Java 1.5 introduces this great feature long present in other languages called an enumerated constant. In theory, it lets you replace ugly java code like this:

public static final int PANCAKE = 0;
public static final int ORANGES = 0;
public static final int CEREAL = 0;
public static final int MILK = 0;

With a simple one line definition:

public enum breakfast { pancake, oranges, cereal, milk };

This is good because it improves the readability and maintability of the code. Enums are also strictly more powerful than a list of integers, as they are actually a shorthand for a class, and contain methods and fields of their own. However, transitioning to using java 1.5 is not as easy as you think. I wanted to use an enum in a switch / case statement, but was having trouble. My first incorrect try was to fully qualify the enum:

switch(visited.operator){
    case BinaryExpression.Operator.and: ... break;
}

This gives the following error:

The enum constant BinaryExpression.Operator.and reference cannot be qualified in a case label

Searching for tutorials online didn’t help–the only one I could find that was truly useful was this one from InformIT. I finally discovered that it inherits the context from the type of the object in the switch() statement, and that cases can simply use unqualified names. The code now becomes:

switch (visited.operator) {
    case and: ... break;
}

One you get used to them, enums in Java 1.5 are great. They really should have been there all along!

27 Responses to “Enumerated Constants in Java”

1. NIHAR RANJAN says:

   2/12/2006 at 10:49 pm

   very good article.teaches complex things in simple and less steps.very clear,good understandable thn its contemporaries. does Elliott Back plan to write a full volume java book with its advanced version ? greatly welcome if he does so.

   Reply
2. Tommy A says:

   5/9/2006 at 10:35 pm

   Thanks for the simple and to-the-point fix. The “[reference] cannot be qualified in a case label” error was driving me crazy..

   I wonder why this happens… Is Java treating the enum constant as a ’static’ field when it is referred to by its full name?

   Thanks,
   Tommy

   Reply
3. Leonardo Barbosa says:

   6/6/2006 at 8:38 pm

   Cool.. Thanks for the article. Saved me time for sure.

   Btw, weird choice import the namespace to the switch isn’t it? anyway, I’m just guessing here..

   []s
   Leonardo

   Reply
4. Mattias says:

   6/9/2006 at 6:37 am

   Just wanted to thank you for solving the problem for me. It was driving me crazy

   Best regards,
   Mattias

   Reply
5. Tommy Driffers says:

   7/27/2006 at 8:01 am

   Awesome!! Gee THANKS!!
   and Ohh please give us more elaboration… thanx a lot
   LOL all of your gago!!!

   Reply
6. Omen 666 says:

   7/27/2006 at 8:02 am

   HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA
   HEHEHEHEHEHEEHHEEHEHEHEHEHEHEHEHEHEHEHEHEHEHEHE
   ****!!!!!!!

   Reply
7. Yaba says:

   8/29/2006 at 4:00 pm

   Thanks a lot. Just stumbled over this.

   Reply
8. Matt Brotherson says:

   10/19/2006 at 10:11 am

   Many thanks for your post!

   Cheers,
   -Matt

   Reply
9. Ton-Yun Fang says:

   4/6/2007 at 6:44 pm

   Thanks so much for figuring this out! The error was driving me nuts because I KNEW you could do enums in Java 1.5….

   Reply
10. Rodrigo says:

    5/19/2007 at 7:59 am

    Gracias!, muy útil.

    Reply
11. Del.irio.us says:

    7/4/2007 at 6:22 am

    Thanks a lot
    Gracias

    Reply
12. bagley says:

    7/6/2007 at 6:35 am

    thx

    Reply
13. Sébastien says:

    7/11/2007 at 7:38 am

    Hi,

    Thanks for the tip, I was just looking to the same stuff with the switch )

    Have a nice day.

    Reply
14. Nate says:

    8/11/2007 at 3:52 pm

    YAPWTY (yet another person who thanks you) for the helpful tip!
    -nate

    Reply
15. Khan says:

    9/12/2007 at 11:10 am

    Thanks so much for this one. I was going crazy trying to figure this out. Your site helped a lot.

    However, I wonder why they did it this way – it does become a bit confusing. Wouldn’t it be easier to use the FQN, that way a programmer would have some ideas about which options are available from the Enum? But now I have to first use the FQN to figure out the option and then delete the first part of it make sure syntax is correct – kind of odd.

    Anyway, thanks again.

    Reply
16. Ivan says:

    4/11/2008 at 1:59 pm

    Thank you very much, the ONLY site that could help me were yours! ^^

    Sometimes Java drives us all mad… I would have used the FQN instead of this.

    Ivan

    Reply
17. Michael says:

    5/26/2008 at 12:52 pm

    Man, I could have spent hours on this. Thanks a bunch

    Reply
18. John says:

    6/4/2008 at 10:03 am

    Thanks for the wonderful advice. You’ve saved me a ton of time.

    Reply
19. Ville Anttonen says:

    8/29/2008 at 5:13 am

    Thanks! Helped me alot..

    Reply
20. Sleeveen says:

    9/10/2008 at 1:58 pm

    Likewise, a big thank you!

    I’m updating a large chunk of code to Java 1.6 and was having exactly this problem – a slightly better error message would solve the problem e.g. Use an unqualified reference in this case. Instead of Enum.CONSTANT just use CONSTANT.

    You don’t have to know everything…. you just have to know where to look

    Reply
21. Anna Skawińska says:

    9/16/2008 at 3:32 am

    Thanks a lot!

    Reply
22. Feniks says:

    11/3/2008 at 12:26 pm

    Thanks for the information.
    I also found this other post, with a link to a bug report with an explanation.
    http://www.davestone.net/blog/post/The-enum-con...

    Reply
23. coder says:

    12/22/2008 at 6:53 pm

    thanks so much! this helped me fix my “The enum constant ___ reference cannot be qualified in a case label” error really fast =]

    Reply
24. Jan says:

    1/7/2009 at 4:29 pm

    this helped me so much!! thx

    Reply
25. Jack says:

    1/27/2009 at 12:48 am

    Thank you Elliot, and thank you google. Saved me 15 ~ 20 minutes figuring this out.

    Reply
26. Aaron S says:

    3/19/2009 at 9:55 pm

    Thank you for your help!

    Reply
27.   Use enumerated constants in Java by Yingliang Du’s Blog says:

    9/24/2009 at 1:24 pm

    [...] on this article, Java 1.5 make it very easy to create enumerated constants. The keyword “enum” is a new [...]

    Reply