java.lang.Class Cast Exception: android.widget.HeaderViewListAdapter

问题背景： 最近写一个项目，根据屏幕左侧的字母查找好友，在网上找了个Demo，现在我想在显示好友的ListView上面增加一个HeaderView，运行的时候报了这个错误。

下面是Demo中的一个自定义View的代码：

public class SideBar extends View {
 private char[] mL;
 private SectionIndexer mSectionIndexter = null;
 private ListView mList;
 private TextView mDialogText;
 private final int mM_nItemHeight = 15;

 public SideBar(Context context) {
  super(context);
  init();
 }

 public SideBar(Context context, AttributeSet attrs) {
  super(context, attrs);
  init();
 }

 public SideBar(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  init();
 }

 private void init() {
  mL = new char[] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
    'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V',
    'W', 'X', 'Y', 'Z' };
 }

 public void setListView(ListView listView) {
  mList = listView;
  HeaderViewListAdapter ha = (HeaderViewListAdapter) listView.getAdapter();
  mSectionIndexter = (SectionIndexer) ha.getWrappedAdapter();
//  mSectionIndexter = (SectionIndexer) listView.getAdapter();
 }

 public void setTextView(TextView mDialogText) {
  this.mDialogText = mDialogText;
 }

 public boolean onTouchEvent(MotionEvent event) {
  super.onTouchEvent(event);
  int i = (int) event.getY();
  int idx = i / mM_nItemHeight;
  if (idx >= mL.length) {
   idx = mL.length - 1;
  } else if (idx < 0) {
   idx = 0;
  }
  if (event.getAction() == MotionEvent.ACTION_DOWN
    || event.getAction() == MotionEvent.ACTION_MOVE) {
   mDialogText.setVisibility(View.VISIBLE);
   mDialogText.setText("" + mL[idx]);
   if (mSectionIndexter == null) {
    mSectionIndexter = (SectionIndexer) mList.getAdapter();
   }
   int position = mSectionIndexter.getPositionForSection(mL[idx]);
   if (position == -1) {
    return true;
   }
   mList.setSelection(position);
  } else {
   mDialogText.setVisibility(View.INVISIBLE);
  }
  return true;
 }

 protected void onDraw(Canvas canvas) {
  Paint paint = new Paint();
  paint.setColor(0xff595c61);
  paint.setTextSize(12);
  paint.setTextAlign(Paint.Align.CENTER);
  float widthCenter = getMeasuredWidth() / 2;
  for (int i = 0; i < mL.length; i++) {
   canvas.drawText(String.valueOf(mL[i]), widthCenter, mM_nItemHeight
     + (i * mM_nItemHeight), paint);
  }
  super.onDraw(canvas);
 }
}

解决方法：

ListView增加HeaderView之后，将原来的Adapter包装成HeaderViewListAdapter，看看HeaderViewListAdapter的文档说明：

ListAdapter used when a ListView has header views. This ListAdapter wraps another one and also keeps track of the header views and their associated data objects.

This is intended as a base class; you will probably not need to use this class directly in your own code.

HeaderViewListAdapter有个方法getWrappedAdapter，该方法能返回被包装的HeaderViewListAdapter的ListAdapter。

到了这里就明白为什么会报ClassCastException异常了。因为ListView的getAdapter方法返回的是HeaderViewListAdapter的实例，而将其转换成BaseAdapter的子类的实例，肯定是不对的。

下面是我给显示好友的ListView设置HeaderView和Adapter的代码：

// 这里要注意，给ListView增加HeaderView和Adapter顺序，不然会报错。

LinearLayout headerView = (LinearLayout) LayoutInflater.from(this).inflate(R.layout.weibo_friends_listheader, null);
 mFriendsContact.addHeaderView(headerView);

 mFriendsContact.setAdapter(new FriendListViewAdapter(this, mNicks));  
 mIndexBar.setListView(mFriendsContact);

 注意上面SideBar这个类中红色标记方法内的代码：

 public void setListView(ListView listView) {
  mList = listView;

  //更改后的，我在获得ListView的Adapter之前进行了一下转换
  HeaderViewListAdapter ha = (HeaderViewListAdapter) listView.getAdapter();
  mSectionIndexter = (SectionIndexer) ha.getWrappedAdapter();

//更改前的
//  mSectionIndexter = (SectionIndexer) listView.getAdapter();
 }