Question

基于@Tom解决方案进行更新：

SongsManager.java

public class SongsManager extends AsyncTask<Void, Void, ArrayList<HashMap<String, String>>>    {

          public interface SongsMasterCallback {         
        void showSongList(List<HashMap<String, String>> result);     
    } 

    private SongsMasterCallback  mCallback;      
    public SongsManager (SongsMasterCallback  callback) 
    {         
        mCallback = callback;    
    }   

    // Constructor
    //public SongsManager(){ }


    ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();
    @Override
    protected ArrayList<HashMap<String, String>>  doInBackground(Void... params) 
          {
             //populating all the data....
       HashMap<String, String> map = new HashMap<String, String>();
             //.........
             songsList.add(map);
    }

       return songsList;
    }

    @Override
    protected void onPostExecute(ArrayList<HashMap<String, String>> result) {
        // TODO Auto-generated method stub
        //super.onPostExecute(result);
        mCallback.showSongList(result); 
    } 
}

main.java

public class Main extends Activity implements SongsMasterCallback {

public void showSongList(List<HashMap<String, String>> result) 
{
     this.songsList = (ArrayList<HashMap<String, String>>) result;     
     // then do something with the list here 
}

@Override
public void onCreate(Bundle savedInstanceState) {

  new SongsManager(this).execute();
  Log.d("songsSize", "string : "+songsList.size()); //it shows 0 ??? 

}

}

更新结束

我正在使用AsyncTask开发一个类，下面是我尝试执行时得到的内容

错误在设计时间，因此没有LogCat

这是班级：

SongManager.java

public class SongsManager extends AsyncTask<Void, Void, ArrayList<HashMap<String, String>>> {


// Constructor
public SongsManager(){ }


@Override
    protected ArrayList<HashMap<String, String>>  doInBackground(Void... params) {
        // TODO Auto-generated method stub

        ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();

        Xml_Parser_Custom_Listview parser = new Xml_Parser_Custom_Listview();
        String xml = parser.getXmlFromUrl(URL); // getting XML from URL
        Document doc = parser.getDomElement(xml); // getting DOM element

        NodeList nl = doc.getElementsByTagName(KEY_NAME);
        // looping through all song nodes <song>
        for (int i = 0; i < nl.getLength(); i++) {
            // creating new HashMap
            HashMap<String, String> map = new HashMap<String, String>();
            Element e = (Element) nl.item(i);
            // adding each child node to HashMap key => value
            //adding the rows into to the map...

            // adding HashList to ArrayList
            songsList.add(map);
        }

        return songsList;
    }


}

main.java文件

尝试执行AsyncTask：

public ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();
songsList = new SongsManager().execute();

这是错误：

Type mismatch: cannot convert from AsyncTask<Void,Void,ArrayList<HashMap<String,String>>> to ArrayList<HashMap<String,String>>

Answer 1

SongsManager.execute()返回的值不是计算结果;这是AsyncTask本身。要发布结果，请在SongsManager中编写方法：

void onPostExecute(ArrayList<HashMap<String,String>> result) {
    songsList = result; // or something similar
    // then update the UI to reflect the results
}

在此方法中，songsList是您现在尝试从new SongsManager.execute()分配返回值的变量。如果SongsManager类看不到此变量（例如，它不是主类的嵌套类），那么您将不得不安排回调：

public class SongsManager
    extends AsyncTask<Void, Void, List<Map<String,String>>
{
    public interface SongsMasterCallback {
        void showSongList(List<Map<String, String>> result);
    }

    private SongsMasterCallback mCallback;

    public SongsMaster(SongsMasterCallback callback) {
        mCallback = callback;
    }

    @Override
    protected List<Map<String, String>>  doInBackground(Void... params) {
        // as before
    }

    @Override
    protected void onPostExecute(List<Map<String, String>> result) {
        mCallback.showSongList(result);
    }
}

然后将您的活动声明为implement SongsManager.SongsManagerCallback并添加方法：

void showSongList(List<Map<String, String>> songList) {
    this.songList = songList;
    // then do something with the list here
}

Answer 2

这是因为在创建/执行AsyncTask时没有得到ArrayList<HashMap<String, String>>。你可以在onPostExecute（）上找回它。您应该像这样创建/执行任务

SongsManager songsList = new SongsManager().execute();

然后像你这样在你的AsyncTask中有一个onPostExecute

protected void onPostExecute(ArrayList<HashMap<String, String>> result) 
{
  // do stuff with the ArrayList<HashMap<String, String>> result parameter.
}

类型不匹配：无法从AsyncTask转换

2 个答案: