Java中java.util和java.time新旧时间日期API的日期计算实战

import java.util.Calendar;

// 获取当前时间
Calendar cal = Calendar.getInstance();
System.out.println(cal.getTime());  // Tue Aug 18 14:30:25 CST 2026

// 获取各字段
int year = cal.get(Calendar.YEAR);         // 2026
int month = cal.get(Calendar.MONTH) + 1;   // 8（月份从 0 开始，必须 +1）
int day = cal.get(Calendar.DAY_OF_MONTH);  // 18
int hour = cal.get(Calendar.HOUR_OF_DAY);  // 14（24 小时制）
int minute = cal.get(Calendar.MINUTE);     // 30

// 日期运算（可变）
cal.add(Calendar.DAY_OF_MONTH, 10);   // +10 天
cal.add(Calendar.MONTH, -2);          // -2 月
cal.set(Calendar.YEAR, 2025);         // 直接设置年份

import java.time.LocalDate;
import java.time.Period;

LocalDate start = LocalDate.of(2024, 1, 1);
LocalDate end = LocalDate.of(2026, 8, 18);

// 计算间隔
Period period = Period.between(start, end);
System.out.println(period.getYears());   // 2
System.out.println(period.getMonths());  // 7
System.out.println(period.getDays());    // 17

// 获取总月数（不常用）
long totalMonths = period.toTotalMonths();  // 31（2年7个月 = 31个月）

// 创建 Period（用于加减）
Period oneYear = Period.ofYears(1);
Period twoMonths = Period.ofMonths(2);
Period tenDays = Period.ofDays(10);
Period oneYearTwoMonths = Period.of(1, 2, 10);  // 1年2个月10天

// 日期加减
LocalDate future = start.plus(oneYear);        // 2025-01-01
LocalDate past = end.minus(tenDays);           // 2026-08-08

// ⚠️ Period.between() 不是简单的“总天数”相减
// 它按“年→月→日”分别计算，受月份天数影响

LocalDate start = LocalDate.of(2026, 1, 31);
LocalDate end = LocalDate.of(2026, 2, 28);
Period p = Period.between(start, end);
System.out.println(p.getMonths());  // 0
System.out.println(p.getDays());    // 28（1月31日 → 2月28日，不是按 28 天算）

import java.time.Duration;
import java.time.LocalDateTime;
import java.time.LocalTime;

LocalDateTime start = LocalDateTime.of(2026, 8, 18, 10, 0, 0);
LocalDateTime end = LocalDateTime.of(2026, 8, 18, 14, 30, 25);

// 计算间隔
Duration duration = Duration.between(start, end);
System.out.println(duration.toDays());        // 0
System.out.println(duration.toHours());       // 4（小时数）
System.out.println(duration.toMinutes());     // 270（分钟数）
System.out.println(duration.getSeconds());    // 15025（总秒数）
System.out.println(duration.toMillis());      // 15025000（总毫秒数）
System.out.println(duration.toNanos());       // 15025000000000（总纳秒数）

// 创建 Duration
Duration fiveHours = Duration.ofHours(5);
Duration thirtyMinutes = Duration.ofMinutes(30);
Duration tenSeconds = Duration.ofSeconds(10);
Duration fiveHoursThirtyMin = Duration.of(5, ChronoUnit.HOURS)
    .plus(Duration.ofMinutes(30));

// 时间加减
LocalTime time = LocalTime.of(10, 0);
LocalTime later = time.plus(fiveHours);        // 15:00:00
LocalDateTime dt = LocalDateTime.now().minus(thirtyMinutes);  // 当前时间 - 30 分钟

// ⚠️ Duration 不能处理 年/月/日（会抛异常）
// ❌ 错误
Duration.between(LocalDate.of(2026,1,1), LocalDate.of(2026,8,18));  
// 编译报错：无法应用于 LocalDate

// ✅ 正确：使用 Period 处理日期
Period.between(LocalDate.of(2026,1,1), LocalDate.of(2026,8,18));

// ✅ 如果必须用 Duration，先转 LocalDateTime
LocalDateTime start = LocalDate.of(2026,1,1).atStartOfDay();
LocalDateTime end = LocalDate.of(2026,8,18).atStartOfDay();
Duration d = Duration.between(start, end);
System.out.println(d.toDays());  // 229（总天数）

import java.time.LocalDateTime;
import java.time.temporal.ChronoUnit;

LocalDateTime start = LocalDateTime.of(2026, 8, 18, 10, 0);
LocalDateTime end = LocalDateTime.of(2026, 8, 20, 14, 30);

// 按不同单位计算差值
long days = ChronoUnit.DAYS.between(start, end);        // 2
long hours = ChronoUnit.HOURS.between(start, end);     // 52
long minutes = ChronoUnit.MINUTES.between(start, end); // 3150
long seconds = ChronoUnit.SECONDS.between(start, end); // 189000
long millis = ChronoUnit.MILLIS.between(start, end);   // 189000000

// 可用的单位
ChronoUnit.YEARS      // 年
ChronoUnit.MONTHS     // 月
ChronoUnit.WEEKS      // 周
ChronoUnit.DAYS       // 天
ChronoUnit.HOURS      // 小时
ChronoUnit.MINUTES    // 分钟
ChronoUnit.SECONDS    // 秒
ChronoUnit.MILLIS     // 毫秒
ChronoUnit.MICROS     // 微秒
ChronoUnit.NANOS      // 纳秒

// 日期加减（用指定单位）
LocalDateTime future = start.plus(3, ChronoUnit.DAYS);  // 加 3 天
LocalDateTime past = end.minus(5, ChronoUnit.HOURS);    // 减 5 小时

import java.time.LocalDate;
import java.time.DayOfWeek;
import java.time.temporal.TemporalAdjusters;

LocalDate today = LocalDate.of(2026, 8, 18);  // 2026-08-18（周二）

// 获取本月最后一天
LocalDate lastDayOfMonth = today.with(TemporalAdjusters.lastDayOfMonth());
System.out.println(lastDayOfMonth);  // 2026-08-31

// 获取本月第一天
LocalDate firstDayOfMonth = today.with(TemporalAdjusters.firstDayOfMonth());
System.out.println(firstDayOfMonth);  // 2026-08-01

// 获取下个周一
LocalDate nextMonday = today.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
System.out.println(nextMonday);  // 2026-08-24

// 获取上个周五
LocalDate previousFriday = today.with(TemporalAdjusters.previous(DayOfWeek.FRIDAY));
System.out.println(previousFriday);  // 2026-08-14

// 获取下个月的第一个周日
LocalDate firstSunday = today.with(TemporalAdjusters.firstInMonth(DayOfWeek.SUNDAY));
System.out.println(firstSunday);  // 2026-08-02

// 获取当月的第 3 个周四
LocalDate thirdThursday = today.with(TemporalAdjusters.dayOfWeekInMonth(3, DayOfWeek.THURSDAY));
System.out.println(thirdThursday);  // 2026-08-20

// 获取明天的开始时间（00:00:00）
LocalDateTime tomorrowStart = today.plusDays(1).atStartOfDay();
System.out.println(tomorrowStart);  // 2026-08-19T00:00

LocalDate today = LocalDate.now();

// ✅ 加减日期（最常用）
LocalDate tomorrow = today.plusDays(1);
LocalDate yesterday = today.minusDays(1);
LocalDate nextMonth = today.plusMonths(1);
LocalDate lastYear = today.minusYears(1);

// ✅ 比较（最常用）
boolean isAfter = today.isAfter(LocalDate.of(2026, 1, 1));
boolean isBefore = today.isBefore(LocalDate.of(2027, 1, 1));
boolean isEqual = today.isEqual(LocalDate.of(2026, 8, 18));

// ✅ 获取时间差（until 方法）
long daysUntil = today.until(LocalDate.of(2027, 1, 1), ChronoUnit.DAYS);
Period period = today.until(LocalDate.of(2027, 1, 1));

import java.time.*;
import java.time.format.DateTimeFormatter;
import java.time.temporal.TemporalAdjusters;

public class DateCalcUtils {
    private static final DateTimeFormatter FORMATTER = 
        DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

    // ===== 计算两个日期之间的差值（年月日）=====
    public static Period betweenDates(LocalDate start, LocalDate end) {
        return Period.between(start, end);
    }

    // ===== 计算两个时间之间的差值（时分秒）=====
    public static Duration betweenTimes(LocalDateTime start, LocalDateTime end) {
        return Duration.between(start, end);
    }

    // ===== 判断是否在同一天 =====
    public static boolean isSameDay(LocalDateTime d1, LocalDateTime d2) {
        return d1.toLocalDate().isEqual(d2.toLocalDate());
    }

    // ===== 获取当月的第一天 00:00:00 =====
    public static LocalDateTime getFirstDayOfMonth() {
        return LocalDate.now().with(TemporalAdjusters.firstDayOfMonth()).atStartOfDay();
    }

    // ===== 获取当月的最后一天 23:59:59 =====
    public static LocalDateTime getLastDayOfMonth() {
        return LocalDate.now().with(TemporalAdjusters.lastDayOfMonth())
            .atTime(LocalTime.MAX);  // 23:59:59.999999999
    }

    // ===== 计算年龄（周岁）=====
    public static int getAge(LocalDate birthDate) {
        return Period.between(birthDate, LocalDate.now()).getYears();
    }

    // ===== 获取两个时间点之间的毫秒数 =====
    public static long getMillisBetween(LocalDateTime start, LocalDateTime end) {
        return Duration.between(start, end).toMillis();
    }

    // ===== 格式化为字符串 =====
    public static String format(LocalDateTime dt) {
        return dt.format(FORMATTER);
    }
}

// 问题1：下面代码分别输出什么？
LocalDate start = LocalDate.of(2024, 1, 31);
LocalDate end = LocalDate.of(2024, 2, 28);
Period p = Period.between(start, end);
System.out.println(p.getMonths());   // A
System.out.println(p.getDays());     // B

// 问题2：计算“当前时间 + 3天2小时30分”，用哪些类？
LocalDateTime now = LocalDateTime.now();
// 请写出代码

// 问题3：下面代码输出什么？
LocalDate today = LocalDate.of(2026, 8, 18);
LocalDate lastDay = today.with(TemporalAdjusters.lastDayOfMonth());
System.out.println(lastDay.getDayOfWeek());  // C