Android 如何从不同的类文件运行asynctask？

当我在一个类文件中使用我的代码时，它可以完美地运行，但当从主活动中分离异步方法时：

GetCategories class:

package ir.reza.smss;


import ir.reza.smss.Fragments.TabFragment1;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.ArrayList;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;

public class GetCategories extends AsyncTask<Void, Void, Void> {

private Context context;
private ArrayList<Category> categoriesList;
ProgressDialog pDialog;
// Url to get all categories
private String URL_CATEGORIES = "http://varzesh-ilam.xzn.ir/app/get_categories.php";

public GetCategories(Context context) {
    this.context = context;
}

@Override
protected void onPreExecute() {
    super.onPreExecute();
    pDialog = new ProgressDialog(context);
    pDialog.setMessage("Loading...");
    pDialog.setCancelable(false);
    pDialog.show();

}

@Override
protected Void doInBackground(Void... arg0) {
    ServiceHandler jsonParser = new ServiceHandler();
    String json = jsonParser.makeServiceCall(URL_CATEGORIES, ServiceHandler.GET);

    Log.e("Response: ", "> " + json);

    if (json != null) {
        try {
            JSONObject jsonObj = new JSONObject(json);
            if (jsonObj != null) {
                JSONArray categories = jsonObj
                        .getJSONArray("categories");                        

                for (int i = 0; i < categories.length(); i++) {
                    JSONObject catObj = (JSONObject) categories.get(i);
                    Category cat = new Category(catObj.getInt("id"),
                            catObj.getString("name"));
                    categoriesList.add(cat);
                }
            }

        } catch (JSONException e) {
            e.printStackTrace();
        }

    } else {
        Log.e("JSON Data", "Didn't receive any data from server!");
    }

    return null;
}

@Override
protected void onPostExecute(Void result) {
    super.onPostExecute(result);
    if (pDialog.isShowing())
        pDialog.dismiss();
    TabFragment1 tb1 = new TabFragment1();
    tb1.populateSpinner();
}



}

我想从另一个类文件运行asyntask

---

我正在使用eclipse，而且我对java和android开发还不熟悉。

java.lang.NullPointerException位于ir.reza.smss.GetCategories.doInBackground（GetCategories.java:64）

使用调试程序检查这些值，并查看传递空对象的位置。（第64行）

可能重复的

01-22 11:17:38.475: I/global(9785): call createSocket() return a new socket.
01-22 11:18:22.465: W/dalvikvm(10293): threadid=12: thread exiting with uncaught     exception (group=0x417e9a08)
01-22 11:18:22.470: E/AndroidRuntime(10293): FATAL EXCEPTION: AsyncTask #1
01-22 11:18:22.470: E/AndroidRuntime(10293): java.lang.RuntimeException: An error     occured while executing doInBackground()
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     android.os.AsyncTask$3.done(AsyncTask.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     java.util.concurrent.FutureTask.finishCompletion(FutureTask.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     java.util.concurrent.FutureTask.setException(FutureTask.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     java.util.concurrent.FutureTask.run(FutureTask.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at java.lang.Thread.run(Thread.java)
01-22 11:18:22.470: E/AndroidRuntime(10293): Caused by: java.lang.NullPointerException
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     ir.reza.smss.GetCategories.doInBackground(GetCategories.java:64)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     ir.reza.smss.GetCategories.doInBackground(GetCategories.java:1)
01-22 11:18:22.470: E/AndroidRuntime(10293):    at     android.os.AsyncTask$2.call(AsyncTask.java)
01-22 11:18:22.470: E/AndroidRuntime(10293):    ... 5 more