嵌套迭代器

Question

我正在尝试构建一个空间入侵者游戏，并开发了这部分代码，使所有的外星人下降时，其中一个击中了屏幕的边缘，然而，我得到了一个相当讨厌的错误信息，当它击中线Enemy de = downwardIterator.next();。游戏仍然按其应有的方式运行，但当它击中这条线时，它会在后台发出这些错误消息。下面是代码：

Iterator<Enemy> iterator = enemyList.iterator();
    while (iterator.hasNext()) {
        Enemy e = iterator.next();

        if(e!=null && e.isActive()){
            e.move();
            e.draw(g);
            if(e.edgeHit()){
                Iterator<Enemy> downwardIterator = enemyList.iterator();
                while (iterator.hasNext()) {
                    Enemy de = downwardIterator.next();
                    de.dropPosition();
                    de.changeDirection();
                }
            }
        }
        else{
            iterator.remove();
        }
    }

e.move只是让敌人左右移动。当任何敌人到达游戏屏幕的边缘时，e.edgeHit会检测到。

当它到达这一行时，这是完整的跟踪：

Exception in thread "AWT-EventQueue-0" java.util.NoSuchElementException
    at java.util.ArrayList$Itr.next(ArrayList.java:834)
    at GamePanel.paintComponent(GamePanel.java:96)
    at javax.swing.JComponent.paint(JComponent.java:1054)
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5219)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1529)
    at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1452)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1249)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5167)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4978)
    at javax.swing.RepaintManager$3.run(RepaintManager.java:808)
    at javax.swing.RepaintManager$3.run(RepaintManager.java:796)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:796)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:769)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:718)
    at javax.swing.RepaintManager.access$1100(RepaintManager.java:62)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1677)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Answer 1

您的第二个迭代器循环不正确。

Iterator<Enemy> downwardIterator = enemyList.iterator();
while (iterator.hasNext()) {
  // ...

应该是

Iterator<Enemy> downwardIterator = enemyList.iterator();
while (downwardIterator.hasNext()) {
  // ...

编辑

您可能更喜欢这样的 loop (它使用隐藏的迭代器)

for (Enemy de : enemyList) {
  // ...

Answer 2

如果需要解释，则调用downwardIterator对象上的downwardIterator()方法，而while循环不检查右侧迭代器对象，以查看是否还剩下任何元素：

Iterator<Enemy> downwardIterator = enemyList.iterator();
while (iterator.hasNext()) { // Check downwardIterator instead of iterator
    Enemy de = downwardIterator.next();
    de.dropPosition();
    de.changeDirection();
}

此外，当您的第一个if条件满足时，如果没有修改enemyList，可以使用增强的for循环来迭代enemyList。