JAVA 日期获得30天前的日期

Calendar calendar=new GregorianCalendar();
System.out.println("当时日期是:"+calendar.getTime());
calendar.add(Calendar.DATE, -30);
System.out.println("30天前是:"+calendar.getTime());

要获得calendar的相应field值, 可以用calendar.get(int field)来取.
field可以是Calendar.Date, Month, Year, DayOfMonth, DayOfWeek等.