERAL","GUARD","HORSES","SOLDIERS"};
    String[] allience = {"GREEN","RED"};
    
    @Override
    public void initialize(URL arg0, ResourceBundle arg1) {
        inputPieceType.getItems().addAll(pieces);
        inputPieceAllience.getItems().addAll(allience);
        
        inputPieceType.setOnAction(this::setPiece);
        inputPieceAllience.setOnAction(this::setAllience);
    }
    
    public void setPiece(ActionEvent event) {
        PieceClicked.setPieceType(inputPieceType.getValue());
    }
    
    public void setAllience(ActionEvent event) {
        PieceClicked.setPieceAllience(inputPieceAllience.getValue());
    }
}

it is also telling me that I have an error in my main class at Parent root = FXMLLoader.load(getClass().getResource("Sample.fxml"));

my main class is this

package application;
    
import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;


public class Main extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception{
            Parent root = FXMLLoader.load(getClass().getResource("Sample.fxml"));
            Scene scene = new Scene(root, 600, 400);
            
            primaryStage.setScene(scene);
            primaryStage.show();
    }
    
    public static void main(String[] args) {
        launch(args);
    }
}

and the error message was this

Exception in Application start method
java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:464)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:363)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at java.base/sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:1051)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:900)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(LauncherImpl.java:195)
    at java.base/java.lang.Thread.run(Thread.java:832)
Caused by: javafx.fxml.LoadException: 
/Users/owenpark/eclipse/java-2020-06/Eclipse.app/Contents/MacOS/computerIA/computerScienceIA/bin/application/Sample.fxml

    at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2707)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2677)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2548)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3323)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3280)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3249)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3222)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3199)
    at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:3192)
    at application.Main.start(Main.java:13)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
Caused by: java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
    at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:273)
    at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
    at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2673)
    ... 14 more
Caused by: java.lang.NullPointerException
    at application.SampleController.initialize(SampleController.java:62)
    ... 25 more
Exception running application application.Main

what could I do to help with this?

edit: my fxmlfile this is the scene with all the choice boxes.

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.ChoiceBox?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.ListView?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.image.Image?>
<?import javafx.scene.image.ImageView?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.text.Text?>

<AnchorPane prefHeight="403.0" prefWidth="839.0" xmlns="http://javafx.com/javafx/15.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.SampleController">
   <children>
      <AnchorPane layoutX="1.0" layoutY="2.0" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="837.0">
         <children>
            <ImageView fitHeight="310.0" fitWidth="312.0" layoutX="14.0" layoutY="45.0">
               <image>
                  <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/thumb-data_illust_Jang_Gi_Pan_1440x1440.jpeg" />
               </image>
            </ImageView>
            <VBox layoutX="443.0" layoutY="43.0" prefHeight="315.0" prefWidth="146.0">
               <children>
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="location: " />
                  <TextField fx:id="input" />
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="piece: " />
                  <ChoiceBox fx:id="inputPieceType" prefWidth="150.0" />
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="Allience: " />
                  <ChoiceBox fx:id="inputPieceAllience" prefWidth="150.0" />
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="enemy piece location: " />
                  <TextField fx:id="inputEnemyPieceLocation" prefHeight="32.0" prefWidth="146.0" />
               </children>
            </VBox>
            <Button layoutX="332.0" layoutY="52.0" mnemonicParsing="false" prefHeight="27.0" prefWidth="33.0">
               <graphic>
                  <ImageView fitHeight="22.0" fitWidth="27.0" pickOnBounds="true" preserveRatio="true">
                     <image>
                        <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/Green_King.png" />
                     </image>
                  </ImageView>
               </graphic>
            </Button>
            <Button layoutX="332.0" layoutY="91.0" mnemonicParsing="false" prefHeight="27.0" prefWidth="33.0">
               <graphic>
                  <ImageView fitHeight="22.0" fitWidth="26.0" pickOnBounds="true" preserveRatio="true">
                     <image>
                        <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/Green_Cha.png" />
                     </image>
                  </ImageView>
               </graphic>
            </Button>
            <Button layoutX="332.0" layoutY="133.0" mnemonicParsing="false" prefHeight="27.0" prefWidth="33.0">
               <graphic>
                  <ImageView fitHeight="21.0" fitWidth="25.0" pickOnBounds="true" preserveRatio="true">
                     <image>
                        <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/Green_Cannon.png" />
                     </image>
                  </ImageView>
               </graphic>
            </Button>
            <Button layoutX="332.0" layoutY="171.0" mnemonicParsing="false" prefHeight="32.0" prefWidth="42.0">
               <graphic>
                  <ImageView fitHeight="23.0" fitWidth="24.0" pickOnBounds="true" preserveRatio="true">
                     <image>
                        <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/green_Elephant.jpeg" />
                     </image>
                  </ImageView>
               </graphic>
            </Button>
            <Button layoutX="332.0" layoutY="214.0" mnemonicParsing="false" prefHeight="31.0" prefWidth="42.0">
               <graphic>
                  <ImageView fitHeight="20.0" fitWidth="37.0" pickOnBounds="true" preserveRatio="true">
                     <image>
                        <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/Green_Horse.png" />
                     </image>
                  </ImageView>
               </graphic>
            </Button>
            <Button layoutX="332.0" layoutY="255.0" mnemonicParsing="false" prefHeight="33.0" prefWidth="42.0">
               <graphic>
                  <ImageView fitHeight="20.0" fitWidth="67.0" pickOnBounds="true" preserveRatio="true">
                     <image>
                        <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/Green_Soldier.png" />
                     </image>
                  </ImageView>
               </graphic>
            </Button>
            <Button layoutX="332.0" layoutY="296.0" mnemonicParsing="false" prefHeight="33.0" prefWidth="42.0">
               <graphic>
                  <ImageView fitHeight="20.0" fitWidth="33.0" pickOnBounds="true" preserveRatio="true">
                     <image>
                        <Image url="@../../../../../../../../../Documents/computer%20science%20IA%20photos/Green_Guard.png" />
                     </image>
                  </ImageView>
               </graphic>
            </Button>
            <Text layoutX="39.0" layoutY="56.0" strokeType="OUTSIDE" strokeWidth="0.0" text="1        2          3         4          5         6        7          8          9" wrappingWidth="270.7308011274751" />
            <Text layoutX="29.0" layoutY="72.0" strokeType="OUTSIDE" strokeWidth="0.0" text="1" wrappingWidth="8.67726926690625" />
            <Text layoutX="29.0" layoutY="98.0" strokeType="OUTSIDE" strokeWidth="0.0" text="2" wrappingWidth="8.67726926690625" />
            <Text layoutX="29.0" layoutY="129.0" strokeType="OUTSIDE" strokeWidth="0.0" text="3" wrappingWidth="8.67726926690625" />
            <Text layoutX="27.0" layoutY="157.0" strokeType="OUTSIDE" strokeWidth="0.0" text="4" wrappingWidth="8.67726926690625" />
            <Text layoutX="27.0" layoutY="189.0" strokeType="OUTSIDE" strokeWidth="0.0" text="5" wrappingWidth="8.67726926690625" />
            <Text layoutX="29.0" layoutY="218.0" strokeType="OUTSIDE" strokeWidth="0.0" text="6" wrappingWidth="8.67726926690625" />
            <Text layoutX="29.0" layoutY="248.0" strokeType="OUTSIDE" strokeWidth="0.0" text="7" wrappingWidth="8.67726926690625" />
            <Text layoutX="29.0" layoutY="279.0" strokeType="OUTSIDE" strokeWidth="0.0" text="8" wrappingWidth="8.67726926690625" />
            <Text layoutX="29.0" layoutY="309.0" strokeType="OUTSIDE" strokeWidth="0.0" text="9" wrappingWidth="8.67726926690625" />
            <Text layoutX="22.0" layoutY="335.0" strokeType="OUTSIDE" strokeWidth="0.0" text="10" wrappingWidth="20.65190019321369" />
            <Button fx:id="enterButton" layoutX="452.0" layoutY="359.0" mnemonicParsing="false" onAction="#enterButton" text="Input" />
            <Button fx:id="clear" layoutX="516.0" layoutY="359.0" mnemonicParsing="false" onAction="#clear" text="clear" />
            <Text layoutX="380.0" layoutY="72.0" strokeType="OUTSIDE" strokeWidth="0.0" text="GENERAL" />
            <Text layoutX="381.0" layoutY="110.0" strokeType="OUTSIDE" strokeWidth="0.0" text="CHARIOT" />
            <Text layoutX="382.0" layoutY="153.0" strokeType="OUTSIDE" strokeWidth="0.0" text="CANNON" />
            <Text layoutX="379.0" layoutY="189.0" strokeType="OUTSIDE" strokeWidth="0.0" text="ELEPHANT" />
            <Text layoutX="382.0" layoutY="234.0" strokeType="OUTSIDE" strokeWidth="0.0" text="HORSE" />
            <Text layoutX="382.0" layoutY="276.0" strokeType="OUTSIDE" strokeWidth="0.0" text="SOLDIER" />
            <Text layoutX="386.0" layoutY="317.0" strokeType="OUTSIDE" strokeWidth="0.0" text="GUARD" />
            <VBox layoutX="589.0" layoutY="42.0" prefHeight="317.0" prefWidth="126.0">
               <children>
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="location:" />
                  <Label fx:id="locationCheck" prefHeight="25.0" prefWidth="126.0" />
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="piece Type: " />
                  <Label fx:id="pieceCheck" prefHeight="36.0" prefWidth="126.0" />
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="Allience: " />
                  <Label fx:id="allianceCheck" prefHeight="31.0" prefWidth="128.0" />
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="enemy piece location:" />
                  <Label fx:id="enemyLocationCheck" prefHeight="33.0" prefWidth="127.0" />
                  <Text strokeType="OUTSIDE" strokeWidth="0.0" text="error message:" wrappingWidth="123.269287109375" />
                  <Label fx:id="errorMessage" prefHeight="35.0" prefWidth="129.0" />
               </children>
            </VBox>
            <Text layoutX="607.0" layoutY="28.0" strokeType="OUTSIDE" strokeWidth="0.0" text="validity check" />
            <Text layoutX="723.0" layoutY="27.0" strokeType="OUTSIDE" strokeWidth="0.0" text="Possible Possition" />
            <Text layoutX="710.0" layoutY="301.0" strokeType="OUTSIDE" strokeWidth="0.0" text="can this piece be taken" />
            <Label fx:id="PieceTake" layoutX="708.0" layoutY="305.0" prefHeight="23.0" prefWidth="126.0" />
         </children>
      </AnchorPane>
      <ListView fx:id="listView" layoutX="715.0" layoutY="40.0" prefHeight="248.0" prefWidth="113.0" />
      <Button fx:id="exitButton" layoutX="779.0" layoutY="361.0" mnemonicParsing="false" onAction="#exitButton" prefHeight="27.0" prefWidth="47.0" text="exit" />
   </children>
</AnchorPane>

sample.fxml file is a different scene and basically it is just a starting scene where the user can choose between two different scenes to go to

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.text.Font?>
<?import javafx.scene.text.Text?>

<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/15.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.SampleController">
   <children>
      <Text layoutX="143.0" layoutY="53.0" strokeType="OUTSIDE" strokeWidth="0.0" text="welcome to janggi!">
         <font>
            <Font size="40.0" />
         </font>
      </Text>
      <Button fx:id="gameButton" layoutX="205.0" layoutY="85.0" mnemonicParsing="false" onAction="#gameButton" prefHeight="74.0" prefWidth="190.0" text="Game Play">
         <font>
            <Font size="22.0" />
         </font>
      </Button>
      <Button fx:id="explanationButton" layoutX="205.0" layoutY="225.0" mnemonicParsing="false" onAction="#explanationButton" prefHeight="74.0" prefWidth="190.0" text="Explanation">
         <font>
            <Font size="22.0" />
         </font>
      </Button>
   </children>
</AnchorPane>

Answer 1

Edit: The actual problem was that SampleController was linked to two different .fxml files and only the Sample.fxml file was being loaded, which didn't contain the ChoiceBoxes. This can be fixed by having two separate controllers, one for each .fxml file, and loading them both.

Old answer: In JavaFX controllers, fields marked with the @FXML tag have their values injected by the JavaFX-runtime from the controls defined in the .fxml file associated with the controller.

In your case, you've got two ChoiceBox<String> instances in the controller, so you'll need two ChoiceBoxes defined in your .fxml-file as well. To allow the runtime to know which control to link to which field in the controller, you'll need to add an fx:id="fieldName" on each of the controls in the .fxml file. Don't forget to change the value from fieldName to the corresponding name of the field in the controller, in your case inputPieceType on one of them and inputPieceAllience on the other.

The .fxml file would then look something like this:

<ChoiceBox fx:id="inputPieceType" [other properties...] />

If you are unsure if the controller class is connected to your .fxml file, one way you can see this is by looking for the fx:controller="com.example.MyController" property inside of your .fxml file. The com.example.MyController will be the full name of the controller class, which in your case should be application.SampleController.

Note that you can also set the controller in code instead of in .fxml by calling fxmlLoader.setController(controller); when loading the .fxml file. Which one you use depends on your use case.

Answer 2

1. Make sure your .fxml file which you are loading in main method is on correct path (classpath).

2. If you verified, then please make sure variable names you declared in controller with @FXML annotation @FXML ChoiceBox inputPieceType, inputPieceAllience; are same or no typo in .fxml file by comparing it. If it does not match then javafx will unable to autowire ChoiceBox class in Controller and resulting null pointer exception.

3. Just a suggestion whenever you make changes(UI Components) using Scene Builder please refresh your .fxml files since eclipse sometimes take time to refresh it.

Answer 3

The issue is that you never say new ChoiceBox<String>, so when you reference them with inputPieceType.getItems().addAll(pieces); or inputPieceAllience.getItems().addAll(allience); on line 62 they are calling methods for an item that was never created.

You could do something like this to initialize the objects:

...
    private int enemyLocation;
    private boolean valid = true;
    
    @FXML 
    ChoiceBox<String> inputPieceType = new ChoiceBox<String>();
    @FXML
    ChoiceBox<String> inputPieceAllience = new ChoiceBox<String>();
    
    String[] pieces = {"CANNON","CHARIOT","ELAPHANT","GENERAL","GUARD","HORSES","SOLDIERS"};
    String[] allience = {"GREEN","RED"};
...