Question

我通过扩展BaseAdapter创建了自定义列表视图和适配器。每次我尝试在适配器上调用notifyDataSetChanged（）时，程序都会发生NullPointerException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);

每次通过网络收到新数据时，我都会尝试在AsyncTask中执行此更新列表视图：

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（）时发生NullPointerException。如果我删除此调用，列表视图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）。所以......我在这里做错了吗？

如果需要，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;
    }
}

列表中的每个项目都是'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

Answer 1

而不是

dataAdapter.notifyDataSetChanged();

你应该试试

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

您的MessageAdapter dataAdapter成员也是私有变量吗？我认为是，否则你应该得到编译错误。
所以，如果确实如此，请确保初始化那个，在onCreate方法中没有一个具有相同名称的其他声明（这样你的私有类成员将保持为null！）。

代码中的这一行表明您为ListView分配了一个本地变量适配器：

MessageAdapter dataAdapter = new MessageAdapter(this, convList);

BaseAdapter在notifyDataSetChanged（）调用时崩溃

1 个答案: