我通过扩展 BaseAdapter 创建了自定义 ListView 和适配器。每次我尝试在适配器上调用 notifyDataSetChanged() 时，程序都会因 NullPointerException 而崩溃。

这是我创建 ListView 和适配器的代码:

private ArrayList<MessageItem> convList = new ArrayList<MessageItem>();

// setup list view
ListView list = (ListView)findViewById(R.id.conv_list);
MessageAdapter dataAdapter = new MessageAdapter(this, convList);
list.setAdapter(dataAdapter);
list.setVisibility(View.VISIBLE);

每当通过网络接收到新数据时，我都会尝试通过在 AsyncTask 中执行此操作来更新 ListView :

protected class ConversationTask extends AsyncTask<Context, String, String>
    {   
        @Override
        protected String doInBackground(Context... arg0) {
            for (;;) {
                while (t_recv.messageQueue.isEmpty());
                publishProgress(t_recv.messageQueue.remove(t_recv.messageQueue.size() - 1));
            }
        }

        @Override
        protected void onProgressUpdate(String... i)
        {
            convList.add(new MessageItem("Name", "Msg"));
            dataAdapter.notifyDataSetChanged();
        }
    }

调用 notifyDataSetChanged() 时出现 NullPointerException。如果我删除此调用， ListView 将仅在我按下后退按钮或显示/隐藏键盘时更新(我猜这会以某种方式触发更新)。那么...我在这里做错了什么吗？

适配器的代码如下:

public class MessageAdapter extends BaseAdapter
{
    private Context context;

    private ArrayList<MessageItem> listMessage;

    public MessageAdapter(Context context, ArrayList<MessageItem> listMessage) {
        this.context = context;
        this.listMessage = listMessage;
    }

    public View getView(int position, View convertView, ViewGroup viewGroup) {
        MessageItem entry = listMessage.get(position);
        if (convertView == null) {
            LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.message, null, false);
        }
        TextView tvName = (TextView) convertView.findViewById(R.id.tvName);
        tvName.setText(entry.getName());

        TextView tvMsg = (TextView) convertView.findViewById(R.id.tvMsg);
        tvMsg.setText(entry.getMsg());

        return convertView;
    }

    public int getCount() {
        return listMessage.size();
    }
    public Object getItem(int position) {
        return listMessage.get(position);
    }
    public long getItemId(int position) {
        return position;
    }
}

列表中的每一项都是一个“MessageItem”，如下所示:

public class MessageItem
{
    private String name;
    private String msg;

    public MessageItem(String name, String msg) {
        this.name = name;
        this.msg = msg;
    }

    // Getter methods
    public String getName() { return name; }
    public String getMsg() { return msg; }
}

此外，错误是:

04-21 13:56:30.762: ERROR/AndroidRuntime(30024): FATAL EXCEPTION: main
04-21 13:56:30.762: ERROR/AndroidRuntime(30024): java.lang.NullPointerException
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at com.client.Conversation.appendMessage(Conversation.java:129)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at com.client.Conversation.access$2(Conversation.java:122)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at com.client.Conversation$ConversationTask.onProgressUpdate(Conversation.java:170)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at com.client.Conversation$ConversationTask.onProgressUpdate(Conversation.java:1)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:432)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at android.os.Looper.loop(Looper.java:143)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at android.app.ActivityThread.main(ActivityThread.java:5068)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at java.lang.reflect.Method.invokeNative(Native Method)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at java.lang.reflect.Method.invoke(Method.java:521)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
04-21 13:56:30.762: ERROR/AndroidRuntime(30024):     at dalvik.system.NativeStart.main(Native Method)

谢谢

最佳答案

代替

dataAdapter.notifyDataSetChanged(); 

你应该试试

((ListView)findViewById(R.id.conv_list)).getAdapter().notifyDataSetChanged(); 

你的 MessageAdapter dataAdapter 成员也是私有(private)变量吗？我想是的，否则你应该得到编译错误。
因此，如果是，请确保你初始化了那个，不要在你的 onCreate 方法中有另一个同名的声明(这样你的私有(private)类成员将保持为空!)。

代码中的这一行建议您为 ListView 分配一个局部变量适配器:

MessageAdapter dataAdapter = new MessageAdapter(this, convList);

关于android - BaseAdapter 在调用 notifyDataSetChanged() 时崩溃，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/5741639/