How can a Java enum start with 1?

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'?

@NathanHughes Have fun with enhanced for-loops `if(DayOfWeek.FOOBARDAY == day) { continue; }` — Stefan Falk, Feb 22 '16 at 18:22
It actually depends on what country you're in. Here in the US, the first day of the week is Sunday. Hence 0, and Monday is 1. In other places, the first day of the week is Monday 0. If you're working on an app that uses a calendar, you have to allow for the first day being location dependent. — durbnpoisn, Feb 22 '16 at 18:23
@durbnpoisn I don't think this is location dependent. `enum` starts always with 0. So how did they manage to let this start by 1? Imagine how that would break stuff if it were location dependent.. — Stefan Falk, Feb 22 '16 at 18:26
@displayname Was your question a technical one about how `enum` works in Java in general? Or was it about why Monday would be assigned a value of `1` as the first day of the week? I suspect the various comments and answers are addressing one question or the other, hence the confusion. Please edit your Question to clarify. — Basil Bourque, Feb 23 '16 at 00:48
Best practice: [Use instance fields instead of ordinals](https://books.google.ca/books?id=ka2VUBqHiWkC&pg=PA158&lpg=PA158&dq=use+instance+fields+instead+of+ordinals&source=bl&ots=y-DoJns1UZ&sig=ACfU3U2HQoFnMIdU82BD_y5lr9SHz8nCJQ&hl=en&sa=X&ved=2ahUKEwi-yqGc5ZTwAhXQKFkFHbkBAywQ6AEwBHoECAUQAw#v=onepage&q=use%20instance%20fields%20instead%20of%20ordinals&f=false) — Leponzo, Apr 23 '21 at 16:38

score 23 · Accepted Answer · answered Feb 22 '16 at 18:32

23

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.

answered Feb 22 '16 at 18:32

MrHappyAsthma

6,332
9
48
78

4

More generally, the documentation for `Enum.ordinal` says something along the lines of "don't call this, it's only meant for EnumSet and EnumMap". – Andy Turner Feb 22 '16 at 18:36

azurefrog · Answer 2 · 2016-02-28T19:14:59.470

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];
}

Yes I know. But how is it possible that the `enum` actually starts with 1? — Stefan Falk, Feb 22 '16 at 18:29
@displayname I've updated my answer to explain how, actually, it doesn't start with 1. — azurefrog, Feb 22 '16 at 18:39

How can a Java enum start with 1?

2 Answers2

Linked