Android实现两次按下返回键退出

@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);    }