Wie kann Listener andere Activity informieren?

public class M_Einstellungen extends PreferenceActivity 
                             implements OnSharedPreferenceChangeListener {
@Override protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        addPreferencesFromResource(R.xml.m_einstellungen);
    }
// ...
    public void onSharedPreferenceChanged(SharedPreferences sPref, String key) {
        ListPreference lp = (ListPreference)getPreferenceScreen() 
                            .findPreference(key);
        lp.setSummary("... wird berechnet in: " +  sPref.getString(key, ""));
    }
}

public class Rechnungen extends Activity {
// ...
@Override public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.rechnungen);
        
        // TemperaturEinheit einstellen: 
        SharedPreferences sharedPreferences = PreferenceManager
              .getDefaultSharedPreferences(getBaseContext());  
        ((TextView) findViewById(R.id.ehTemp)).setText(sharedPreferences
              .getString(getBaseContext().getString(R.string.keyTemp),"??"));  
         // Berechnungen + Ausgabe ... über diverse TextWatcher ...
    }
@Override public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.optionen, menu);
        return true;
    }
@Override public boolean onOptionsItemSelected(MenuItem item){
        Intent intent;
        switch (item.getItemId()) {
        case R.id.m_einstellungen:
            intent = new Intent( this, M_Einstellungen.class );
            startActivity(intent);            
            break;
        }
   }
// div TextWatcher ...
}