java - android.util.GregorianCalendar : will c. add(Calendar.DAY_OF_MONTH, 1) 回滚还是前进？

Question

如果日历是该月的最后一天(例如 7 月 31 日)，则会

c.add(Calendar.DAY_OF_MONTH, 1);

将 c 设置为同月(七月)的开始，还是会将 c 提前到下个月(八月)？

Answer 1

查看父类(super class)java.util.Calendar的文档，在名为“字段操作”的部分中(强调我的):

add(f, delta) adds delta to field f. This is equivalent to calling set(f, get(f) + delta) with two adjustments:

Add rule 1. The value of field f after the call minus the value of field f before the call is delta, modulo any overflow that has occurred in field f. Overflow occurs when a field value exceeds its range and, as a result, the next larger field is incremented or decremented and the field value is adjusted back into its range.

因此 add(Calendar.DAY_OF_MONTH, 1) 会将 7 月 31 日更改为8 月 1 日。

相反，文档继续:

roll(f, delta) adds delta to field f without changing larger fields. This is equivalent to calling add(f, delta) with the following adjustment:

Roll rule. Larger fields are unchanged after the call. A larger field represents a larger unit of time. DAY_OF_MONTH is a larger field than HOUR.

因此，roll(Calendar.DAY_OF_MONTH, 1) 会将 7 月 31 日更改为7 月 1 日。