GregorianCalendar add, roll方法
细细体味 roll 和 add 的区别
直接上代码：
Using roll method
Case 1:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.FEBRUARY);
cal.set(Calendar.DAY_OF_MONTH, 29);
System.out.println(cal.getTime()); ==> Wed Feb 29 16:44:30 SGT 2012
cal.roll(Calendar.DAY_OF_YEAR, true);
System.out.println(cal.getTime()); ==> Thu Mar 01 16:44:30 SGT 2012
Case 2:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.FEBRUARY);
cal.set(Calendar.DAY_OF_MONTH, 29);
System.out.println(cal.getTime()); ==> Wed Feb 29 16:48:14 SGT 2012
cal.roll(Calendar.DAY_OF_MONTH, true);
System.out.println(cal.getTime()); ==> Wed Feb 01 16:48:14 SGT 2012
Case 3:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.DECEMBER);
cal.set(Calendar.DAY_OF_MONTH, 31);
System.out.println(cal.getTime()); ==> Mon Dec 31 16:50:12 SGT 2012
cal.roll(Calendar.DAY_OF_MONTH, true);
System.out.println(cal.getTime()); ==> Sat Dec 01 16:50:12 SGT 2012
Case 4:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.DECEMBER);
cal.set(Calendar.DAY_OF_MONTH, 31);
System.out.println(cal.getTime()); ==> Mon Dec 31 16:52:44 SGT 2012
cal.roll(Calendar.DAY_OF_YEAR, true);
System.out.println(cal.getTime()); ==> Sun Jan 01 16:52:44 SGT 2012
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Using add method
Case 1:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.FEBRUARY);
cal.set(Calendar.DAY_OF_MONTH, 29);
System.out.println(cal.getTime()); ==> Wed Feb 29 16:55:01 SGT 2012
cal.add(Calendar.DAY_OF_YEAR, 1);
System.out.println(cal.getTime()); ==> Thu Mar 01 16:55:01 SGT 2012
Case 2:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.FEBRUARY);
cal.set(Calendar.DAY_OF_MONTH, 29);
System.out.println(cal.getTime()); ==> Wed Feb 29 16:57:06 SGT 2012
cal.add(Calendar.DAY_OF_MONTH, 1);
System.out.println(cal.getTime()); ==> Thu Mar 01 16:57:06 SGT 2012
Case 3:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.DECEMBER);
cal.set(Calendar.DAY_OF_MONTH, 31);
System.out.println(cal.getTime()); ==> Mon Dec 31 16:59:12 SGT 2012
cal.add(Calendar.DAY_OF_MONTH, 1);
System.out.println(cal.getTime()); ==> Tue Jan 01 16:59:12 SGT 2013
Case 4:
GregorianCalendar cal = new GregorianCalendar();
cal.set(Calendar.YEAR, 2012);
cal.set(Calendar.MONTH, Calendar.DECEMBER);
cal.set(Calendar.DAY_OF_MONTH, 31);
System.out.println(cal.getTime()); ==> Mon Dec 31 16:59:48 SGT 2012
cal.add(Calendar.DAY_OF_YEAR, 1);
System.out.println(cal.getTime()); ==> Tue Jan 01 16:59:48 SGT 2013