1､case包含多个常量

单个的写法：

public void daysOfMonth(int month) {
    switch (month) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            System.out.println("this month has 31 days");
            break;
        case 4:
        case 6:
        case 9:
            System.out.println("this month has 30 days");
            break;
        case 2:
            System.out.println("February can have 28 or 29 days");
            break;
        default:
            System.out.println("invalid month");
    }
}

多个常量的写法：

switch (month) {
    case 1, 3, 5, 7, 8, 10, 12 -> System.out.println("this month has 31 days");
    case 4, 6, 9 -> System.out.println("this month has 30 days");
    case 2 -> System.out.println("February can have 28 or 29 days");
    default -> System.out.println("invalid month");
}

新语法“ case L->”，开关块代码看起来更加清晰，简洁和可读。可以使用多个常量来减少代码，并且不必显式使用break语句来跳出，从而减少了容易出错的代码（没有失败，也没有中断）。

switch新语法中，箭头标签后面的代码可以是表达式，块或throw语句。例如：

switch (month) {
    case 1, 3, 5, 7, 8, 10, 12 -> System.out.println("this month has 31 days");
    case 4, 6, 9 -> System.out.println("this month has 30 days");
    case 2 -> {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter year: ");
        int year = scanner.nextInt();
        if (year % 4 == 0)
            System.out.println("This february has 29 days");
        else
            System.out.println("This February has 28 days");
    }
    default -> throw new IllegalArgumentException("Invalid month");
}

注意：新语法的switch表达式，由于代码不会进入下一种情况，因此我们不必每种情况使用break中断。

2､switch-case中返回值

从Java 14开始，可以在表达式中使用switch case块，也就是switch块可以返回一个值。对于传统的switch语句，我们必须使用临时变量，如以下代码示例所示：

int days = 0;
switch (month) {
    case 1:
    case 3:
    case 5:
    case 7:
    case 8:
    case 10:
    case 12:
        days = 31;
        break;
    case 4:
    case 6:
    case 9:
        days = 30;
        break;
    case 2:
        days = 28;
        break;
    default:
        throw new IllegalArgumentException("Invalid month");
}

在switch-case中返回值:

int days = switch (month) {
        case 1, 3, 5, 7, 8, 10, 12 -> 31;
        case 4, 6, 9 -> 30;
        case 2 -> 28;
        default -> 0;
};

或

System.out.println(
    switch (month) {
        case 1, 3, 5, 7, 8, 10, 12 -> 31;
        case 4, 6, 9 -> 30;
        case 2 -> 28;
        default -> 0;
    }
);

或

int days = switch (month) {
        case 1, 3, 5, 7, 8, 10, 12 -> 31;
        case 4, 6, 9 -> 30;
        case 2 -> {
            Scanner scanner = new Scanner(System.in);
            System.out.print("Enter year: ");
            int year = scanner.nextInt();
            if (year % 4 == 0)
                yield 29;
            else
                yield 28;
        }
        default -> 0;
};

或

int days = switch (month) {
    case 1, 3, 5, 7, 8, 10, 12:
        yield 31;
    case 4, 6, 9:
        yield 30;
    case 2: {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter year: ");
        int year = scanner.nextInt();
        if (year % 4 == 0)
            yield 29;
        else
            yield 28;
    }
    default: yield 0;
};

注意：不能在Switch代码块中同时使用“case L:”和“case L->”。

相关文档：

Java JDK13(Java 13) switch新特性语法使用及示例代码

Java中switch用法示例