How can a Java enum start with 1? [closed]

Question

Taking a look at the source code of java.time.DayOfWeek reveals:

public enum DayOfWeek implements TemporalAccessor, TemporalAdjuster {

    /**
     * The singleton instance for the day-of-week of Monday.
     * This has the numeric value of {@code 1}.
     */
    MONDAY,
    /**
     * The singleton instance for the day-of-week of Tuesday.
     * This has the numeric value of {@code 2}.
     */
    TUESDAY,

    // ..
}

So how can DayOfWeek.MONDAY = 1? I am asking because I use GWT which does not support the new java.time stuff yet (or never will be idk). So what I did I made my own version of DayOfWeek but that one got a public int getValue() { return ordinal() + 1; } which is really annoying since I have to call it everytime.

That is why I am curious why the above version starts with 1. The other thing I'd like to know is why the f this has to start with 1 and not with 0 just like every other enum does. They could have gone for Monday = 0, Tuesday = 1, etc. but no! They instead switched from Sunday = 0, Monday = 1, etc. to that thing up there.

---

No, this is not an option

public enum MyDayOfWeek {

    DUMMY,

    MONDAY, TUESDAY, // ..
    ;
}

Simply because this would be annoying:

for(MyDayOfWeek day : MyDayOfWeek.value() {
   if(MyDayOfWeek.DUMMY == day) {
      continue;
   }
}

[SUMMARY]

If user-defined enums always start with the first element having the value '0' then how does the first element of the java.time.DayOfWeek enum start with '1', and is there a way to have my enum start with '1' instead of '0'?

Answer 1

To address "how does the enum start with 1"?

It doesn't.

Enums start with 0. They add 1 to the return value:

public int getValue() {
    return ordinal() + 1;
}

Note: In the comments they clearly state:

Do not use ordinal() to obtain the numeric representation of DayOfWeek. Use getValue() instead.

Answer 2

That is why I am curious why the above version starts with 1.

According to the comments in the code, it's because they wanted to follow the ISO-8601 week date standard:

DayOfWeek is an enum representing the 7 days of the week - Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday.
In addition to the textual enum name, each day-of-week has an int value. The int value follows the ISO-8601 standard, from 1 (Monday) to 7 (Sunday). It is recommended that applications use the enum rather than the int value to ensure code clarity.

The other thing I'd like to know is why the f this has to start with 1 and not with 0 just like every other enum does.

It doesn't.

The enum itself is 0-indexed like any other in Java. It just fiddles with the values to give you 1-7:

public int getValue() {
    return ordinal() + 1;
}

or to turn a 1-7 into the appropriate enum value:

public static DayOfWeek of(int dayOfWeek) {
    if (dayOfWeek < 1 || dayOfWeek > 7) {
        throw new DateTimeException("Invalid value for DayOfWeek: " + dayOfWeek);
    }
    return ENUMS[dayOfWeek - 1];
}