Question

我制作了以下活动类来制作列表视图。

但是我在启动它的时候有NPE。 java文件和logcat如下：

活动类

public class CategoryListActivity extends Activity implements OnItemClickListener{
     //URL to get JSON Array
    private static String url = "http://code-explore.com/app/webroot/sp-codeexplore/getCategoryList.php?platform=Android";

    //JSON Node Names 
    private static final String TAG_CATEGORY = "categories";
    private static final String TAG_ID = "cat_id";
    private static final String TAG_TITLE = "cat_title";
    private static final String TAG_NO = "total_apps";

    JSONArray categories = null;

    public final static String CAT_ID = "com.smartmux.codeexplore.MESSAGE";

    ArrayList<String> list = new ArrayList<String>();

    ProgressDialog pDialog = new ProgressDialog(this);



ListView listView;
List<RowItem> rowItems = new ArrayList<RowItem>();

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);

    setContentView(R.layout.category);

    new SM_AsyncTask().execute(url);

}

    public void returnHome(View view) {
        onBackPressed();
    }



    class SM_AsyncTask extends AsyncTask<String, Integer, List<RowItem>> {
         ProgressDialog pDialog=null;

     @Override
     protected void onPreExecute() {

            pDialog.setMessage("Please wait..");
            pDialog.setIndeterminate(true);
            pDialog.setCancelable(false);
            pDialog.show();       
            }

    @Override
    protected List<RowItem> doInBackground(
            String... params) {
         // Creating new JSON Parser
        JSONParser jParser = new JSONParser();

        // Getting JSON from URL
        JSONObject json = jParser.getJSONFromUrl(url);

        try {
            // Getting JSON Array
            categories = json.getJSONArray(TAG_CATEGORY);

            for (int i = 0; i < categories.length(); i++) {
              JSONObject c = categories.getJSONObject(i);
              String title = c.getString(TAG_TITLE);
              String number = c.getString(TAG_NO);
              String id = c.getString(TAG_ID);
              list.add(id);


              RowItem item = new RowItem(title, number);
              rowItems.add(item);


            }

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

        return rowItems;
    }

    }

    protected void onPostExecute(List<RowItem> result) {

    pDialog.dismiss();

    listView = (ListView) findViewById(R.id.listCategory);
    ArrayAdapterForCategory adapter = new ArrayAdapterForCategory(this,
            R.layout.row_cat_list, result);
    listView.setAdapter(adapter);
    listView.setOnItemClickListener(this);
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position,
            long id) {

        String catId = list.get(position);
        Intent intent = new Intent(CategoryListActivity.this, AppListActivity.class);
        intent.putExtra(CAT_ID, catId);
        startActivity(intent);

    }
}

logcat的

12-03 08:36:59.370: E/AndroidRuntime(2082): FATAL EXCEPTION: main
12-03 08:36:59.370: E/AndroidRuntime(2082): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.smartmux.codeexplore/com.smartmux.codeexplore.CategoryListActivity}: java.lang.NullPointerException
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2137)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.os.Looper.loop(Looper.java:137)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.ActivityThread.main(ActivityThread.java:5103)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at java.lang.reflect.Method.invokeNative(Native Method)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at java.lang.reflect.Method.invoke(Method.java:525)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at dalvik.system.NativeStart.main(Native Method)
12-03 08:36:59.370: E/AndroidRuntime(2082): Caused by: java.lang.NullPointerException
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:146)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:103)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:143)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.AlertDialog.<init>(AlertDialog.java:98)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.ProgressDialog.<init>(ProgressDialog.java:77)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at com.smartmux.codeexplore.CategoryListActivity.<init>(CategoryListActivity.java:39)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at java.lang.Class.newInstanceImpl(Native Method)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at java.lang.Class.newInstance(Class.java:1130)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
12-03 08:36:59.370: E/AndroidRuntime(2082):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2128)
12-03 08:36:59.370: E/AndroidRuntime(2082):     ... 11 more

基本上，调用AsyncTask时会出现问题。

Answer 1

更改为

ProgressDialog pDialog;
ListView listView;
List<RowItem> rowItems = new ArrayList<RowItem>();

@Override
public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);

    setContentView(R.layout.category);
    pDialog = new ProgressDialog(this); // initialize in onCreate

删除

ProgressDialog pDialog=null; // already declared

在Asynctask中

另外

 @Override
 protected void onPreExecute() {
 super.onPreExcute()

和

 @Override
 protected void onPostExecute(List<RowItem> result) {
 super.onPostExecute(result);
 listView = (ListView) findViewById(R.id.listCategory);
 ArrayAdapterForCategory adapter = new ArrayAdapterForCategory(CategoryListActivity.this,
        R.layout.row_cat_list, result);
 listView.setAdapter(adapter);
 listView.setOnItemClickListener(CategoryListActivity.this);
 }

更改此

  return rowItems;
}
}

要

  return rowItems;
}

也改变。你错放了}

      startActivity(intent);
     }
 }

要

      startActivity(intent);
      }
    } 
 }

Answer 2

在Asynctask中删除此行：

ProgressDialog pDialog = null;或者
之后的init进度对话框

在android中的AsyncTask期间出现NullPointerException

2 个答案: