AWT-EventQueue-1 NullExceptionPointer

Question

我在用瓷砖做迷宫游戏。迷宫每隔一段时间就会改变形状。在这个时候，在迷宫重建之前，铁射线就会变得空了。我得到NPE错误后，在不同的时间间隔和一段时间后，游戏停止。可能是因为有些方法在实际重新创建之前尝试访问。我尝试将if(.=null)放在某些地方，甚至尝试同步某些部分(我不太熟悉)。

编辑:谢谢Arvind指出了这个问题。我设法修复了它，但我仍然会学学更多的帮助，因为这不是最好和最整洁的修复，我认为。在某些情况下，Tile等于空。我只是简单地检查了它，并将其放入for循环中以避免错误。

非常感谢您的任何帮助和建议，提前！

有了这个错误，游戏仍然在运行：

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
at shiftingmaze.Game.paintTiles(Game.java:161)
at shiftingmaze.Game.paint(Game.java:115)
at shiftingmaze.Game.update(Game.java:107)
at sun.awt.RepaintArea.updateComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$400(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

过了一段时间，它就会因为这个错误而冻结：

Exception in thread "Thread-4" java.lang.NullPointerException
at shiftingmaze.Game.updateTiles(Game.java:153)
at shiftingmaze.Game.run(Game.java:86)
at java.lang.Thread.run(Unknown Source)

下面是代码的一些部分：

@Override
public void start() {

    hero = new Hero();

    Timer t = new Timer();

    t.scheduleAtFixedRate(new TimerTask() {
        public void run() {
            tilearray.clear();
            shiftMaze();
        }
    }, 0, 1000);

    Thread thread = new Thread(this);
    thread.start();
}

@Override
public void stop() {

}

@Override
public void destroy() {

}

@Override
public void run() {

    while (true) {
        hero.update();
        updateTiles();
        repaint();
        try {
            Thread.sleep(17);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

@Override
public void update(Graphics g) {

    if (image == null) {
        image = createImage(this.getWidth(), this.getHeight());
        second = image.getGraphics();
    }

    second.setColor(getBackground());
    second.fillRect(0, 0, getWidth(), getHeight());
    second.setColor(getForeground());
    paint(second);
    g.drawImage(image, 0, 0, this);
}

@Override
public void paint(Graphics g) {

    g.drawImage(background, 0, 0, this);
    paintTiles(g);
    // g.drawRect((int)Hero.rect.getX(), (int)Hero.rect.getY(),
    // (int)Hero.rect.getWidth(), (int)Hero.rect.getHeight());
    // g.drawRect((int)Hero.bigRect.getX(), (int)Hero.bigRect.getY(),
    // (int)Hero.bigRect.getWidth(), (int)Hero.bigRect.getHeight());
    g.drawImage(character, hero.getHeroX(), hero.getHeroY(), this);
}

public void shiftMaze() {

    final int MAZEROW = 24;
    final int MAZECOL = 40;

    for (int i = 0; i < MAZEROW * MAZECOL / 2; i++) {
        int n = rand.nextInt(MAZECOL - 2) + 1;
        int m = rand.nextInt(MAZEROW - 2) + 1;
        if (!(n == 1 && m == 1) && !(n == MAZECOL - 2 && m == MAZEROW - 2)) {
            Tile t = new Tile(n, m);
            tilearray.add(t);
        }
    }

    for (int i = 0; i < MAZECOL; i++) {
        for (int j = 0; j < MAZEROW; j++) {
            if (i == 0 || i == MAZECOL - 1 || j == 0 || j == MAZEROW - 1)
                if (!(i == 1 && j == 0)
                        && !(i == MAZECOL - 2 && j == MAZEROW - 1)) {
                    Tile t = new Tile(i, j);
                    tilearray.add(t);
                }
        }
    }
}

public void updateTiles() {

    for (int i = 0; i < tilearray.size(); i++) {
        Tile t = (Tile) tilearray.get(i);
        t.update();
    }
}

public void paintTiles(Graphics g) {

    for (int i = 0; i < tilearray.size(); i++) {
        Tile t = (Tile) tilearray.get(i);
        g.drawImage(t.getTileImage(), t.getTileX(), t.getTileY(), this);
    }
}

这是来自Tile类的：

public Tile(int x, int y) {

    tileX = x * 20;
    tileY = y * 20;
    tileImage = Game.getWall();

    r = new Rectangle();
}

public void update() {

    r.setBounds(tileX, tileY, 20, 20);
    if (r.intersects(Hero.bigRect))
        checkCollision(Hero.rect);
}

Answer 1

问题在于：

public void updateTiles() {
    for (int i = 0; i < tilearray.size(); i++) {
        Tile t = (Tile) tilearray.get(i);//<-- tile is null in some cases
        t.update();
    }
}

要避免这种情况，有许多选择：

选项1：子类ArrayList并覆盖方法add()和addAll()，并使用null检查。

选项2：填充tilearray之后，使用以下行删除空值

    tilearray.removeAll(Collections.singleton(null));

编辑

如果您正在寻找第二个选项，那么您应该尝试如下：

public void shiftMaze() {
    //<-- to do here
    tilearray.removeAll(Collections.singleton(null));//<-- at the end
}

Answer 2

谢谢你的帮助，我试过了，但还是有些无效。同时，我做了以下工作，以消除错误。它几乎一直工作，但很少我仍然得到NPE或索引的范围外的错误.我加快了游戏到100毫厘娱乐，以获得错误检查在长期内。

for (int i = 0; i < tilearray.size(); i++) {
    Tile t = (Tile) tilearray.get(i);
    if (t == null && i < tilearray.size() - 1) {
        System.out.println("t = null @ updateTiles(), moving to next element");
        continue;
    }
    if (t == null && i >= tilearray.size()) {
        System.out.println("t = null @ updateTiles(), no elements left");
        break;
    }
    t.update();