按名称获取枚举常量

Created: November-22, 2018

假设我们有一个 enum DayOfWeek：

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

枚举是使用内置的静态 valueOf() 方法编译的，该方法可用于通过名称查找常量：

String dayName = DayOfWeek.SUNDAY.name();
assert dayName.equals("SUNDAY");

DayOfWeek day = DayOfWeek.valueOf(dayName);
assert day == DayOfWeek.SUNDAY;

使用动态枚举类型也可以这样做：

Class<DayOfWeek> enumType = DayOfWeek.class;
DayOfWeek day = Enum.valueOf(enumType, "SUNDAY");
assert day == DayOfWeek.SUNDAY;

如果指定的枚举没有具有匹配名称的常量，则这两个 valueOf() 方法都将抛出 IllegalArgumentException。

Guava 库提供了一个帮助方法 Enums.getIfPresent() ，它返回一个 Guava Optional 来消除显式的异常处理：

DayOfWeek defaultDay = DayOfWeek.SUNDAY;
DayOfWeek day = Enums.valueOf(DayOfWeek.class, "INVALID").or(defaultDay);
assert day == DayOfWeek.SUNDAY;