设置Java日历的值不能提供预期的日期时间

Question

我有一个小时，分钟，日期和毫秒时间戳，并且正在尝试创建一个表示时间的Date对象。时间戳记是美国东部夏令时。

在剖析问题时，我创建了一些简单的测试代码以查看发生了什么，并观察到以下内容：

    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());

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

有人知道为什么会这样吗？

Answer 1

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;
}