Android实现两次按上返回键退出

@Override    public boolean onKeyDown(int keyCode, KeyEvent event)     {			     if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN)			     {			             					     if((System.currentTimeMillis()-exitTime) > 2000)  //System.currentTimeMillis()无论何时调用，肯定大于2000					     {					      Toast.makeText(getApplicationContext(), "再按一次退出程序",Toast.LENGTH_SHORT).show();                                					      exitTime = System.currentTimeMillis();					     }					     else					     {					         finish();					         System.exit(0);					     }					             					     return true;			     }			     return super.onKeyDown(keyCode, event);    }