在异步任务中：尝试在空对象引用上调用虚拟方法'void android.widget.ListView.setAdapter(android.widget.ListAdapter)'

Question

我想要什么

当点击地图中的标记时，显示带有列表视图（包含文本）的警报对话框

LogCat错误

05-01 23:20:21.098 11392-11654/com.example.talha.atmlocaterproject W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at com.example.talha.atmlocaterproject.MainActivity$override.retreive_message(MainActivity.java:372)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at com.example.talha.atmlocaterproject.MainActivity$override.access$dispatch(MainActivity.java)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at com.example.talha.atmlocaterproject.MainActivity.retreive_message(MainActivity.java:0)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at com.example.talha.atmlocaterproject.MainActivity$GetAtms.doInBackground(MainActivity.java:300)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at com.example.talha.atmlocaterproject.MainActivity$GetAtms.doInBackground(MainActivity.java:257)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at android.os.AsyncTask$2.call(AsyncTask.java:288)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
05-01 23:20:21.099 11392-11654/com.example.talha.atmlocaterproject W/System.err:     at java.lang.Thread.run(Thread.java:818)

我所做的是在OnMarkerClick（）中调用AsyncTask从云中获取消息。

@Override
public boolean onMarkerClick(Marker marker) {
    String atm_title = marker.getTitle();
    option = "retreive_message";
    new GetAtms(atm_title).execute();      //here is the call to asyncTask
    return false;
} 

现在在doInBackground（）方法中，我从云中检索消息并设置适配器

DBCollection c5  = db.getCollection("ATM_message");
    String[] atm_message = new String[50];
    try{
        Log.d("retreive message","before lopping");
        DBCursor cursor = c5.find();
        String atm_title,message,username;
        int message_adding_counter = 0;
        while (cursor.hasNext()) {
            DBObject doc = cursor.next();
            atm_title = "" + doc.get("atm_name");
            message = "" + doc.get("message");
            username ="" + doc.get("username");
            Log.d("retreive message","between lopping");
            if(atm_title.equalsIgnoreCase(title)){
                atm_message[message_adding_counter]=message;
                Log.d("retreive message"," adding message");
            }
        }
        Log.d("retreive message","" + atm_message[0]);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,R.layout.list_item,R.id.textView,atm_message);
        listview.setAdapter(adapter);
        showDailogListView(listview);

showDailogListView（）

public void showDailogListView(View view){
    AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
    builder.setCancelable(true);
    builder.setPositiveButton("OK",null);
    builder.setView(view);
    AlertDialog dailog = builder.create();
    dailog.show();
}

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">

<TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="New Text"
    android:padding="10dp"
    android:id="@+id/textView"
    android:layout_gravity="center_horizontal" />
</LinearLayout>

OnCreate（）

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mProgressDialog= new ProgressDialog(this);
    try {
        MapFragment mapFragment = (MapFragment) getFragmentManager().findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Answer 1

尝试使用onPostExecute来处理您的UI:

更改AsyncTask:的签名

public class GetAtms extends AsyncTask<Void, Void, String[]> {
    private ArrayAdapter<String> mAdapter;
    public GetAtms(String title, ArrayAdapter<String> adapter)
    {
        mAdapter = adapter;
        //the rest of the constructor codes
    }
    //rest of your code

让您的doInBackground将结果字符串数组返回给onPostExecute

DBCollection c5  = db.getCollection("ATM_message");
String[] atm_message = new String[50];
try{
    Log.d("retreive message","before lopping");
    DBCursor cursor = c5.find();
    String atm_title,message,username;
    int message_adding_counter = 0;
    while (cursor.hasNext()) {
        DBObject doc = cursor.next();
        atm_title = "" + doc.get("atm_name");
        message = "" + doc.get("message");
        username ="" + doc.get("username");
        Log.d("retreive message","between lopping");
        if(atm_title.equalsIgnoreCase(title)){
            atm_message[message_adding_counter]=message;
            Log.d("retreive message"," adding message");
        }
    }
    Log.d("retreive message","" + atm_message[0]);
    return atm_message;

在onPostExecute do:中

public void onPostExecute(String[] data)
{
    mAdapter.clear();
    mAdapter.addAll(data);
}

最后更改您的onMarkerClick:

public boolean onMarkerClick(Marker marker) {
    String atm_title = marker.getTitle();
    option = "retreive_message";
    ListView listview = (ListView) findViewById(...) //your listview resource
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,R.layout.list_item,R.id.textView,null);
    listView.setAdapter(adapter);
    new GetAtms(atm_title,adapter).execute();      //here is the call to asyncTask
    return false;
} 

Answer 2

更好的方法是从doInBackground方法返回数组适配器，并在onPreExecute方法中设置进度对话框；

doInBackground(){
//your code
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,R.layout.list_item,R.id.textView,atm_message);
return adapter;
}
@Override
    protected void onPostExecute(ArrayAdapter<String> result) {
      return result;
    }

//////////////

public boolean onMarkerClick(Marker marker) {
    String atm_title = marker.getTitle();
    option = "retreive_message";
   ArrayAdapter<String> adapter=(new GetAtms(atm_title)).execute();      
   listview.setAdapter(adapter);
    return false;
} 

Answer 3

如果你面对空对象引用，请首先检查所有文本视图是否正确findviewbyid完成，其次无论你设置了什么适配器，请在postexecute方法中进行，它都会起作用。