我想说是懒惰。数组从0开始(每个人都知道);一年中的月份是一个数组，这让我相信Sun的一些工程师只是懒得在Java代码中加入这个小细节。

基于C的语言在某种程度上复制了C。tm结构(在time.h中定义)有一个整数字段tm_mon，(注释的)范围为0-11。

基于C的语言从索引0开始数组。因此，这对于以tm_mon作为索引输出月份名称数组中的字符串非常方便。

对我来说，没有人比mindpro.com更能解释这一点:

Gotchas java.util.GregorianCalendar has far fewer bugs and gotchas than the old java.util.Date class but it is still no picnic. Had there been programmers when Daylight Saving Time was first proposed, they would have vetoed it as insane and intractable. With daylight saving, there is a fundamental ambiguity. In the fall when you set your clocks back one hour at 2 AM there are two different instants in time both called 1:30 AM local time. You can tell them apart only if you record whether you intended daylight saving or standard time with the reading. Unfortunately, there is no way to tell GregorianCalendar which you intended. You must resort to telling it the local time with the dummy UTC TimeZone to avoid the ambiguity. Programmers usually close their eyes to this problem and just hope nobody does anything during this hour. Millennium bug. The bugs are still not out of the Calendar classes. Even in JDK (Java Development Kit) 1.3 there is a 2001 bug. Consider the following code: GregorianCalendar gc = new GregorianCalendar(); gc.setLenient( false ); /* Bug only manifests if lenient set false */ gc.set( 2001, 1, 1, 1, 0, 0 ); int year = gc.get ( Calendar.YEAR ); /* throws exception */ The bug disappears at 7AM on 2001/01/01 for MST. GregorianCalendar is controlled by a giant of pile of untyped int magic constants. This technique totally destroys any hope of compile-time error checking. For example to get the month you use GregorianCalendar. get(Calendar.MONTH)); GregorianCalendar has the raw GregorianCalendar.get(Calendar.ZONE_OFFSET) and the daylight savings GregorianCalendar. get( Calendar. DST_OFFSET), but no way to get the actual time zone offset being used. You must get these two separately and add them together. GregorianCalendar.set( year, month, day, hour, minute) does not set the seconds to 0. DateFormat and GregorianCalendar do not mesh properly. You must specify the Calendar twice, once indirectly as a Date. If the user has not configured his time zone correctly it will default quietly to either PST or GMT. In GregorianCalendar, Months are numbered starting at January=0, rather than 1 as everyone else on the planet does. Yet days start at 1 as do days of the week with Sunday=1, Monday=2,… Saturday=7. Yet DateFormat. parse behaves in the traditional way with January=1.

真正的原因

You would think that when we deprecated most of Date and added the new Calendar class, we would have fixed Date's biggest annoyance: the fact that January is month 0. We certainly should have, but unfortunately we didn't. We were afraid that programmers would be confused if Date used zero-based months and Calendar used one-based months. And a few programmers probably would have been. But in hindsight, the fact that Calendar is still zero-based has caused an enormous amount of confusion, and it was probably the biggest single mistake in the Java international API's.

引用自Laura Werner的Java国际日历，链接在底部。

更好的选择:java.time

这可能只是重复其他人所说的，抛弃旧的和设计糟糕的Calendar类，使用java。时间，现代Java日期和时间API。从1月的1到12月的12，月份的编号始终是合理的。

如果您从尚未升级到java的遗留API中获取Calendar。时，要做的第一件事是转换为现代的ZonedDateTime。根据你的需要，你可以从那里做进一步的转换。在大多数情况下，您获得的Calendar对象实际上总是GregorianCalendar子类的一个实例(因为Calendar类本身是抽象的)。demonstreate:

    Calendar oldfashionedCalendarObject = Calendar.getInstance();
    ZonedDateTime zdt
            = ((GregorianCalendar) oldfashionedCalendarObject).toZonedDateTime();
    
    System.out.println(zdt);
    System.out.format("Month is %d or %s%n", zdt.getMonthValue(), zdt.getMonth());

当我刚刚在我的时区运行时输出:

2021 - 03 - 17 - t23:18:47.761 + 01:00[欧洲/哥本哈根] 月是3号或MARCH

链接

Laura Werner编写的Java国际日历 Oracle教程:Date Time解释如何使用java.time。

设置月份为“日历”。MARCH，或者比较，看看它是否== Calendar。比如六月。

Date和Calendar类可以追溯到Java的早期，当时人们还在摸索，它们被广泛认为设计得不是很好。

如果今天用相同的设计创建Calendar，而不是int类型的Calendar。JUNE等等，他们会用枚举。

因为用月份计算要简单得多。

12月之后的一个月就是1月，但是要算出这个，通常你需要用月份的数字来计算

12 + 1 = 13 // What month is 13?

我知道!我可以通过使用模量12来快速解决这个问题。

(12 + 1) % 12 = 1

在11月之前的11个月里，这一切都很好……

(11 + 1) % 12 = 0 // What month is 0?

你可以在加上月份之前再减去1，然后做模数，最后再加1……也就是解决潜在的问题。

((11 - 1 + 1) % 12) + 1 = 12 // Lots of magical numbers!

现在让我们考虑0 - 11月的问题。

(0 + 1) % 12 = 1 // February
(1 + 1) % 12 = 2 // March
(2 + 1) % 12 = 3 // April
(3 + 1) % 12 = 4 // May
(4 + 1) % 12 = 5 // June
(5 + 1) % 12 = 6 // July
(6 + 1) % 12 = 7 // August
(7 + 1) % 12 = 8 // September
(8 + 1) % 12 = 9 // October
(9 + 1) % 12 = 10 // November
(10 + 1) % 12 = 11 // December
(11 + 1) % 12 = 0 // January

所有月份的工作都是一样的，没有必要进行变通。