android – AsyncTask – 扩展和doInBackground需要哪些参数？

使用AsyncTask的代码有什么问题？特别是：
– 我需要在fetchSchools中放入哪些参数
– 我需要在doInBACkground中放入哪些参数？

我发现了许多“有用的”示例,但它们都在这些参数中使用了伪代码,并没有解释我实际需要放在那里的内容.

“我得到Eclipse错误的方法fetchSchools必须实现继承的抽象方法AsynchTask …”

我不需要传递任何东西,我希望它返回一个字符串.

public class fetchSchools extends AsyncTask<Void,Void,String> {

public String doInBACkground(String retval) {
       StringBuilder builder = new StringBuilder();
        httpClient client = new DefaulthttpClient();

        httpGet httpGet = new httpGet("http://www.domain/schools.PHP");
        try 
     {
          httpResponse response = client.execute(httpGet);
          StatusLine statusLine = response.getStatusLine();
          int statusCode = statusLine.getStatusCode();
          if (statusCode == 200) {
            httpentity entity = response.getEntity();
            InputStream content = entity.getContent();
            BufferedReader reader = new BufferedReader(new InputStreamReader(content));
            String
     line;

            int a=0;
            while ((line = reader.readLine()) != null) {
              builder.append(linE);
            Log.i(MainActivity.class.getName(),"Reading in: " + a +" : "+ linE);
            a++;
            }
          } else {
            Log.e(MainActivity.class.toString(),"Failed to download file");
          }
        } catch (ClientProtocolException E) {
          e.printStackTrace();
        } catch (IOException E)
     {
          e.printStackTrace();
        }

        return builder.toString(); 
}

protected void onPostExecute() {
}

}

AsyncTask<String,String> {

public class fetchSchools extends AsyncTask<Void,String> {
    @Override
    protected String doInBACkground(Void... arg0) {
      StringBuilder builder = new StringBuilder();
      httpClient client = new DefaulthttpClient();
      // ...

     return builder.toString();
    }
    protected void onPostExecute(String retval) 
    {

    }
  }