当我尝试更新onPostExecute（…）方法中的字段时，Android应用程序崩溃

Question

I am trying to set the value of myString in onPostExecute(..) method. Well, I am sure that myString is set there. Next I want to set a ListView containing also myString which was set in the onPostExecute(..) method. But when I run the application it crashes, and it crashes only because I am adding myString in the ArrayList .

What is wrong with setting values in onPostExecute(..) method? How can I fix it? Thanks in advance.

public class SearchTwitter extends ListActivity {
private ArrayList<String> events;
private ArrayAdapter<String> adapter;
private String myString;

public void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.search);
    searchTweets();

    events = new ArrayList<String>();
    events.add("Item");
    events.add(myString); // Application crashes because of that line

    adapter = new ArrayAdapter<String> (this, android.R.layout.simple_list_item_1, events);

    setListAdapter(adapter);
}
private void searchTweets()
{
    new SearchTweets().execute("Hello World");
}

class SearchTweets extends AsyncTask<String, String, List<Status>> {
    @Override
    protected void onPostExecute(List<Status> result) 
    {
          myString = result.get(0).getUser().getScreenName();
    }
    @Override
    protected List<Status> doInBackground(String... params) 
    {
        //Do the work here and return a List<Status>
    }    
}

EDIT 1

06-22 10:39:51.073: E/AndroidRuntime(2696): FATAL EXCEPTION: main
06-22 10:39:51.073: E/AndroidRuntime(2696): Process: com.bledi.android.twittertest, PID: 2696
06-22 10:39:51.073: E/AndroidRuntime(2696): java.lang.NullPointerException
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:394)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.AbsListView.obtainView(AbsListView.java:2240)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.ListView.measureHeightOfChildren(ListView.java:1263)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.ListView.onMeasure(ListView.java:1175)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.View.measure(View.java:16497)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:719)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:455)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.View.measure(View.java:16497)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.View.measure(View.java:16497)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.View.measure(View.java:16497)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.View.measure(View.java:16497)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1916)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1113)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1295)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.Choreographer.doCallbacks(Choreographer.java:574)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.Choreographer.doFrame(Choreographer.java:544)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.os.Handler.handleCallback(Handler.java:733)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.os.Handler.dispatchMessage(Handler.java:95)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.os.Looper.loop(Looper.java:136)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at android.app.ActivityThread.main(ActivityThread.java:5017)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at java.lang.reflect.Method.invokeNative(Native Method)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at java.lang.reflect.Method.invoke(Method.java:515)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
06-22 10:39:51.073: E/AndroidRuntime(2696):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

Your onCreate() should look like this:

public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.search);
    searchTweets();

}

and your onPostExecute() should look like this:

@Override
protected void onPostExecute(List<Status> result) {

    myString = result.get(0).getUser().getScreenName();

    events = new ArrayList<String>();
    events.add("Item");
    events.add(myString);

    adapter = new ArrayAdapter<String> (SearchTwitter.this, android.R.layout.simple_list_item_1, events);

    setListAdapter(adapter);
}

Try this. It should work.