NullPointerException Error on simple program

Question

I am trying to make a basic game in which a dot is moved randomly around the screen, and when it is clicked, the user gets some points. When I run the program, I get a NullPointerException at the start method. I am wondering what I am doing wrong as well as any types for improving the program. Thank you!

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.stage.Popup;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.Pane;
import javafx.scene.shape.Circle;
import javafx.scene.text.Text;
import java.util.Random;
import java.util.Timer;
import java.util.TimerTask;


public class CatchTheDot extends Application{

//create ball for game
public static Circle dot;

//create pane to run game
public static Pane window;

//create score counter
int score = 0;

//creat random
Random random = new Random();

@Override
public void start(Stage primaryStage) throws Exception {

    window.getChildren().add(dot);

    // create scene and place on pane
    Scene s = new Scene(window, 800, 800);
    primaryStage.setTitle("Catch The Dot");
    primaryStage.setScene(s);
    primaryStage.show();

    //move dot
    Timer timer = new Timer();
    timer.schedule(new TimerTask() 
        {
        @Override
        public void run(){
        window.getChildren().remove(dot);
        int ranX = random.nextInt(800-1); // random value from 0 to width
        int ranY = random.nextInt(800-1);  // random value from 0 to height
        window.getChildren().add(ranY, dot);
        }
        }, 0, 5000);        


    // create listener
    dot.setOnMouseClicked(new EventHandler<MouseEvent>()
        {
        public void handle(MouseEvent e){
        if(e.getSource()==dot)
        {
            score = score + 10;
            if(score == 50)
            {
                popUp(primaryStage);
            }
        }
    }
});
}
public void popUp(final Stage primaryStage)
{
    primaryStage.setTitle("You won!");
    final Popup popup = new Popup();
    popup.setX(300);
    popup.setY(200);
    Text t = new Text("You won! Nice job!");
    Text tt = new Text("Play again?");
    Button yes = new Button("yes");
    Button no = new Button("no");
    popup.getContent().addAll(t, tt, yes, no);
    yes.setOnAction(e -> Yes());
    no.setOnAction(e -> No());
}

public void Yes()
{
    restartGame();
}
public void No()
{
    System.exit(0);
}
public void restartGame()
{
    score = 0;
}
public static void main(String[] args)
{
    launch(args);
}

}

Error log:

Exception in Application start method
java.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$156(LauncherImpl.java:182)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
    at CatchTheDot.start(CatchTheDot.java:34)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$163(LauncherImpl.java:863)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$176(PlatformImpl.java:326)
    at com.sun.javafx.application.PlatformImpl.lambda$null$174(PlatformImpl.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$175(PlatformImpl.java:294)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
Exception running application CatchTheDot

The likely cause is that `dot` is never instantiated, but as others point out, you would be better served by doing some basic debugging. — Paul Lammertsma, Apr 18 '16 at 20:38
I don't see your `window` variable initialized anywhere, and you're calling its method — Michał Szydłowski, Apr 18 '16 at 20:38
`window.getChildren().add(dot);`... Both `window` and `dot` seem to be null there. — OneCricketeer, Apr 18 '16 at 20:39
Please do not use PasteBin. You will get warnings when you approach the max content length of a question. — OneCricketeer, Apr 18 '16 at 20:42

score 2 · Answer 1 · answered Apr 18 '16 at 20:43

2

Window and Dot have been declared but not defined

answered Apr 18 '16 at 20:43

Rana

1,675
3
25
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How do I define them? Sorry if this is a stupid question... – Adam7397 Apr 18 '16 at 20:46

Michał Szydłowski · Accepted Answer · 2016-04-18T20:48:41.577

1

In line 34, as the trace says:

window.getChildren().add(dot);

either window or dot (or both) are not initialized by then. Hence the null pointer exception.

edited Apr 18 '16 at 20:48

answered Apr 18 '16 at 20:42

Michał Szydłowski

3,261
5
33
55

Am I not initializing them at the beginning of the code? I thought that is what I was doing.... – Adam7397 Apr 18 '16 at 20:44
You are declaring them, in that you are telling the compiler that a specific variable should be treated as an object of a specific class. You never initialized it. – Rabbit Guy Apr 18 '16 at 20:45
Declaring a variable specifies its type, to initialize it, you need to assign any value to them. How can the compiler evaluate an expression with this variable if it doesn't know its value? – Michał Szydłowski Apr 18 '16 at 20:48
Ok I understand what are saying, for some reason I thought both was happening all ready.. Thank you both for your help – Adam7397 Apr 18 '16 at 20:50
Great. Consider accepting one of the answers if your problem is solved. – Michał Szydłowski Apr 18 '16 at 20:53
He can't. Marked as duplicate. – Rabbit Guy Apr 18 '16 at 20:54

NullPointerException Error on simple program

2 Answers2