java - 如何仅使用 LocalDate 检查每月 13 日是否是星期五？

Question

我想仅使用 LocalDate 来检查从 2019 年 1 月 1 日到 2029 年 12 月 12 日期间每月 13 日是否为星期五的情况。如何将 LocalDate.getDayOfMonth() 转换为 String 表示周一/周二/.../周六/周日？

    for (int i = 2019; i < 2030; i++) {
        for (int j = 1; j < 13; j++) {
            LocalDate date = LocalDate.of(i, j, 13);
            int dayOfMonth = date.getDayOfMonth();


        }
    }

Answer 1

您可以使用enum java.time.DayOfWeek来检查它是否是星期五:

public static void main(String[] args) {
    for (int i = 2019; i < 2030; i++) {
        for (int j = 1; j < 13; j++) {
            LocalDate date = LocalDate.of(i, j, 13);
            DayOfWeek dayOfWeek = date.getDayOfWeek();

            if (dayOfWeek == DayOfWeek.FRIDAY) {
                System.out.println("Beware of bad luck in " 
                        + j + "/" + i + "…");
            }
        }
    }
}

输出:

Beware of bad luck in 9/2019…
Beware of bad luck in 12/2019…
Beware of bad luck in 3/2020…
Beware of bad luck in 11/2020…
Beware of bad luck in 8/2021…
Beware of bad luck in 5/2022…
Beware of bad luck in 1/2023…
Beware of bad luck in 10/2023…
Beware of bad luck in 9/2024…
Beware of bad luck in 12/2024…
Beware of bad luck in 6/2025…
Beware of bad luck in 2/2026…
Beware of bad luck in 3/2026…
Beware of bad luck in 11/2026…
Beware of bad luck in 8/2027…
Beware of bad luck in 10/2028…
Beware of bad luck in 4/2029…
Beware of bad luck in 7/2029…

如果您想获取 DayOfWeek 的 String 表示形式，您可以这样做:

String weekday = dayOfWeek.getDisplayName(TextStyle.FULL_STANDALONE, Locale.getDefault());

本例中选择的是系统默认Locale中使用的全名，在我的Locale中是"Freitag"，但是您可以使用 Locale.US 或其他替代。

同样适用于java.time.Month，请参阅这个新的(但仅略有不同)示例:

public static void main(String[] args) {
    for (int i = 2019; i < 2030; i++) {
        for (int j = 1; j < 13; j++) {
            LocalDate date = LocalDate.of(i, j, 13);
            DayOfWeek dayOfWeek = date.getDayOfWeek();

            // get the day of week as String (full name, US style)
            String weekday = dayOfWeek.getDisplayName(TextStyle.FULL_STANDALONE, Locale.US);
            // get the name of the Month as String (full name, US style)
            Month month = Month.of(j);

            if (dayOfWeek == DayOfWeek.FRIDAY) {
                System.out.println("Beware of bad luck on " 
                        + weekday 
                        + " 13th, " 
                        + month.getDisplayName(TextStyle.FULL, Locale.US) 
                        + " " 
                        + i);
            }
        }
    }
}