java 时间日期的判定和时间的加减

http://blog.csdn.net/zhengqiqiqinqin/article/details/20630415

比如现在有个String str =”2013-07-18”;
现在要在这个时间上加8个月就变成了“2014-03-18”
怎么完成这种时间的转换加减

String str = "2013-07-18";
SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
try {
    Date myDate = formatter.parse(str);
    Calendar c = Calendar.getInstance();
    c.setTime(myDate);
    c.add(Calendar.MONTH, 8);
    myDate = c.getTime();
    System.out.println(formatter.format(myDate));
} catch (ParseException e1) {
    e1.printStackTrace();
}