1. switch表达式(预览)

在JDK12中，switch表达式可以通过函数表达式方式省略break关键字， JDK13中又对switch表达式进行了增强，增加了yield关键词用于返回值，相比break，语义更加明确了。

enum DayOfWeek {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
}

public static String test(DayOfWeek dayOfWeek) {
    return
        switch (dayOfWeek) {
            case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY: {
                yield "工作日";
            }
            case SATURDAY, SUNDAY: {
                yield "周末";
            }
        };
}