java 为什么我只能有一个 Calendar 对象实例
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Why can i have only one instance of Calendar object
提问by Gabriel
I was just wondering...
我只是想知道...
why can i have only one instance of Calendar object. Is there a reason for it to be a singleton?
为什么我只能有一个 Calendar 对象实例。有没有理由让它成为单身人士？
I have tried to read the documentationbut they didn't mention why this is needed. And a quick google search didn't give me any answers.
我试图阅读文档，但他们没有提到为什么需要这样做。快速的谷歌搜索没有给我任何答案。
回答by Codemwnci
Calendar is not a singleton, it is an abstract class. The getInstancemethod is a Factory method that returns a concrete implementation of the Calendar class.
Calendar 不是单例，它是一个抽象类。该getInstance方法是一个工厂方法，它返回 Calendar 类的具体实现。
Search Google for java.util.Calendar source code, and you will see how it works.
在 Google 上搜索 java.util.Calendar 源代码，您将看到它是如何工作的。
回答by MByD
It is not singleton.
它不是单例。
This:
这：
public static void main(String args[]) {
Calendar c1, c2;
c1 = Calendar.getInstance();
c2 = Calendar.getInstance();
c1.add(Calendar.MONTH, 1);
System.out.println(c1);
System.out.println(c2);
}
Outputs:
输出：
java.util.GregorianCalendar[time=?,areFieldsSet=false,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Jerusalem",offset=7200000,dstSavings=3600000,useDaylight=true,transitions=143,lastRule=java.util.SimpleTimeZone[id=Asia/Jerusalem,offset=7200000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=26,startDayOfWeek=6,startTime=7200000,startTimeMode=0,endMode=1,endMonth=8,endDay=13,endDayOfWeek=0,endTime=7200000,endTimeMode=0]],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2011,MONTH=5,WEEK_OF_YEAR=21,WEEK_OF_MONTH=3,DAY_OF_MONTH=19,DAY_OF_YEAR=139,DAY_OF_WEEK=5,DAY_OF_WEEK_IN_MONTH=3,AM_PM=0,HOUR=10,HOUR_OF_DAY=10,MINUTE=21,SECOND=27,MILLISECOND=839,ZONE_OFFSET=7200000,DST_OFFSET=3600000]
java.util.GregorianCalendar[time=1305789687839,areFieldsSet=true,areAllFieldsSet=true,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Jerusalem",offset=7200000,dstSavings=3600000,useDaylight=true,transitions=143,lastRule=java.util.SimpleTimeZone[id=Asia/Jerusalem,offset=7200000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=3,startMonth=2,startDay=26,startDayOfWeek=6,startTime=7200000,startTimeMode=0,endMode=1,endMonth=8,endDay=13,endDayOfWeek=0,endTime=7200000,endTimeMode=0]],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2011,MONTH=4,WEEK_OF_YEAR=21,WEEK_OF_MONTH=3,DAY_OF_MONTH=19,DAY_OF_YEAR=139,DAY_OF_WEEK=5,DAY_OF_WEEK_IN_MONTH=3,AM_PM=0,HOUR=10,HOUR_OF_DAY=10,MINUTE=21,SECOND=27,MILLISECOND=839,ZONE_OFFSET=7200000,DST_OFFSET=3600000]
(Which is different as you can see)
（正如你所见，这是不同的）
BTW, quick search for source code returns:
顺便说一句，快速搜索源代码返回：
public static synchronized Calendar getInstance() {
return new GregorianCalendar();
}
回答by Joachim Sauer
Did you think that it is a singleton because it has a getInstance()method? That's not the case!
你认为它是单例是因为它有getInstance()方法吗？事实并非如此！
getInstance()returns a new instance each time.
getInstance()每次返回一个新实例。
回答by Stephen C
You can have as many instances of Calendaras you want ... modulo that it is an abstract class, so you are talking about of instances of child classesof Calendar.
您可以拥有任意数量的实例Calendar...取模它是一个抽象类，因此您正在谈论Calendar的子类的实例。
Perhaps you think that the getInstance()method returns a singleton object? It doesn't. It creates and returns a new object each time you call it.
也许您认为该getInstance()方法返回一个单例对象？它没有。每次调用它时，它都会创建并返回一个新对象。
(The javadoc doesn't explicitly state that the calendar is not a singleton, but it says "The Calendar returned is based on the current time ...". That implies that it is returning a new object each time ... because the current time keeps changing. And anyway, that's what the method does if you'd care to look at the source code.)
（javadoc 没有明确说明日历不是单例，但它说“返回的日历基于当前时间......”。这意味着它每次都返回一个新对象......因为当前时间不断变化。无论如何，如果您想查看源代码，这就是该方法所做的。）
