@Override     public boolean onKeyDown(int keyCode, KeyEvent event) {         if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {             showDialog(APP_EXIT);             return true;         } else             return super.onKeyDown(keyCode, event);     }         @Override     protected Dialog onCreateDialog(int id) {         if (id == APP_EXIT) {             return new AlertDialog.Builder(BaseActivity.this)                     .setMessage("是否退出程序?")                     .setTitle("用户退出")                     .setPositiveButton("确定",                             new DialogInterface.OnClickListener() {                                     public void onClick(DialogInterface dialog,                                         int which) {                                     dialog.dismiss();                                     android.os.Process                                             .killProcess(android.os.Process                                                     .myPid());                                     finish();                                     }                             })                     .setNegativeButton("取消",                             new DialogInterface.OnClickListener() {                                     public void onClick(DialogInterface dialog,                                         int which) {                                     dialog.dismiss();                                     }                             }).create();             }         return null;         }