Calendar 类 set() 方法使用要点

相关源码：

/*
 * ...... 
 * <h3>Getting and Setting Calendar Field Values</h3>
 *
 * <p>The calendar field values can be set by calling the <code>set</code>
 * methods. Any field values set in a <code>Calendar</code> will not be
 * interpreted until it needs to calculate its time value (milliseconds from
 * the Epoch) or values of the calendar fields. Calling the
 * <code>get</code>, <code>getTimeInMillis</code>, <code>getTime</code>,
 * <code>add</code> and <code>roll</code> involves such calculation.
 * ......
 */
public abstract class Calendar implements Serializable, Cloneable, Comparable<Calendar> {
    ......
}

• 可以通过set方法修改Calendar实例各个Field的值；
• 只有在真正需要使用calendar的值之前，这种改变才会生效；
• get()、getTimeMillis()、getTime()、add()、roll()方法可以触发set方法的修改。

例子：

public static void main(String[] args) {
        Calendar calendar = Calendar.getInstance();
        System.out.println(calendar);

        calendar.set(Calendar.DAY_OF_MONTH, 1);
        System.out.println(calendar);

        calendar.getTime();
        System.out.println(calendar);
}

输出1	输出2	输出3
java.util.GregorianCalendar[ time=1608696480267, areFieldsSet=true, areAllFieldsSet=true, lenient=true, firstDayOfWeek=1, minimalDaysInFirstWeek=1, ERA=1, YEAR=2020, MONTH=11, WEEK_OF_YEAR=52, WEEK_OF_MONTH=4, DAY_OF_MONTH=23, DAY_OF_YEAR=358, DAY_OF_WEEK=4, DAY_OF_WEEK_IN_MONTH=4, AM_PM=1, HOUR=0, HOUR_OF_DAY=12, MINUTE=8, SECOND=0, MILLISECOND=267, ZONE_OFFSET=28800000, DST_OFFSET=0]	java.util.GregorianCalendar[ time=?, areFieldsSet=false, areAllFieldsSet=true, lenient=true, firstDayOfWeek=1, minimalDaysInFirstWeek=1, ERA=1, YEAR=2020, MONTH=11, WEEK_OF_YEAR=52, WEEK_OF_MONTH=4, DAY_OF_MONTH=1, DAY_OF_YEAR=358, DAY_OF_WEEK=4, DAY_OF_WEEK_IN_MONTH=4, AM_PM=1,HOUR=0, HOUR_OF_DAY=12, MINUTE=8, SECOND=0, MILLISECOND=267, ZONE_OFFSET=28800000, DST_OFFSET=0]	java.util.GregorianCalendar[ time=1606795680267, areFieldsSet=true, areAllFieldsSet=false, lenient=true, firstDayOfWeek=1, minimalDaysInFirstWeek=1, ERA=1, YEAR=2020, MONTH=11, WEEK_OF_YEAR=49, WEEK_OF_MONTH=1, DAY_OF_MONTH=1, DAY_OF_YEAR=336, DAY_OF_WEEK=3, DAY_OF_WEEK_IN_MONTH=1, AM_PM=1, HOUR=0, HOUR_OF_DAY=12, MINUTE=8, SECOND=0, MILLISECOND=267, ZONE_OFFSET=28800000, DST_OFFSET=0]