java-switch表达式

import java.util.Calendar;
 
class DaysInMonth {
    public static void main(String[] args) {
        Calendar today = Calendar.getInstance();
        int month = today.get(Calendar.MONTH);
        int year = today.get(Calendar.YEAR);
 
        int daysInMonth;
        switch (month) {
            case Calendar.JANUARY:
            case Calendar.MARCH:
            case Calendar.MAY:
            case Calendar.JULY:
            case Calendar.AUGUST:
            case Calendar.OCTOBER:
            case Calendar.DECEMBER:
                daysInMonth = 31;
                break;
            case Calendar.APRIL:
            case Calendar.JUNE:
            case Calendar.SEPTEMBER:
            case Calendar.NOVEMBER:
                daysInMonth = 30;
                break;
            case Calendar.FEBRUARY:
                if (((year % 4 == 0) && !(year % 100 == 0))
                        || (year % 400 == 0)) {
                    daysInMonth = 29;
                } else {
                    daysInMonth = 28;
                }
                break;
            default:
                throw new RuntimeException(
                    "Calendar in JDK does not work");
        }
 
        System.out.println(
            "There are " + daysInMonth + " days in this month.");
    }
}

import java.util.Calendar;
 
class DaysInMonth {
    public static void main(String[] args) {
        Calendar today = Calendar.getInstance();
        int month = today.get(Calendar.MONTH);
        int year = today.get(Calendar.YEAR);
 
        int daysInMonth = switch (month) {
            case Calendar.JANUARY,
                 Calendar.MARCH,
                 Calendar.MAY,
                 Calendar.JULY,
                 Calendar.AUGUST,
                 Calendar.OCTOBER,
                 Calendar.DECEMBER -> 31;
            case Calendar.APRIL,
                 Calendar.JUNE,
                 Calendar.SEPTEMBER,
                 Calendar.NOVEMBER -> 30;
            case Calendar.FEBRUARY -> {
                if (((year % 4 == 0) && !(year % 100 == 0))
                        || (year % 400 == 0)) {
                    yield 29;
                } else {
                    yield 28;
                }
            }
            default -> throw new RuntimeException(
                    "Calendar in JDK does not work");
        };
 
        System.out.println(
                "There are " + daysInMonth + " days in this month.");
    }
}