Android点击返回按钮弹出提示“点击两次退出程序”

    //定义一个用于判断退出时的时间    private long mExitTime;    //对返回键进行监听    @Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {            exit();            return true;        }        return super.onKeyDown(keyCode, event);    }    public void exit() {        if ((System.currentTimeMillis() - mExitTime) > 2000) {            Toast.makeText(LoginActivity.this, "再按一次退出程序", Toast.LENGTH_SHORT).show();            mExitTime = System.currentTimeMillis();        } else {            //MyConfig.clearSharePre(this, "users");            finish();            System.exit(0);        }    }