java - 错误: Exception in thread "JavaFX Application Thread"

Question

这个问题已经有答案了:

Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop (31 个回答)

已关闭 7 年前。

我有一个程序，其中包括一艘宇宙飞船(大圆圈)，它发射一颗子弹(小圆圈)，过了一会儿就消失了。每次运行代码的特定部分时，它都会生成错误(但它会继续运行)。

所以我确定问题出在以下几行

private ArrayList<Bullet> bullets = new ArrayList<Bullet>();
//[...]
for (Bullet bullet : bullets) {
  if (bullet.getLoops() > 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);
  }
}

这是我得到的错误:

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)

我很乐意根据要求提供所需的任何其他信息。

干杯。(抱歉我的英语不好，我来自德国，但几年前我在学校就开始学英语了。)

PS:如果有人需要主类的完整代码:

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<Bullet> bullets = new ArrayList<Bullet>();

    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<MouseEvent>() {
                    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<KeyEvent>() {
                    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<KeyEvent>() {
                    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 && x >= 16) {
                    p.setX(x - 3);
                }
                if (right && x <= 500 - 16) {
                    p.setX(x + 3);
                }
                if (up && y >= 16) {
                    p.setY(y - 3);
                }
                if (down && y <= 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() > 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;
    }
}

其他类只处理一些 int- 和 double-vars

Answer 1

你不能这么做。您将获得 ConcurrentModificationException 。

什么是ConcurrentModificationException ？

来自官方Javadoc :

This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.

基本上，您正在迭代您的列表。如果您在某一时刻决定删除一个元素，这可能会导致不可预测的行为。为了避免这种情况，抛出此异常。

如何修复它？

只需使用 <a href="https://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html" rel="noreferrer noopener nofollow">Iterator</a>而不是循环你的ArrayList 。 Iterator使用方法add(T element)和remove(T element) .