ListView aktualisieren

public class Foo extends Activity {

    private ListView listView;
    private SimpleCursorAdapter adapter;
    
    public void onCreate(Bundle savedInstanceState) {
        //...
        
      saveButton.setOnClickListener(new OnClickListener() {
                    
            public void onClick(View v) {
                save(); // neuen Eintrag bei Buttonklick in der DB speichern
            }
        });
        
        fillData(); // alle aktuellen Einträge anzeigen

    }
    
    private void fillData() {
        String[] projection = {Table.COLUMN_ID, A.COLUMN_A};
        String[] from = {A.COLUMN_A};
        int[] to = {R.id.tx_a_entry};;
        Cursor cursor = getContentResolver().query(MyContentProvider.CONTENT_URI_A, projection, null,  null,  null);
        
        if(cursor.getCount() == 0) {            
        }
        else {
            adapter = new SimpleCursorAdapter(this, R.layout.a_row, cursor, from, to, 0);
            listView.setAdapter(adapter);                        
        }
        
    }
    
    private void save() {
        String str = ((EditText) findViewById(R.id.ed_enter_a)).getText().toString();
        ContentValues values = new ContentValues();
        values.put(Table.COLUMN_A, str);
        
        getContentResolver().insert(MyContentProvider.CONTENT_URI_A, values);
        adapter.notifyDataSetChanged(); // funktioniert nicht. Warum?

    }
    
    
}