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()不管何时调用，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);
    }