Thread and handler not work in android

Question

I use loading before load completed listview

This code worked :

@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); final ListView listContent = (ListView) findViewById(R.id.listContent); adapter = new AdapterNote(G.tasks); listContent.setAdapter(adapter); Commands.readData(); adapter.notifyDataSetChanged(); }

But after changing to the below code , application crashed and force closed :

@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); startDialog(); } ProgressDialog pd; private void startDialog() { pd = ProgressDialog.show(NoteActivity.this, "title", "loading"); new Thread(new Runnable() { @Override public void run() { final ListView listContent = (ListView) findViewById(R.id.listContent); adapter = new AdapterNote(G.tasks); listContent.setAdapter(adapter); Commands.readData(); adapter.notifyDataSetChanged(); handler.sendEmptyMessage(0); } }).start(); } Handler handler = new Handler() { @Override public void handleMessage(Message msg) { pd.dismiss(); } };

In log cat these errors occurred :

03-08 00:27:09.734: E/AndroidRuntime(334): FATAL EXCEPTION: main 03-08 00:27:09.734: E/AndroidRuntime(334): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.NoteProject.download.app/com.NoteProject.download.app.NoteNoteProjectActivity}: java.lang.NullPointerException 03-08 00:27:09.734: E/AndroidRuntime(334): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.app.ActivityThread.access$2300(ActivityThread.java:125) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.os.Handler.dispatchMessage(Handler.java:99) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.os.Looper.loop(Looper.java:123) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.app.ActivityThread.main(ActivityThread.java:4627) 03-08 00:27:09.734: E/AndroidRuntime(334): at java.lang.reflect.Method.invokeNative(Native Method) 03-08 00:27:09.734: E/AndroidRuntime(334): at java.lang.reflect.Method.invoke(Method.java:521) 03-08 00:27:09.734: E/AndroidRuntime(334): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 03-08 00:27:09.734: E/AndroidRuntime(334): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 03-08 00:27:09.734: E/AndroidRuntime(334): at dalvik.system.NativeStart.main(Native Method) 03-08 00:27:09.734: E/AndroidRuntime(334): Caused by: java.lang.NullPointerException 03-08 00:27:09.734: E/AndroidRuntime(334): at com.NoteProject.download.app.NoteNoteProjectActivity$2.onScroll(NoteNoteProjectActivity.java:79) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.widget.AbsListView.invokeOnItemScrollListener(AbsListView.java:675) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.widget.AbsListView.setOnScrollListener(AbsListView.java:664) 03-08 00:27:09.734: E/AndroidRuntime(334): at com.NoteProject.download.app.NoteNoteProjectActivity.onCreate(NoteNoteProjectActivity.java:69) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 03-08 00:27:09.734: E/AndroidRuntime(334): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627) 03-08 00:27:09.734: E/AndroidRuntime(334): ... 11 more

How to fix it ?

In commands.readdata() , return listview from web

Thx

You cannot update a adapter this way, I have had the same problem in my current project. — Apurva
– Apurva, Commented Mar 8, 2015 at 12:55
possible duplicate of What is a Null Pointer Exception, and how do I fix it? — Simon
– Simon, Commented Mar 8, 2015 at 12:58

littlet · Accepted Answer · 2015-03-08 13:03:51Z

adapter.notifyDataSetChanged(); must be run at UI thread, this will works:

 private void startDialog() { pd = ProgressDialog.show(NoteActivity.this, "title", "loading"); final ListView listContent = (ListView) findViewById(R.id.listContent); adapter = new AdapterNote(G.tasks); listContent.setAdapter(adapter); new Thread(new Runnable() { @Override public void run() { Commands.readData(); handler.sendEmptyMessage(0); } }).start(); } Handler handler = new Handler() { @Override public void handleMessage(Message msg) { adapter.notifyDataSetChanged(); pd.dismiss(); } };

Simar · Accepted Answer · 2015-03-08 13:01:11Z

Updatinh the liatview is a UI operation and it needs to be performed on the main (UI) thread.

Kunal · Accepted Answer · 2015-03-08 13:08:02Z

You can use Async task to load listview in background. Code written below works for me fine.

 @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); final ListView listContent = (ListView) findViewById(R.id.listContent); new AsyncTask<Void, Void, Void>() { @Override protected void onPreExecute() { pd = ProgressDialog.show((NoteActivity.this, "title", "loading", true, false); }// End of onPreExecute method @Override protected Void doInBackground(Void... params) { //reading data for listview Commands.readData(); return null; }// End of doInBackground method @Override protected void onPostExecute(Void result) { pd.dismiss(); //setting adapter here adapter = new AdapterNote(G.tasks); listContent.setAdapter(adapter);}//End of onPostExecute method }.execute((Void[]) null); }

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