learning java Calendar类

//Calendar.MONTH ，这是一个特殊于日历的值。
//在格里高利历和罗马儒略历中一年中的第一个月是 JANUARY，它为 0；最后一个月取决于一年中的月份数。
//
//所以这个值的初始值为0，所以我们用它来表示日历月份时需要加1
        var c = Calendar.getInstance();
        System.out.println(c.get(Calendar.DAY_OF_WEEK));
        System.out.println(c.get(Calendar.YEAR));
        System.out.println(c.get(Calendar.MONTH));
        System.out.println(c.get(Calendar.DATE));
        System.out.println(c.get(Calendar.HOUR));
        System.out.println(c.get(Calendar.MINUTE));
        System.out.println(c.get(Calendar.SECOND));

        c.set(,,,,,);
        System.out.println(c.get(Calendar.YEAR));
        System.out.println(c.get(Calendar.MONTH));
        System.out.println(c.get(Calendar.DATE));
        System.out.println(c.get(Calendar.HOUR));
        System.out.println(c.get(Calendar.MINUTE));
        System.out.println(c.get(Calendar.SECOND));
        // add 会进位
        c.add(Calendar.YEAR, -);
        System.out.println(c.getTime());
        // roll 当修改的字段超出它的允许范围时，上一级字段不会增大。
        c.roll(Calendar.MONTH, -);
        System.out.println(c.getTime());

output:

6
2019
6
26
8
54
16
2003
10
23
0
32
23
Sat Nov 23 12:32:23 CST 2002
Mon Dec 23 12:32:23 CST 2002