Java 8 全新日期時間 API (java.time) 20 大實戰範例完整指南

Java 8 日期時間 API 核心類概覽

所有類：**不可變、線程安全、基於 ISO-8601 日曆系統**。

20 大實戰範例

1. 獲取當前日期

LocalDate today = LocalDate.now();
System.out.println(today); // 2024-01-15

2. 解析日期組件

int year = today.getYear();
int month = today.getMonthValue(); // 1-12
int day = today.getDayOfMonth();
DayOfWeek dow = today.getDayOfWeek(); // MONDAY...

3. 構建指定日期

LocalDate birthday = LocalDate.of(2000, 5, 20); // 2000-05-20
// 無月份 0 基索引陷阱，直觀易用

4. 日期比較

if (today.equals(birthday)) System.out.println("生日快樂！");
if (today.isAfter(birthday)) System.out.println("生日已過");
if (today.isBefore(birthday)) System.out.println("生日未到");

5. 週期性事件檢查（MonthDay）

MonthDay birthdayMd = MonthDay.from(birthday);
MonthDay todayMd = MonthDay.from(today);
if (todayMd.equals(birthdayMd)) System.out.println("生日快樂！");
// 忽略年份，適用於周年紀念、節假日

6. 獲取當前時間

LocalTime now = LocalTime.now(); // 14:30:45.123456789
int hour = now.getHour();
int minute = now.getMinute();

7. 時間加減（plus/minus）

LocalTime later = now.plusHours(2).plusMinutes(30);
LocalDate nextWeek = today.plus(1, ChronoUnit.WEEKS);
LocalDate nextMonth = today.plusMonths(1);
LocalDate nextYear = today.plusYears(1);

8. 日期加減

LocalDate lastYear = today.minusYears(1);
LocalDate nextYear = today.plusYears(1);

9. Clock 時鐘抽象（測試友好）

Clock utcClock = Clock.systemUTC();
Clock defaultClock = Clock.systemDefaultZone();
Clock fixedClock = Clock.fixed(Instant.parse("2024-01-01T00:00:00Z"), ZoneId.of("UTC"));

// 注入測試
public class Scheduler {
    private final Clock clock;
    public Scheduler(Clock clock) { this.clock = clock; }
    public boolean isDue(LocalDate deadline) {
        return !LocalDate.now(clock).isBefore(deadline);
    }
}

10. 時區轉換

ZoneId ny = ZoneId.of("America/New_York");
LocalDateTime local = LocalDateTime.now();
ZonedDateTime nyTime = local.atZone(ny); // 當地時間視為紐約時間
ZonedDateTime utcTime = nyTime.withZoneSameInstant(ZoneOffset.UTC); // 轉 UTC

11. 固定日期模式（YearMonth）

YearMonth ym = YearMonth.now();
int days = ym.lengthOfMonth(); // 當月天數，自動處理閏年
YearMonth expiry = YearMonth.of(2025, 12); // 信用卡過期

12. 閏年判斷

if (today.isLeapYear()) System.out.println("閏年");

13. 兩日期間隔（Period）

LocalDate release = LocalDate.of(2024, 3, 15);
Period p = Period.between(today, release);
System.out.println(p.getMonths() + "月" + p.getDays() + "天");

14. 偏移量日期時間

OffsetDateTime odt = OffsetDateTime.of(
    LocalDateTime.of(2024, 1, 15, 10, 30),
    ZoneOffset.of("+08:00")
);

15. Instant 時間戳

Instant now = Instant.now(); // 2024-01-15T02:30:45.123Z
// 舊 API 互轉
Date date = Date.from(now);
Instant ins = date.toInstant();

16. 預定義格式化解析

String iso = "20240115";
LocalDate parsed = LocalDate.parse(iso, DateTimeFormatter.BASIC_ISO_DATE);
// 2024-01-15

17. 自定義格式化解析

String custom = "Apr 18 2024";
DateTimeFormatter fmt = DateTimeFormatter.ofPattern("MMM dd yyyy", Locale.ENGLISH);
LocalDate holiday = LocalDate.parse(custom, fmt);

18. 日期時間格式化輸出

LocalDateTime now = LocalDateTime.now();
DateTimeFormatter fmt = DateTimeFormatter.ofPattern("MMM dd yyyy hh:mm a");
String formatted = now.format(fmt); // Jan 15 2024 10:30 AM

19. 時區格式化

ZonedDateTime zdt = ZonedDateTime.now(ZoneId.of("Asia/Taipei"));
DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss z");
System.out.println(zdt.format(fmt)); // 2024-01-15 10:30:45 CST

20. 練習：下一個工作日計算

public LocalDate nextBusinessDay(LocalDate date) {
    LocalDate next = date.plusDays(1);
    while (next.getDayOfWeek() == DayOfWeek.SATURDAY 
        || next.getDayOfWeek() == DayOfWeek.SUNDAY) {
        next = next.plusDays(1);
    }
    // 可擴展：排除節假日表
    return next;
}

關鍵差異速查

避坑指南

• Month enum 從 1 開始（JANUARY=1），無 0 基陷阱
• DayOfWeek enum：MONDAY=1 ... SUNDAY=7
• 格式化模式：MM 月份，mm 分鐘；HH 24制，hh 12制
• yyyy 年份，YYYY 週年（慎用）
• 解析失敗拋 DateTimeParseException，格式化失敗拋 DateTimeException
• 線程安全：DateTimeFormatter 可靜態共享，SimpleDateFormat 不可