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