XML : How to force an update of the main activity from 'outside' in android?

This question is a kind of followup on this unanswered question with more and better details.

Basically, I have an android main activity showing some data which are taken from a database (via SQLiteDatabase). Now I have a PreferenceActivity which is shown when the user wants to change some preference, or delete the content of the database. In the latter case the content of the database is deleted - but how do I make the main activity to be updated? If I look at the main activity after I pressed the button to delete the content of the database, I expect to show the main activity zero entries!

Here is the relevant part of the main activity:

  public class MainActivity extends AppCompatActivity  {        SharedPreferences.OnSharedPreferenceChangeListener mPrefListener;      private SharedPreferences settings;        @Override      protected void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);            Intent intent = new Intent(this, AlarmService.class);          startService(intent);          setContentView(R.layout.activity_main);            Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);          setSupportActionBar(toolbar);            settings = PreferenceManager.getDefaultSharedPreferences(this);          mPrefListener = new SharedPreferences.OnSharedPreferenceChangeListener() {              @Override              public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {                  Log.d("TEST", "testtest");                    Toast.makeText(MainActivity.this, "TOAST MAINACTIVITY", Toast.LENGTH_LONG).show();                    finish();                  startActivity(getIntent());              }          };            // Register the listener on the SharedPreferences          settings.registerOnSharedPreferenceChangeListener(mPrefListener);    

Here is the content of UserSettingActivity.java:

  public class UserSettingActivity extends PreferenceActivity {        private Preference myPreference;        @Override      protected void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          addPreferencesFromResource(R.xml.preferences);          myPreference = findPreference("reset");          myPreference.setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {              public boolean onPreferenceClick(Preference arg0) {                  AlertDialog.Builder alertDialog = new AlertDialog.Builder(UserSettingActivity.this);                  alertDialog.setMessage("Are you sure to delete the database?");                  alertDialog.setCancelable(true);                  alertDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener() {                      public void onClick(DialogInterface dialog, int which) {                          final DBAdapter db = new DBAdapter(UserSettingActivity.this);                          db.open();                          db.resetDatabase();                        } });                  alertDialog.setNegativeButton("No", new DialogInterface.OnClickListener() {                      public void onClick(DialogInterface dialog, int which) {                          dialog.cancel();                      } });                  alertDialog.show();                  return false;              }          });        }    }    

And finally, the xml file describing the layout of the settings in xml/preferences.xml:

  <?xml version="1.0" encoding="utf-8"?>  <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">      <EditTextPreference android:title="Your Name"          android:key="username"          android:summary="Please provide your username"></EditTextPreference>      <CheckBoxPreference android:title="Application Updates"          android:defaultValue="false"          android:summary="This option if selected will allow the application to check for latest versions."          android:key="applicationUpdates" />        <Preference          android:key="reset"          android:title="Reset database"          android:summary="This will remove every entry in the database"           />  </PreferenceScreen>    

When I click on the button reset on the Settings screen and confirm this choice, the database is deleted resetDatabase does a DROP and a CREATE TABLE of the two tables in the database). However, the Toast 'TOAST MAINACTIVITY' is never shown on the display!

What do I miss here to make this work as I expect it to work? How do I trigger a method/function/something in my main activity from the UserSettingActivity?