Error: Exception in thread "JavaFX Application Thread"
<p>I have a Program including a spaceship (big circle) which fires a bullet (small circle), that disappears after a while.
Every time it runs over a specific part of the code it generates an error(, but it continues to run).</p>

<p>So I'm sure that the problem is in the following lines</p>

<pre><code>private ArrayList&lt;Bullet&gt; bullets = new ArrayList&lt;Bullet&gt;();
//[...]
for (Bullet bullet : bullets) {
  if (bullet.getLoops() &gt; 50) {

    bullets.remove(bullet); // specially in this line,
                            // because if I delete this, the error doesn't show up!
                            // I think it has something to do with the ArrayList.

  } else {
    bullet.next();
    gc.strokeOval(bullet.getX() - 3, bullet.getY() - 3, 6, 6);
  }
}
</code></pre>

<p>and this is the error I get:</p>

<pre><code>Exception in thread "JavaFX Application Thread" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    at java.util.ArrayList$Itr.next(ArrayList.java:851)
    at sample.Main$4.handle(Main.java:131)
    at javafx.animation.AnimationTimer$AnimationTimerReceiver.lambda$handle$484(AnimationTimer.java:57)
    at java.security.AccessController.doPrivileged(Native Method)
    at javafx.animation.AnimationTimer$AnimationTimerReceiver.handle(AnimationTimer.java:56)
    at com.sun.scenario.animation.AbstractMasterTimer.timePulseImpl(AbstractMasterTimer.java:357)
    at com.sun.scenario.animation.AbstractMasterTimer$MainLoop.run(AbstractMasterTimer.java:267)
    at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:506)
    at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:490)
    at com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$404(QuantumToolkit.java:319)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
    at java.lang.Thread.run(Thread.java:745)
</code></pre>

<p>I will be delighted to supply any other information needed upon request.</p>

<p>Cheers.
(Sorry for my English, I'm from Germany, but I have English in school since a few Years.)</p>

<p>PS: if anybody needs the full code of the mainclass:</p>

<pre><code>package sample;

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.fxml.FXMLLoader;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.input.KeyEvent;
import javafx.scene.input.MouseEvent;
import javafx.scene.paint.Color;
import javafx.scene.text.Font;
import javafx.scene.text.FontWeight;
import javafx.stage.Stage;

import java.util.ArrayList;

public class Main extends Application {

    private Player p = new Player(100, 100, 0);
    private ArrayList&lt;Bullet&gt; bullets = new ArrayList&lt;Bullet&gt;();

    private static boolean left;
    private static boolean right;
    private static boolean up;
    private static boolean down;
    private static boolean shoot;

    @Override
    public void start(Stage primaryStage) throws Exception {
        FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        Group root0 = new Group();
        Scene theScene = new Scene(root0, 500, 500);
        primaryStage.setScene(theScene);
        Canvas canvas = new Canvas(500, 500);
        root0.getChildren().add(canvas);


        theScene.setOnMouseMoved(
                new EventHandler&lt;MouseEvent&gt;() {
                    public void handle(MouseEvent e) {
                        Vector a = new Vector(p.getX(), -p.getY());
                        Vector b = new Vector(e.getX(), -e.getY());
                        Vector c = new Vector(p.getX(), -p.getY() + 1);
                        p.setRotation(Vector.getAngle(a, b, c));
                    }
                }
        );
        theScene.setOnKeyPressed(
                new EventHandler&lt;KeyEvent&gt;() {
                    public void handle(KeyEvent e) {
                        String code = e.getCode().toString();
                        if (code.equals("A")) {
                            left = true;
                        }
                        if (code.equals("D")) {
                            right = true;
                        }
                        if (code.equals("W")) {
                            up = true;
                        }
                        if (code.equals("S")) {
                            down = true;
                        }
                        if (code.equals("SPACE")) {
                            if (!PlayerShot()) shoot = true;
                        }
                    }
                }
        );
        theScene.setOnKeyReleased(
                new EventHandler&lt;KeyEvent&gt;() {
                    public void handle(KeyEvent e) {
                        String code = e.getCode().toString();
                        if (code.equals("A")) {
                            left = false;
                        }
                        if (code.equals("D")) {
                            right = false;
                        }
                        if (code.equals("W")) {
                            up = false;
                        }
                        if (code.equals("S")) {
                            down = false;
                        }
                        if (code.equals("SPACE")) {
                            shoot = false;
                        }
                    }
                }
        );
        GraphicsContext gc = canvas.getGraphicsContext2D();

        Font theFont = Font.font("Helvetica", FontWeight.BOLD, 24);
        gc.setFont(theFont);
        gc.setStroke(Color.BLACK);
        gc.setLineWidth(1);

        new AnimationTimer() {
            public void handle(long currentNanoTime) {
                // Clear the canvas
                gc.setFill(new Color(0.85, 0.85, 1.0, 1.0));
                gc.fillRect(0, 0, 500, 500);

                int x = p.getX();
                int y = p.getY();
                if (left &amp;&amp; x &gt;= 16) {
                    p.setX(x - 3);
                }
                if (right &amp;&amp; x &lt;= 500 - 16) {
                    p.setX(x + 3);
                }
                if (up &amp;&amp; y &gt;= 16) {
                    p.setY(y - 3);
                }
                if (down &amp;&amp; y &lt;= 500 - 16) {
                    p.setY(y + 3);
                }
                if (shoot) {
                    bullets.add(new Bullet(p.getX(), p.getY(), p.getRotationInVelocity(), p));
                    shoot = false;
                }
                gc.setStroke(Color.BLUE);
                gc.strokeLine(p.getX(), p.getY(), p.getX() + Math.sin(p.getRotation()) * 16, p.getY() - Math.cos(p.getRotation()) * 16);
                gc.strokeOval(p.getX() - 16, p.getY() - 16, 32, 32);
                gc.setStroke(Color.RED);
                for (Bullet bullet : bullets) {
                    if (bullet.getLoops() &gt; 50) {
                        bullets.remove(bullet);
                    } else {
                        bullet.next();
                        gc.strokeOval(bullet.getX() - 3, bullet.getY() - 3, 6, 6);
                    }
                }
            }
        }.start();

        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }

    // Hat der Spieler gerade eben schon geschossen?
    public boolean PlayerShot() {
        for (Bullet bullet : bullets) {
            if(bullet.getShooter() == p)return true;
        }
        return false;
    }
}
</code></pre>

<p>the other classes just handle a few int- and double-vars</p>