Null Pointer Exception when closing a JavaFX Dialog

Question

I have a simple program that starts with a basic stage. I then select a menu item that launches a dialog box. I have a button on the dialog box that does nothing but close the current stage. I was expecting that the dialog would close, and the focus would return to the initial stage. However, I get a null pointer exception when I click on the Create Game button. I am fairly new to Java and am probably missing something obvious. Please help.

HWFX.JAVA

package Main;

import java.util.Date;

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.fxml.FXMLLoader;
import javafx.scene.Group;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.TableView;
import javafx.scene.control.TextField;
import javafx.scene.layout.GridPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class HWFX extends Application {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
    launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        // TODO Auto-generated method stub
    FXMLLoader loader = new FXMLLoader(getClass().getResource("/fxml/Main.fxml"));
    Parent root = loader.load();
    MainController controller = (MainController) loader.getController();
    controller.init(primaryStage);

    Scene scene = new Scene(root,600,400);
    scene.getStylesheets().add("/fxml/styles/mainfx.css");

    primaryStage.setScene(scene);
    primaryStage.setTitle("Hello World FX");
    primaryStage.show();

    }

MainController.java

package Main;

import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;

import javafx.application.Platform;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.Parent;
import javafx.scene.control.Button;
import javafx.scene.control.TextField;
import javafx.stage.FileChooser;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

import java.sql.*;

import javax.sql.*;

import oracle.jdbc.*;
import oracle.jdbc.pool.OracleDataSource;

public class MainController implements Initializable {

    private Stage stage;
    @FXML
    javafx.scene.layout.AnchorPane AnchorPane;
    @FXML
    javafx.scene.layout.GridPane GridPane;
    @FXML
    javafx.scene.layout.GridPane GridPaneMain;
    @FXML
    Button TestButton1;
    Button TestButton2;
    Button TestButton3;
    @FXML
    javafx.scene.control.TextField PlayerName;


    @Override
    public void initialize(URL arg0, ResourceBundle arg1) {
        // TODO Auto-generated method stub

    }

    public void doExit() {
        Platform.exit();
    }

    public void openFile() {
        FileChooser fileChooser = new FileChooser();
        fileChooser.setTitle("Open text file");     
        fileChooser.setInitialDirectory(new File(System.getProperty("user.home")));
        fileChooser.getExtensionFilters().addAll(
                new FileChooser.ExtensionFilter("Text Files","*.txt"),
                new FileChooser.ExtensionFilter("All Files","*.*")
                );
        File file = fileChooser.showOpenDialog(stage);

        if (file != null){
            System.out.println("Choosen File: " + file);
        }
    }

    public void ButtonClicked() throws SQLException {
        System.out.println("Click!");
        final TextField F1 = new TextField();
        F1.setPromptText("F1");
        F1.setPrefColumnCount(10);
        F1.getText();
//      javafx.scene.layout.GridPane.setConstraints(F1,10,10);
        GridPaneMain.add(F1, 0, 0);

        OracleDataSource ods = new OracleDataSource();

        // Set the user name, password, driver type and network protocol
        ods.setUser("ppmcdev");
        ods.setPassword("ppmc");
        ods.setURL("jdbc:oracle:thin:@172.21.51.56:1521:INAVD");
        ods.setDriverType("oci8");
        ods.setNetworkProtocol("ipc");
        ods.setTNSEntryName("INAVD");

        // Retrieve a connection
        Connection conn = ods.getConnection();
        getUserName(conn);
        // Close the connection
        conn.close();
        conn = null;
    }

      public void CreateGame(ActionEvent event) throws IOException {
          System.out.println("Create Game Menu");
          CreateGameDialogController CreateGameDialog = new CreateGameDialogController();   
          try {
              CreateGameDialog.showCreateGameDialog(stage);
          } catch (Exception e) {
              e.printStackTrace();
          }
      }

    public void init(Stage stage) {
        // TODO Auto-generated method stub
        this.stage = stage;
    }
}

Main.fxml

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

<?import javafx.scene.input.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import java.lang.*?>
<?import javafx.scene.image.*?>

<AnchorPane fx:id="AnchorPane" 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" fx:controller="Main.MainController">
   <children>
      <BorderPane layoutX="-2.0" layoutY="-3.0" prefHeight="400.0" prefWidth="606.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
         <center>
            <TextArea prefHeight="200.0" prefWidth="200.0" BorderPane.alignment="CENTER" />
         </center>
         <top>
            <MenuBar BorderPane.alignment="CENTER">
              <menus>
                <Menu text="_File">
                  <items>
                        <MenuItem onAction="#openFile" text="_Open">
                           <accelerator>
                              <KeyCodeCombination alt="UP" code="O" control="DOWN" meta="UP" shift="UP" shortcut="UP" />
                           </accelerator>
                        </MenuItem>
                    <MenuItem onAction="#doExit" text="E_xit">
                           <accelerator>
                              <KeyCodeCombination alt="UP" code="X" control="DOWN" meta="UP" shift="UP" shortcut="UP" />
                           </accelerator>
                        </MenuItem>
                  </items>
                </Menu>
                  <Menu text="_Game">
                    <items>
                        <MenuItem onAction="#CreateGame" text="_Create">
                           <accelerator>
                              <KeyCodeCombination alt="DOWN" code="C" control="UP" meta="UP" shift="UP" shortcut="UP" />
                           </accelerator></MenuItem>
                    </items>
                  </Menu>
              </menus>
            </MenuBar>
         </top>
         <bottom>
            <Button mnemonicParsing="false" onAction="#ButtonClicked" text="ClickMeButton" BorderPane.alignment="CENTER" />
         </bottom>
      </BorderPane>
      <GridPane id="GridPane" fx:id="GridPane" layoutX="20.0" layoutY="70.0" prefHeight="68.0" prefWidth="594.0" AnchorPane.bottomAnchor="30.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="6.0">
        <columnConstraints>
          <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
        </columnConstraints>
        <rowConstraints>
          <RowConstraints fillHeight="false" minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
        </rowConstraints>
         <padding>
            <Insets top="40.0" />
         </padding>
         <children>
            <Button id="TestButton1" alignment="TOP_LEFT" contentDisplay="BOTTOM" mnemonicParsing="false" prefHeight="22.0" prefWidth="94.0" text="Test Button 1" />
            <Button id="TestButton2" contentDisplay="BOTTOM" mnemonicParsing="false" prefHeight="25.0" prefWidth="91.0" text="Test Button 2">
               <GridPane.margin>
                  <Insets left="100.0" />
               </GridPane.margin>
            </Button>
            <Button id="addPlayer" fx:id="addPlayer" alignment="BASELINE_RIGHT" mnemonicParsing="false" onAction="#addPlayer" text="Add Player">
               <GridPane.margin>
                  <Insets left="300.0" />
               </GridPane.margin>
            </Button>
            <TextField fx:id="PlayerName" prefHeight="25.0" prefWidth="152.0" promptText="Player Name">
               <GridPane.margin>
                  <Insets left="400.0" />
               </GridPane.margin>
            </TextField>
         </children>
      </GridPane>
      <GridPane id="GridPaneMain" fx:id="GridPaneMain" gridLinesVisible="true" layoutY="29.0" prefHeight="308.0" prefWidth="600.0">
        <columnConstraints>
          <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
          <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" />
        </columnConstraints>
        <rowConstraints>
          <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
          <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
          <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
        </rowConstraints>
      </GridPane>
   </children>
</AnchorPane>

CreateGameDialogController.java

package Main;

import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;

import javafx.application.Platform;
import javafx.stage.Modality;
import javafx.stage.Stage;
import javafx.event.ActionEvent;
import javafx.fxml.*;
import javafx.scene.layout.*;
import javafx.scene.*;

public class CreateGameDialogController implements Initializable {

    @FXML
    private javafx.scene.layout.AnchorPane AnchorPane;
    @FXML
    private javafx.scene.control.Slider NumberOfPlayers;

    private Stage myParent;
    private Stage CreateGameDialogStage;

    public void showCreateGameDialog(Stage parentStage) {
    this.myParent = parentStage;
    try {
        CreateGameDialogStage = new Stage();
        AnchorPane page = (AnchorPane) FXMLLoader.load(CreateGameDialogController.class.getResource("/fxml/CreateGameDialog.fxml"));
        Scene scene = new Scene(page);
        CreateGameDialogStage.setScene(scene);
        CreateGameDialogStage.setTitle("Create Game");
        CreateGameDialogStage.initOwner(this.myParent);
        CreateGameDialogStage.initModality(Modality.WINDOW_MODAL);
        CreateGameDialogStage.show();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    } 

    public void CreateGame(ActionEvent event) {
        try {
        System.out.println("Create Game");
        CreateGameDialogStage.close();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    @Override
    public void initialize(URL location, ResourceBundle resources) {
        // TODO Auto-generated method stub

    }

}

CreateGameDialog.fxml

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

<?import javafx.scene.text.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>

<AnchorPane id="AnchorPane" prefHeight="427.0" prefWidth="602.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="Main.CreateGameDialogController">
   <children>
      <BorderPane prefHeight="200.0" prefWidth="200.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
         <left>
            <Pane prefHeight="427.0" prefWidth="257.0" BorderPane.alignment="CENTER">
               <children>
                  <Slider id="NumberOfPlayers" fx:id="NumberOfPlayers" blockIncrement="1.0" layoutX="14.0" layoutY="42.0" majorTickUnit="1.0" max="8.0" min="3.0" minorTickCount="0" orientation="VERTICAL" prefHeight="310.0" prefWidth="14.0" showTickLabels="true" showTickMarks="true" snapToTicks="true" value="1.0" />
                  <Label alignment="CENTER" contentDisplay="CENTER" prefHeight="29.0" prefWidth="257.0" text="Select the Players" textAlignment="CENTER">
                     <font>
                        <Font name="System Bold" size="14.0" />
                     </font>
                  </Label>
                  <Button id="CreateGameButton" layoutX="79.0" layoutY="388.0" mnemonicParsing="false" onAction="#CreateGame" prefHeight="25.0" prefWidth="102.0" text="Create Game" />
               </children>
            </Pane>
         </left>
         <right>
            <ScrollPane prefHeight="427.0" prefWidth="360.0" BorderPane.alignment="CENTER" />
         </right>
      </BorderPane>
   </children>
</AnchorPane>

When I click the Create Game button, I get:

Create Game
java.lang.NullPointerException
    at Main.CreateGameDialogController.CreateGame(CreateGameDialogController.java:47)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.Trampoline.invoke(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.MethodUtil.invoke(Unknown Source)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(Unknown Source)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(Unknown Source)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
    at com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
    at javafx.event.Event.fireEvent(Unknown Source)
    at javafx.scene.Node.fireEvent(Unknown Source)
    at javafx.scene.control.Button.fire(Unknown Source)
    at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(Unknown Source)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(Unknown Source)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(Unknown Source)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(Unknown Source)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
    at com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
    at com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
    at javafx.event.Event.fireEvent(Unknown Source)
    at javafx.scene.Scene$MouseHandler.process(Unknown Source)
    at javafx.scene.Scene$MouseHandler.access$1500(Unknown Source)
    at javafx.scene.Scene.impl_processMouseEvent(Unknown Source)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Unknown Source)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(Unknown Source)
    at com.sun.glass.ui.View.handleMouseEvent(Unknown Source)
    at com.sun.glass.ui.View.notifyMouse(Unknown Source)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$141(Unknown Source)
    at com.sun.glass.ui.win.WinApplication$$Lambda$38/2075313.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Answer 1

Inside MainController, the problem in CreateGame() is in this line:

CreateGameDialogController CreateGameDialog = new CreateGameDialogController();

When you are using FXML Controllers, instead of creating a controller object using new, always load the FXML and get the controller from the FXMLoader

You must try to load CreateGameDialog.fxml inside CreateGame() instead of loading the FXML in its own controller.

ProTip - Use camel casing while coding in Java