怎样用java获得想要的日期？

public static void main(String[] args) throws Exception {

        SimpleDateFormat sdf=new SimpleDateFormat("yyyyMMdd");
        String str="20150101";
        Date dt=sdf.parse(str);
        Calendar rightNow = Calendar.getInstance();
        rightNow.setTime(dt);
        //rightNow.add(Calendar.YEAR,-1);//日期减1年
        rightNow.add(Calendar.MONTH,9);//日期加9个月
        //rightNow.add(Calendar.DAY_OF_YEAR,10);//日期加10天
        Date dt1=rightNow.getTime();
        String reStr = sdf.format(dt1);
        System.out.println(reStr);

    }

}