java中字符串的格式以及日期的加减运算

String str ="2009-1-1"
java.text.SimpleDateFormat sdf  = new java.text.SimpleDateFormat("yyyy-MM-dd");
java.util.Date date =sdf.parse(str);
str=sdf.format(date); // str 的格式就变成了 "2009-01-01"
 

java.util.Date date =new java.util.Date();
Calendar calendar =new java.util.GregorianCalendar();
calendar.setTime(date);
calendar.add(Calendar.DAY_OF_MONTH ,+1);//当前月+1
calendar.add(Calendar.MONTH,-12);//去年当前月
int year = calendar.get(Calendar.YEAR);
int month =calendar.get(Calendar.MONTH);//取到的月份是上个月的
int month_day = calendar.get(Calendar.DAY_OF_MONTH);