java 指定日期後n天

RT

　　算時間本來就是我的弱項：不廢話了，貼代碼

　　想傳什麼參數自己在改改就ok，傳入String，放回String   

public class Text {
    public static void main(String[] args) {
        String s =dayAdd("20180131");
        System.out.println(s);
    }
    public static String dayAdd(String now){
        SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
        String datastr = null;
        try {
            Date d = sdf.parse(now);
            Calendar ca = Calendar.getInstance();
            ca.setTime(d);
            
            ca.set(ca.get(Calendar.YEAR),ca.get(Calendar.MONTH),ca.get(Calendar.DAY_OF_MONTH)+7);
            
            datastr = sdf.format(ca.getTime());
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return datastr;
    }
}