1. 概述
计算两个日期之间的月份间隔是常见的编程任务。Java标准库和第三方库都提供了相应的类和方法来实现这个功能。
本教程将深入探讨如何使用传统Date API、Date Time API和Joda-Time库来计算Java中两个日期之间的月份间隔。
2. 日期值的影响
计算月份间隔时，日期中的"日"值会影响最终结果。默认情况下，Java标准库和Joda-Time库都会考虑日值：
✅ 如果结束日期的日值小于开始日期的日值，最后一个月不会被计入完整月份：
LocalDate startDate = LocalDate.parse("2023-05-31");
LocalDate endDate = LocalDate.parse("2023-11-28");
上述代码中，结束日期的日值(28)小于开始日期的日值(31)，因此最后一个月不会被计入。
✅ 如果结束日期的日值大于或等于开始日期的日值，最后一个月会被计入完整月份：
LocalDate startDate = LocalDate.parse("2023-05-15");
LocalDate endDate = LocalDate.parse("2023-11-20");
在某些场景下，我们可能需要完全忽略日值，只计算月份差值。后续章节将展示如何实现这两种计算方式。
3. 使用传统Date API
使用传统Date API时，需要自定义方法计算月份间隔。我们可以通过Calendar类结合Date类来实现。
3.1. 忽略日值
下面是一个忽略日值的计算方法：
int monthsBetween(Date startDate, Date endDate) {
if (startDate == null || endDate == null) {
throw new IllegalArgumentException("Both startDate and endDate must be provided");
}
Calendar startCalendar = Calendar.getInstance();
startCalendar.setTime(startDate);
int startDateTotalMonths = 12 * startCalendar.get(Calendar.YEAR)
+ startCalendar.get(Calendar.MONTH);
Calendar endCalendar = Calendar.getInstance();
endCalendar.setTime(endDate);
int endDateTotalMonths = 12 * endCalendar.get(Calendar.YEAR)
+ endCalendar.get(Calendar.MONTH);
return endDateTotalMonths - startDateTotalMonths;
}
核心逻辑：
- 将年份转换为月份数并加上当前月份值
- 计算两个日期的总月份差值
单元测试示例：
@Test
void whenCalculatingMonthsBetweenUsingLegacyDateApi_thenReturnMonthsDifference() throws ParseException {
MonthInterval monthDifference = new MonthInterval();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date startDate = sdf.parse("2016-05-31");
Date endDate = sdf.parse("2016-11-30");
int monthsBetween = monthDifference.monthsBetween(startDate, endDate);
assertEquals(6, monthsBetween);
}
3.2. 考虑日值
考虑日值的计算方法：
int monthsBetweenWithDayValue(Date startDate, Date endDate) {
if (startDate == null || endDate == null) {
throw new IllegalArgumentException("Both startDate and endDate must be provided");
}
Calendar startCalendar = Calendar.getInstance();
startCalendar.setTime(startDate);
int startDateDayOfMonth = startCalendar.get(Calendar.DAY_OF_MONTH);
int startDateTotalMonths = 12 * startCalendar.get(Calendar.YEAR)
+ startCalendar.get(Calendar.MONTH);
Calendar endCalendar = Calendar.getInstance();
endCalendar.setTime(endDate);
int endDateDayOfMonth = endCalendar.get(Calendar.DAY_OF_MONTH);
int endDateTotalMonths = 12 * endCalendar.get(Calendar.YEAR)
+ endCalendar.get(Calendar.MONTH);
return (startDateDayOfMonth > endDateDayOfMonth)
? (endDateTotalMonths - startDateTotalMonths) - 1
: (endDateTotalMonths - startDateTotalMonths);
}
关键点：
- 比较开始日期和结束日期的日值
- 如果开始日值大于结束日值，结果月份需要减1
单元测试示例：
@Test
void whenCalculatingMonthsBetweenUsingLegacyDateApiDayValueConsidered_thenReturnMonthsDifference() throws ParseException {
MonthInterval monthDifference = new MonthInterval();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date startDate = sdf.parse("2016-05-31");
Date endDate = sdf.parse("2016-11-28");
int monthsBetween = monthDifference.monthsBetweenWithDayValue(startDate, endDate);
assertEquals(5, monthsBetween);
}
由于结束日值(28)小于开始日值(31)，最后一个月未被计入。
4. 使用Date Time API
Java 8引入的Date Time API提供了更优雅的解决方案，主要通过Period类和ChronoUnit枚举实现。
4.1. Period类
Period.between()方法默认考虑日值：
@Test
void whenCalculatingMonthsBetweenUsingPeriodClass_thenReturnMonthsDifference() {
Period diff = Period.between(LocalDate.parse("2023-05-25"), LocalDate.parse("2023-11-23"));
assertEquals(5, diff.getMonths());
}
⚠️ 由于结束日值(23)小于开始日值(25)，结果为5个月而非6个月。
要忽略日值，可将日期调整为每月第一天：
@Test
void whenCalculatingMonthsBetweenUsingPeriodClassAndAdjsutingDatesToFirstDayOfTheMonth_thenReturnMonthsDifference() {
Period diff = Period.between(LocalDate.parse("2023-05-25")
.withDayOfMonth(1), LocalDate.parse("2023-11-23")
.withDayOfMonth(1));
assertEquals(6, diff.getMonths());
}
4.2. ChronoUnit枚举
ChronoUnit.MONTHS.between()同样默认考虑日值：
@Test
void whenCalculatingMonthsBetweenUsingChronoUnitEnum_thenReturnMonthsDifference() {
long monthsBetween = ChronoUnit.MONTHS.between(
LocalDate.parse("2023-05-25"),
LocalDate.parse("2023-11-23")
);
assertEquals(5, monthsBetween);
}
忽略日值的两种方式：
调整为每月第一天：
@Test void whenCalculatingMonthsBetweenUsingChronoUnitEnumdSetTimeToFirstDayOfMonth_thenReturnMonthsDifference() { long monthsBetween = ChronoUnit.MONTHS.between(LocalDate.parse("2023-05-25") .withDayOfMonth(1), LocalDate.parse("2023-11-23") .withDayOfMonth(1)); assertEquals(6, monthsBetween); }使用
YearMonth.from()：@Test void whenCalculatingMonthsBetweenUsingChronoUnitAndYearMonth_thenReturnMonthsDifference() { long diff = ChronoUnit.MONTHS.between( YearMonth.from(LocalDate.parse("2023-05-25")), LocalDate.parse("2023-11-23") ); assertEquals(6, diff); }
5. 使用Joda-Time库
Joda-Time库提供了Months.monthsBetween()方法。首先添加依赖：
<dependency>
<groupId>joda-time</groupId>
<artifactId>joda-time</artifactId>
<version>2.12.5</version>
</dependency>
默认考虑日值的计算：
@Test
void whenCalculatingMonthsBetweenUsingJodaTime_thenReturnMonthsDifference() {
DateTime firstDate = new DateTime(2023, 5, 25, 0, 0);
DateTime secondDate = new DateTime(2023, 11, 23, 0, 0);
int monthsBetween = Months.monthsBetween(firstDate, secondDate).getMonths();
assertEquals(5, monthsBetween);
}
忽略日值的方法：
@Test
void whenCalculatingMonthsBetweenUsingJodaTimeSetTimeToFirstDayOfMonth_thenReturnMonthsDifference() {
DateTime firstDate = new DateTime(2023, 5, 25, 0, 0).withDayOfMonth(1);
DateTime secondDate = new DateTime(2023, 11, 23, 0, 0).withDayOfMonth(1);
int monthsBetween = Months.monthsBetween(firstDate, secondDate).getMonths();
assertEquals(6, monthsBetween);
}
6. 总结
本文介绍了三种计算日期月份差的方法：
| 方法 | 优点 | 缺点 |
|---|---|---|
| 传统Date API | 兼容旧系统 | 代码繁琐，易出错 |
| Date Time API | 简洁高效，无依赖 | 需Java 8+ |
| Joda-Time | 功能强大 | 需额外依赖 |
✅ 推荐使用Date Time API，因为它：
- 提供了清晰的API设计
- 无需外部依赖
- 支持多种计算方式
是否考虑日值取决于具体业务需求：
- 计算完整月份间隔时考虑日值
- 计算月份差值时忽略日值
完整示例代码可在GitHub获取。