android - 有检查null怎么会有NullPointerException？

Question

注意:我知道这个问题是重复的，您可能被鼓励投反对票，但是，

• 它发生在 Android Framework 代码中
• 即使存在空检查且没有多线程，它也会发生。

---

我最近在 Playstore 上部署了我的 APK 并收到了极少数用户的 NullPointerException(4):

 Exception java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.Object.equals(java.lang.Object)' on a null object reference
android.widget.AdapterView.getPositionForView (AdapterView.java:607)
com.snap.testsnapboard.AlbumSelectActivity$1.onItemClick (AlbumSelectActivity.java:114)
android.widget.AdapterView.performItemClick (AdapterView.java:305)
android.widget.AbsListView.performItemClick (AbsListView.java:1146)
android.widget.AbsListView$PerformClick.run (AbsListView.java:3057)
android.widget.AbsListView.onTouchUp (AbsListView.java:3876)
android.widget.AbsListView.onTouchEvent (AbsListView.java:3641)
android.view.View.dispatchTouchEvent (View.java:8481)
android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2400)
android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2093)
android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2406)
android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2107)
android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2406)
android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2107)
android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2406)
android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2107)
android.view.ViewGroup.dispatchTransformedTouchEvent (ViewGroup.java:2406)
android.view.ViewGroup.dispatchTouchEvent (ViewGroup.java:2107)
com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent (PhoneWindow.java:2369)
com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent (PhoneWindow.java:1719)
android.app.Activity.dispatchTouchEvent (Activity.java:2785)
com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent (PhoneWindow.java:2330)
android.view.View.dispatchPointerEvent (View.java:8676)
android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent (ViewRootImpl.java:4141)
android.view.ViewRootImpl$ViewPostImeInputStage.onProcess (ViewRootImpl.java:4007)
android.view.ViewRootImpl$InputStage.deliver (ViewRootImpl.java:3562)
android.view.ViewRootImpl$InputStage.onDeliverToNext (ViewRootImpl.java:3615)
android.view.ViewRootImpl$InputStage.forward (ViewRootImpl.java:3581)
android.view.ViewRootImpl$AsyncInputStage.forward (ViewRootImpl.java:3698)
android.view.ViewRootImpl$InputStage.apply (ViewRootImpl.java:3589)
android.view.ViewRootImpl$AsyncInputStage.apply (ViewRootImpl.java:3755)
android.view.ViewRootImpl$InputStage.deliver (ViewRootImpl.java:3562)
android.view.ViewRootImpl$InputStage.onDeliverToNext (ViewRootImpl.java:3615)
android.view.ViewRootImpl$InputStage.forward (ViewRootImpl.java:3581)
android.view.ViewRootImpl$InputStage.apply (ViewRootImpl.java:3589)
android.view.ViewRootImpl$InputStage.deliver (ViewRootImpl.java:3562)
android.view.ViewRootImpl.deliverInputEvent (ViewRootImpl.java:5825)
android.view.ViewRootImpl.doProcessInputEvents (ViewRootImpl.java:5799)
android.view.ViewRootImpl.enqueueInputEvent (ViewRootImpl.java:5770)
android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent (ViewRootImpl.java:5915)
android.view.InputEventReceiver.dispatchInputEvent (InputEventReceiver.java:185)
android.os.MessageQueue.nativePollOnce (MessageQueue.java)
android.os.MessageQueue.next (MessageQueue.java:143)
android.os.Looper.loop (Looper.java:122)
android.app.ActivityThread.main (ActivityThread.java:5254)
java.lang.reflect.Method.invoke (Method.java)
java.lang.reflect.Method.invoke (Method.java:372)
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run (ZygoteInit.java:902)
com.android.internal.os.ZygoteInit.main (ZygoteInit.java:697)

此异常源自行 while ((v = (View) listItem.getParent()) != null && !v.equals(this)) {

在 AdapterView.java 中:

/**
     * Returns the position within the adapter's data set for the view, where
     * view is a an adapter item or a descendant of an adapter item.
     * <p>
     * <strong>Note:</strong> The result of this method only reflects the
     * position of the data bound to <var>view</var> during the most recent
     * layout pass. If the adapter's data set has changed without a subsequent
     * layout pass, the position returned by this method may not match the
     * current position of the data within the adapter.
     *
     * @param view an adapter item, or a descendant of an adapter item. This
     *             must be visible in this AdapterView at the time of the call.
     * @return the position within the adapter's data set of the view, or
     *         {@link #INVALID_POSITION} if the view does not correspond to a
     *         list item (or it is not currently visible)
     */
    public int getPositionForView(View view) {
        View listItem = view;
        try {
            View v;
            while ((v = (View) listItem.getParent()) != null && !v.equals(this)) {
                listItem = v;
            }
        } catch (ClassCastException e) {
            // We made it up to the window without find this list view
            return INVALID_POSITION;
        }

        if (listItem != null) {
            // Search the children for the list item
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                if (getChildAt(i).equals(listItem)) {
                    return mFirstPosition + i;
                }
            }
        }

        // Child not found!
        return INVALID_POSITION;
    }

其他值得一提的事情:

• 我有一个 ListView ，它通过 addFooterView()
附加了一个页脚 View
• 我想检查此页脚 View 是否已被单击，但什么都不做，因为此 View 仅包含一些不是通常列表项的图形。

• 为此，我编写了如下代码:

              // Since we have a Footer view attached to the ListView, the position selected
              // might not always map 1-to-1 with the ArrayList we are using to represent the
              // Album model objects list. Lets check if the Footer view has been selected and
              // in this case, do nothing.
  
              if(myListView.getPositionForView(loadMoreView) == arg2){
                  // Blank return
                  return;
              }
  

这是最终调用AdapterView.java 中方法的代码。现在，当 while 循环中存在 null 检查且所有 UI 处理仅发生在主线程中时，为什么会出现 NullPointerException？

Answer 1

我想告诉你一些关于空指针异常的小技巧。

如果 listItem 为 null，它将创建异常...如果 listItem 不为 null 但 listItem.getParent() 为 null，它也会创建异常...或者当您将任何对象转换为另一个对象且该对象为 null ..它抛出异常。

(v = (View) listItem.getParent()) != null && !v.equals(this)

在您的例子中，listItem.getParent() 为空，这就是它抛出异常的原因。

你应该这样做:

if(v != null && listItem != null && listItem.getParent() != null && (v = (View) listItem.getParent()) != null && !v.equals(this) {

}

我希望它不会抛出异常。