GregorianCalendar add, roll方法
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细细体味 roll 和 add 的区别
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直接上代码：
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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
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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
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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
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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
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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
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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
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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
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