设置Java Calendar的值不会给出预期的日期时间

Question

我有一个小时,分钟,日期和毫秒时间戳,我正在尝试创建一个表示时间的Date对象.时间戳在东部夏令时提供.

在剖析问题时,我创建了一些简单的测试代码来查看发生的情况,并观察了以下内容:

    Date today = new Date();
    int hour = 4, min  = 0, sec  = 0, ms   = 64;
    boolean print = true;

    Calendar cal = GregorianCalendar.getInstance();
    if(print)
        System.out.println("After initializing, time is: "+cal.getTime());
    cal.clear();
    if(print)
        System.out.println("After clearing, time is: "+cal.getTime());
    cal.setTime(today);
    if(print)
        System.out.println("After setting date, time is: "+cal.getTime());
    cal.set(Calendar.HOUR_OF_DAY,hour);
    if(print)
        System.out.println("After setting hour, time is: "+cal.getTime());
    cal.set(Calendar.MINUTE,min);
    if(print)
        System.out.println("After setting minute, time is: "+cal.getTime());
    cal.set(Calendar.SECOND,sec);
    if(print)
        System.out.println("After setting second, time is: "+cal.getTime());
    cal.set(Calendar.MILLISECOND,ms);
    if(print)
        System.out.println("After setting milliseconds, time is: "+cal.getTime());
    cal.setTimeZone(TimeZone.getTimeZone("EDT"));

    System.out.println("After setting time zone, time is: "+cal.getTime());

这会产生输出:

After initializing, time is: Tue Jan 07 16:01:59 EST 2014
After clearing, time is: Thu Jan 01 00:00:00 EST 1970
After setting date, time is: Tue Jan 07 16:01:59 EST 2014
After setting hour, time is: Tue Jan 07 04:01:59 EST 2014
After setting minute, time is: Tue Jan 07 04:00:59 EST 2014
After setting second, time is: Tue Jan 07 04:00:00 EST 2014
After setting milliseconds, time is: Tue Jan 07 04:00:00 EST 2014
After setting time zone, time is: Tue Jan 07 04:00:00 EST 2014

但是,如果我稍微更改代码:

boolean print = false;

我得到以下(不同)结果(!)

After setting time zone, time is: Mon Jan 06 23:00:00 EST 2014

有谁知道为什么会这样？

Answer 1

如gtgaxiola所述:来自日历文档

在Field Manipulation部分下:

set(f,value)将日历字段f更改为值.此外,它设置内部成员变量以指示日历字段f已更改.虽然日历字段f立即更改,但在下一次调用get(),getTime(),getTimeInMillis(),add()或roll()之前,不会重新计算日历的时间值(以毫秒为单位).

问题是您的getTime()调用会重新计算日期,但setTimeZone(..)不会将内部成员变量设置isTimeSet为false.因此,您的第一个输出中的最后一行对您来说是错误的,因为您希望考虑时区而不是.

Answer 2

从日历文档

在Field Manipulation部分下:

set(f,value)将日历字段f更改为值.此外,它设置内部成员变量以指示日历字段f已更改.虽然日历字段f立即更改,但在下一次调用get(),getTime(),getTimeInMillis(),add()或roll()之前,不会重新计算日历的时间值(以毫秒为单位).

因此,对set()的多次调用不会触发多次不必要的计算.作为使用set()更改日历字段的结果,其他日历字段也可能会更改,具体取决于日历字段,日历字段值和日历系统.此外,在重新计算日历字段后,get(f)不一定返回通过调用set方法设置的值.具体情况由具体的日历类决定.

Answer 3

您需要先设置时区.请参阅下面的GregorianCalendar.setTimeZone的定义:

public void setTimeZone(TimeZone value)
{
    zone = value;
    sharedZone = false;
    /* Recompute the fields from the time using the new zone.  This also
     * works if isTimeSet is false (after a call to set()).  In that case
     * the time will be computed from the fields using the new zone, then
     * the fields will get recomputed from that.  Consider the sequence of
     * calls: cal.setTimeZone(EST); cal.set(HOUR, 1); cal.setTimeZone(PST).
     * Is cal set to 1 o'clock EST or 1 o'clock PST?  Answer: PST.  More
     * generally, a call to setTimeZone() affects calls to set() BEFORE AND
     * AFTER it up to the next call to complete().
     */
    areAllFieldsSet = areFieldsSet = false;
}