Android “再按一次退出“

@Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) 
        {
            if ((System.currentTimeMillis() - mExitTime) > 2000) { //第一种：判断2次按后退键间隔是否在2秒内
            /*if(event.getRepeatCount() == 0) //第二种，判断是否重复按后退键
            {*/
                System.out.println(mExitTime);
                Object mHelperUtils;
                showTips(R.drawable.tips_smile, "再按一次返回桌面");
                mExitTime = System.currentTimeMillis();

            } else {
                finish();
            }
            return true; 
        }
        return super.onKeyDown(keyCode, event);
    }