 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Java Forum / General / August 2007Inner static puzzle
I have always wondered why static inner classes can have static members but ordinary inner classes cannot. It seems a strange anti-orthogonality. Why? It would at first glance seem more work to treat them specially. Is there some difficulty in implementation? Is there some reason why stylistically the statics would be wicked?  SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > I have always wondered why static inner classes can have static > members but ordinary inner classes cannot. It seems a strange [quoted text clipped - 3 lines] > Is there some difficulty in implementation? Is there some reason why > stylistically the statics would be wicked? I speculate it was to simplify notational cruft and difficulties of implementation. I've always been confused by whether a putative inner static would need an enclosing instance, leading to weird notational idiomata: new Foo().Bar.illegalStaticString = ""; or whether such a static would be static across all enclosing instances of the inner class objects: Foo.Bar.illegalStaticString = ""; Would a static inner-class method have access to any enclosing instance attributes? I suspect the choices matter at a low level in either the compiler or the JVM and would've been difficult. SignatureLew >or whether such a static would be static across all enclosing instances of the >inner class objects: I assumed they would be. The statics would be primarily for static final constants local only to that inner class. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com Lew wrote: >> or whether such a static would be static across all enclosing instances of the >> inner class objects: > I assumed they would be. The statics would be primarily for static > final constants local only to that inner class. But compile-time constants are legal: <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.3> > Inner classes may not declare static members, unless they are compile-time constant fields (§15.28). ... > class HasStatic{ > static int j = 100; [quoted text clipped - 9 lines] > interface NeverInner{} // interfaces are never inner > } SignatureLew >But compile-time constants are legal: >http://java.sun.com/docs/books/jls/ >third_edition/html/classes.html#8.1.3 >Inner classes may not declare static members, >unless they are compile-time constant fields (15.28) Seems to me javac rejected something of this form in an inner class. private static final int X; static { int temp = something; X = expression; } SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >> But compile-time constants are legal: >> http://java.sun.com/docs/books/jls/ [quoted text clipped - 8 lines] > X = expression; > } That's not a compile-time constant. A compile-time constant's value is set in the declaration, and result in the field's value being explcitly stored in the class file, so that it's available at compile time. That's what causes this odd-seeming behavior: Class1.java: public static final int i = 12; Class2.java: System.out.println(Class1.i); This will, of course, print "12". If you change i's value to 13 and recompile Class1, it continues to print 12 until you recompile Class2. >>> But compile-time constants are legal: >>> http://java.sun.com/docs/books/jls/ [quoted text clipped - 24 lines] > This will, of course, print "12". If you change i's value to 13 and > recompile Class1, it continues to print 12 until you recompile Class2. Furthermore, the JLS also states: <http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.3> > Inner classes may not declare static initializers (§8.7) or member interfaces. Logical or not, the rules are laid out. SignatureLew >> I have always wondered why static inner classes can have static >> members but ordinary inner classes cannot. It seems a strange [quoted text clipped - 22 lines] > I suspect the choices matter at a low level in either the compiler or the > JVM and would've been difficult. I disagree. I've always considered the restriction arbitrary, and think the behavior is both clear and easy to implement. There's already a model for its behavior: a local or anonymous class defined inside a static method of the containing class. The resulting rules are: 1. Static members of an inner class would be truly static, that is, scoped to the class rather than to an enclosing instance, and could be referenced as Foo.Bar.member. 2. There is no enclosing instance inside a static method; attempts to access instance methods of the containing class would be illegal. And this all falls naturally out of the implementation. >I have always wondered why static inner classes can have static > members but ordinary inner classes cannot. It seems a strange [quoted text clipped - 3 lines] > Is there some difficulty in implementation? Is there some reason why > stylistically the statics would be wicked? As others have suggested, the problem lays in the "staticness" of the construct: class Outer{ class Inner { static int member; // illegal } } Is there a "member" per Outer instance (in which case, it should just be a member of Outer, not stuck in Inner, scoping be damned), or is there just one "member"? >Is there a "member" per Outer instance (in which case, it should just be a >member of Outer, not stuck in Inner, scoping be damned), or is there just >one "member"? statics are always one per class. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > I have always wondered why static inner classes can have static > members but ordinary inner classes cannot. It seems a strange > anti-orthogonality. My guess is that it's for consistency with the idea that an inner class (a non-static nested class) is not /one/ class, but one class /per instance/ of the outer class. Following that idea, everything you describe become necessary. You cannot have a static field shared by all the inner classes, since a static belongs to only one class[1]. /L [1] ... although you could implement it by implementing statics on the inner class as non-statics on instances of the outer class. THAT would be hackery, though. SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' >> I have always wondered why static inner classes can have static >> members but ordinary inner classes cannot. It seems a strange [quoted text clipped - 3 lines] > class (a non-static nested class) is not /one/ class, but one class > /per instance/ of the outer class. If that idea had any support, I might agree with you. >>> I have always wondered why static inner classes can have static >>> members but ordinary inner classes cannot. It seems a strange [quoted text clipped - 5 lines] > > If that idea had any support, I might agree with you. Heh. I wish I could find anybody but myself saying it ... but that might be just me googleing for the way I would formulate it. I found a two year old discussion here, where I said essentially the same, to essentially the same question. <URL:http://groups.google.com/group/comp.lang.java.programmer/browse_frm/thread/29346 b15b8b3ac80/034bb32c24a0f75b?lnk=st&q=%22statics+in+inner+classes%22&rnum=3&hl=e n#034bb32c24a0f75b> and another one in August 2005 too ... at least I am consistent :) <URL:http://groups.google.com/group/comp.lang.java.programmer/browse_frm/thread/cc69f 69cabb66d34/2e0efe80921ddf57?lnk=st&q=%22Nested+Class+Language+Lawyers%22&rnum=1 &hl=en#2e0efe80921ddf57> Maybe I have just believed this for so long that I thought everybody else did too :) It is consistent with everything in the design of inner classes, so it's not like I have had it challenged. Ah well. It was a good idea. /L SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' >>>> I have always wondered why static inner classes can have static >>>> members but ordinary inner classes cannot. It seems a strange [quoted text clipped - 20 lines] > > Ah well. It was a good idea. I was being snippy, and I apologize for that. Anyway, we had this discussion back in February starting with this post: http://groups.google.com/group/comp.lang.java.programmer/msg/ff5c4bf25469be1a and the consensus, I think, was that while some language in the JLS suggests that there's an inner class per instance of the outer class, that the idea can't be sustained. >My guess is that it's for consistency with the idea that an inner >class (a non-static nested class) is not /one/ class, but one class >/per instance/ of the outer class. There may be many instances, but only one inner class. Have a look at the generated class files. There is only one per inner class, instance or static, not one per instance of the outer class. 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
|
©2009 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.