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.时间戳在Eastern Daylight Time中提供.
在解剖这个问题时,我创建了一些简单的测试代码来看看发生了什么,并且已经观察到以下内容：
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());
这产生的输出：
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
有人知道为什么会发生这种情况吗？
解决方法
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;
}
