 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Java Forum / General / November 2007enums and modifiers
Hello all, I was messing around with enums and am confused about the modifiers I can apply. I am using Eclipse and am not sure if I am misunderstanding the error. Consider this enum enum Planets {Mercury, Venus} I can put private/protected/public in front of it. At the moment I consider it to have package access. This makes perfect sense I can also add static in front of it. Enum is a class that extends Object so that kind of makes sense. However I cannot add final in front of it. Now I dont believe you can extends enums (why would you want to) so having final might seem a little pointless but i do not see why you cannot have it for completeness. (You can add final to variables in Iiterfaces which are implicitly final) Also and more confusingly when I put final in front of it then Eclipse says it is an invalid modifier and that I can only have private/protected/public/ abstract/static. When I put abstract in front I get a compile error. Is Eclipse wrong? Can you put an abstract in front of enums in some cases? Is there any reason you cannot put final in front? (Assuming you can use abstract then i can see why final isnt allowed). What use would an abstract have? abstract enum Planet {Mercury, Venus} //does not compile Any help is greatly appreciated Thanks R ah so the Enum class is abstract. This I believe is what the compiler uses when it come across the enum keyword. That would explain why I cannot use final. Is the fact that the class is abstract confusing Eclipse which tells me I can use abstract? > ah so the Enum class is abstract. This I believe is what the compiler > uses when it come across the enum keyword. That would explain why I > cannot use final. Is the fact that the class is abstract confusing > Eclipse which tells me I can use abstract? Don't know what's up with Eclipse, but the JLS clearly states: > Enum types (§8.9) must not be declared abstract; > doing so will result in a compile-time error. and > An enum type is implicitly final > unless it contains at least one enum constant that has a class body. > In any case, it is a compile-time error to explicitly declare an enum type to be final. Also, > Nested enum types are implicitly static. > It is permissable to explicitly declare a nested enum type to be static. There are more restrictions, too. <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.9> It's not to do with the implementation of enums as extension of Enum, rather, with the language's laws.  SignatureLew Hi Lew and Joshua, Thanks for the replies. It is much clearer now. I am studying for my SCJP and there is nothing like messing around with code to make you confused. Thanks again Richard > > ah so the Enum class is abstract. This I believe is what the compiler > > uses when it come across the enum keyword. That would explain why I [quoted text clipped - 22 lines] > -- > Lew > I can put private/protected/public in front of it. At the moment I > consider it to have package access. This makes perfect sense Only if it is internal to a class. Top-level source declarations can only have a package-protected or public access level, same as classes, interfaces, and annotations. > I can also add static in front of it. Enum is a class that extends > Object so that kind of makes sense. Once again, only if internal to a class. > However I cannot add final in > front of it. Now I dont believe you can extends enums (why would you > want to) so having final might seem a little pointless but i do not > see why you cannot have it for completeness. (You can add final to > variables in Iiterfaces which are implicitly final) enums != interfaces. The short answer: enums have nuanced finality/abstractness qualifiers. Specifically, an enum of the sort: enum Foo { ONE { public boolean getVar() { return true; } }, TWO { public boolean getVar() { return false; } } public abstract boolean getVar(); } is actually composed with three classes internally (Foo, Foo$ONE, and Foo$TWO) of which the first one is abstract and the other two are not. Since an enum can actually be abstract in some cases and final in others, neither qualifier is valid. See the JLS for more information. > Also and more > confusingly when I put final in front of it then Eclipse says it is an > invalid modifier and that I can only have private/protected/public/ > abstract/static. When I put abstract in front I get a compile error. See above. > Is Eclipse wrong? Can you put an abstract in front of enums in some > cases? Is there any reason you cannot put final in front? (Assuming > you can use abstract then i can see why final isnt allowed). What use > would an abstract have? No, Eclipse is not wrong; it follows the JLS precisely in this instance. JLS 3 §8.9 Enums: Enum types (§8.9) must not be declared abstract; doing so will result in a compile-time error. It is a compile-time error for an enum type E to have an abstract method m as a member unless E has one or more enum constants, and all of E's enum constants have class bodies that provide concrete implementations of m. It is a compile-time error for the class body of an enum constant to declare an abstract method. An enum type is implicitly final unless it contains at least one enum constant that has a class body. In any case, it is a compile-time error to explicitly declare an enum type to be final. Nested enum types are implicitly static. It is permissable to explicitly declare a nested enum type to be static. Those are the full rules for modifiers attached to enums. SignatureBeware of bugs in the above code; I have only proved it correct, not tried it. -- Donald E. Knuth >I was messing around with enums and am confused about the modifiers I >can apply See http://mindprod.com/jgloss/enum.html If you look at how enums get turned into ordinary classes, some of the strange enum rules begin to make sense. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
|
Reply to this Message
|
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.