Method to limit potential values of an Enum

Question

Is it possible to limit the valid enum values that a method can accept.

Say, for example, I have an enum like this:

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

And say I have a method that only should accept an inner set of those values:

//Here dayOfWeek should only be Monday - Friday.
public void setWorkDayAlarm(WEEKDAY dayOfWeek) {
}

I know you can obviously switch on the valid values, and in the default case not do anything. But is there a way (or a pattern) to define in the interface of the method, that it only accepts a certain subset of valid enums in Java (5+)?

Answer 1

I don't know that the type system can help you here. What about something like this?

public enum DayOfWeek {
    SUNDAY(false), MONDAY(true), TUESDAY(true), WEDNESDAY(true),
    THURSDAY(true), FRIDAY(true), SATURDAY(false);

    private final boolean isWeekday;

    private DayOfWeek(boolean isWeekday) {
        this.isWeekday = isWeekday;
    }

    public boolean isWeekday() {
        return isWeekday;
    }
}

Then check the argument:

public void setWorkDayAlarm(DayOfWeek dayOfWeek) {
    if (!dayOfWeek.isWeekday())
        throw new IllegalArgumentException("Need weekday, got " + dayOfWeek);

    // ...
}

Answer 2

The language itself provides no way to build subsets of an enum. That would defeat the purpose of an enum in the first place - to have a defined set of values for a given domain.

What you can do: Define another appropriate enum for that domain and provide reasonable cross-mappings. To get the idea:

public enum WORKDAY {
    MONDAY(WEEKDAY.MONDAY),
    TUESDAY(WEEKDAY.TUESDAY),
    WEDNESDAY(WEEKDAY.WEDNESDAY),
    THURSDAY(WEEKDAY.THURSDAY),
    FRIDAY(WEEKDAY.FRIDAY);

    final public WEEKDAY weekday;

    WORKDAY(WEEKDAY wd){
        this.weekday = wd;
    }
    static WORKDAY convertWeekday(WEEKDAY weekday){
        // in real live use a static EnumMap
        for(WORKDAY workday: values())
            if( workday.weekday == weekday )
                return workday;
        return null; // or an exception
    }
}

public void setWorkDayAlarm(WORKDAY workDay){ ... }

The good thing: The caller of the conversion method must deal with these values which are not mappable. Your method stays clean.

Answer 3

Since the method you provided there is public, it is acceptable to throw an IllegalArgumentException if a caller gives it an invalid value. For a private method, you could have a debug assertion fail.