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Java Forum / First Aid / January 2006Hmmm....I apparently have the wrong idea about extending classes
I was under the impression that you extended classes so that you could have the same variables as the class you're extending from. But when I try this: public class A { private String x = "Hi"; private String y = " there!"; } public class B extends A { public void printIt() { System.out.print(x + y); } } public class C { public static void main(String[] args) { C drive = new C(); drive.runIt(); } public void runIt() { B b = new B(); b.printIt(); } } I get this: C:\Documents and Settings\HP_Owner\My Documents\JunkDrawer\JavaPrograms \javaExperiments\ExtendingExample\B.java:6: x has private access in A System.out.print(x + y); ^ C:\Documents and Settings\HP_Owner\My Documents\JunkDrawer\JavaPrograms \javaExperiments\ExtendingExample\B.java:6: y has private access in A System.out.print(x + y); ^ 2 errors Process javac exited with code 1 So my thinking was wrong but now I'm kind of like WTF. Why is it saying in A instead of B? Isn't the purpose in extending so that you can use the variables in the subclass?  SignatureNoodles Jefferson mhm31x9 Smeeter#29 WSD#30 sTaRShInE_mOOnBeAm aT HoTmAil dOt CoM NP: "Informer" -- Snow "Our earth is degenerate in these latter days, bribery and corruption are common, children no longer obey their parents and the end of the world is evidently approaching." --Assyrian clay tablet 2800 B.C. > I was under the impression that you extended classes so that you could > have the same variables as the class you're extending from. [quoted text clipped - 7 lines] > > } "protected" is what you want. SignatureSean Be careful about reading health books. You may die of a misprint. --Mark Twain > > I was under the impression that you extended classes so that you could > > have the same variables as the class you're extending from. [quoted text clipped - 9 lines] > > "protected" is what you want. Oh duh. Thanks. SignatureNoodles Jefferson mhm31x9 Smeeter#29 WSD#30 sTaRShInE_mOOnBeAm aT HoTmAil dOt CoM NP: "Informer" -- Snow "Our earth is degenerate in these latter days, bribery and corruption are common, children no longer obey their parents and the end of the world is evidently approaching." --Assyrian clay tablet 2800 B.C. SMC skrev: Snip. > "protected" is what you want. Don't use, "protected," unless you can't use the default access modifer. .ed -- www.EdmundKirwan.com - Home of The Fractal Class Composition. What are you talking about? protected and the default access modifier -- package private, have different meaning. I don't understand your advice. Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ public members are accessible from any method. private members only from methods in the same class. protected members are accessible from methods in extending classes, subclasses (in addition to methods in the class itself) package members, when there's no specifier written, are accessible from methods in classes that are in the same package. --- Paul Hamaker, SEMM http://javalessons.com > protected members are accessible from methods in extending classes, Protected members are freely accessible in all classes in the same package, and are also inherited by extensions, and accessible with certain restrictions in extensions in other packages. Really? Protected members are accessible by package classes that don't extend? I didn't know that. Why in the world is that? I'm gonna try this out. It boggles me why that would be. Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ If I was making a language I would change that. Thanks for the info. One other aspect I would change is the ability to access inner class's private members from outer classes. Like, I think the following should not be allowed: package experiment; class InnerClassPrivatesExposedBug { private static class Inner { private char datum; } InnerClassPrivatesExposedBug() { Inner inner; (inner = new Inner()).datum = 'F'; System.out.println("inner's datum: " + inner.datum); } public static void main(String args[]) { new InnerClassPrivatesExposedBug(); } } Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ On 16 Jan 2006 07:30:50 -0800, "opalpa@gmail.com opalinski from opalpaweb" <opalpa@gmail.com> wrote, quoted or indirectly quoted someone who said : >Really? Protected members are accessible by package classes that don't >extend? I didn't know that. > >Why in the world is that? > >I'm gonna try this out. It boggles me why that would be. Protected is a bit more public that default. The idea is if you extend a package you know more what you are doing that if you merely use it. see http://mindprod.com/jgloss/scope.html By the way, has Sun standardized on a proper name for default scope? Even if there is not a keyword, there still needs to be a name for it in documentation on the scope. "default" is not a name; it is an adjective. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. They call it "package private". http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol.html This table shows things off pretty concisely: Specifier Class Package Subclass World private Y N N N no specifier Y Y N N protected Y Y Y N public Y Y Y Y This is what I thought was going on, and personally I want this: Specifier Class Package Subclass World private Y N N N no specifier Y Y N N protected Y N Y N public Y Y Y Y I would change protected line's package visibility. I thought that's what it was too. For what... ten years I thought that is what it was. Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ > By the way, has Sun standardized on a proper name for default scope? > Even if there is not a keyword, there still needs to be a name for it > in documentation on the scope. "default" is not a name; it is an > adjective. "default" could be both a name and an adjective, just like "public" and "protected" and "private" are also both names and adjectives. - Oliver opalpa@gmail.com opalinski from opalpaweb skrev: > What are you talking about? protected and the default access modifier > -- package private, have different meaning. I don't understand your [quoted text clipped - 3 lines] > opalpa@gmail.com > http://www.geocities.com/opalpaweb/ (After Paulus's excellent email, this is probably unnecessary, but as I was asked ...) All I meant was a designer should restrict access as much as possible. So if Noodles was designing a base class to be extended by other classes in other packages in the system, then he should mark the methods as, "protected." If, however, Noodles was designing a base class to be extended only by other classes in the same package, then he should mark the methods with the default access modifier. .ed -- www.EdmundKirwan.com - Home of The Fractal Class Composition. Paulus provided a list of definitions. Perhaps my question was not clear because I know those definitions well (his email did not give me more info). I don't think of package private as being along a gradient with the other access modifiers. The situation you are describing iamfrac, is a little strange, say we have a package X with class C in it, and consider the following two cases: 1. C is a public class 2. C is a package private class In case 2, classes outside of package X cannot extend C so protected and package private have different meaning. protected would be more restrictive, allowing only subclass of C within X to use a method. In case 1, we are making a public class and want to only allow extending classes in X to access certain methods? That does not make sense to me. There are a few different designs that come to mind, one is to have a package private subclass in X that has those methods and uses protected on them. --- Using package private for protected in a class allows non-subclass in package to call methods which they should be restricted from calling. In short I disagree with your advice, finding it overly simplistic and a potential root of problems. Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ Verified fact that package private is on gradient with other access modifiers. Learned something pretty big today. Thanks for posting your messages. This new fact and ability to access private members of inner classes are my least two favorite things about the Java language. I think the language is excellent, but I would change these things in a language I would create. My other peeves are: * "!" reads poorly; i would prefer "not" * import static is file scope and not class scope * enhanced for syntax should use "in" instead of ":" * 1 gig heap ceiling All in all, not too bad, but still peeves. Thanks Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ > Verified fact that package private is on gradient with other access > modifiers. Learned something pretty big today. Thanks for posting > your messages. JDK 1.0 had private protected which did what protected looks as if it might do. It was removed in 1.1, and never really worked anyway (apparently). I'm not keen on using protected or package private (except types) in most cases. Certainly variables should be kept private. > This new fact and ability to access private members of inner classes > are my least two favorite things about the Java language. I think the Inner classes accessing privates of the outer classes is necessary. The other way around seems completely unnecessary to me. > language is excellent, but I would change these things in a language I > would create. > > My other peeves are: > * "!" reads poorly; i would prefer "not" && as and, and: or and-then and || as or, or: or or-else or something. I'd ditch the likes of ~, &, |, ^, <<, %, / on integers, /=, etc., altogether. > * import static is file scope and not class scope You aren't suggesting putting more than one outer class in the same file, are you? I suppose it'd work applicable to members and constructors. > * enhanced for syntax should use "in" instead of ":" I want to keep in for my InputStreams, Readers and similar! I certainly don't want Microsoft-style grammar sensitive keyword hacks. They could have added a foreach keyword, but it seems to work as it is. > * 1 gig heap ceiling Is there? Tom Hawtin SignatureUnemployed English Java programmer http://jroller.com/page/tackline/ About 1% of the time I have multiple outer classes in same file. The better example is when I want "import static" to be in an inner class only. I believe it is possible that "import static", "lack of private protected", and "inner class privates being exposed" might be a reality of implementation. In other words those are desireable aspects however the additional effort needed to carry them out put them outside of feasible. I don't know. I got my fingers crossed for the future. Maybe sun will put a copy of compiler into open source one day and I can pay someone to make these mods -- or maybe do it myself. Don't know. I don't know how grammar sensitive keyword hacks cause problems. I'll look into this. Even this one small change? Interesting blog, btw. and let me get more info on the 1 gig heap thing. Last I read about it that is what I understood. Although personally I seem to approach the boundry I've yet to need to breach it and am completely knowledable about it. Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ > I believe it is possible that "import static", "lack of private > protected", and "inner class privates being exposed" might be a reality [quoted text clipped - 4 lines] > can pay someone to make these mods -- or maybe do it myself. Don't > know. Isn't the Eclipse JDT compiler open source? But if you did add a random feature to it, it'd no longer be Java. Tom Hawtin SignatureUnemployed English Java programmer http://jroller.com/page/tackline/ On Mon, 16 Jan 2006 17:31:04 +0000, Thomas Hawtin <usenet@tackline.plus.com> wrote, quoted or indirectly quoted someone who said : >&& as and, and: or and-then and || as or, or: or or-else or something. >I'd ditch the likes of ~, &, |, ^, <<, %, / on integers, /=, etc., >altogether. If you do that, much of the class library could not be written in Java, e.g .fiddling byte sex. dealing with unsigned bytes. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. >> This new fact and ability to access private members of inner classes >> are my least two favorite things about the Java language. I think the > > Inner classes accessing privates of the outer classes is necessary. The > other way around seems completely unnecessary to me. Isn't inner-classes-accessing-privates-of-the-outer-classes emulated by the compiler creating virtual protected accessors for those proviate fields anyway? That's what some of Eclipse's warnings would seem to have you believe. - Oliver In article <1137400803.935472.234170@f14g2000cwb.googlegroups.com>, iamfractal@hotmail.com took the hamburger, threw it on the grill, and I said "Oh wow"... > SMC skrev: > [quoted text clipped - 4 lines] > Don't use, "protected," unless you can't use the default access > modifer. How are you supposed to extend classes without it? Surely you don't want your variable public. SignatureNoodles Jefferson mhm31x9 Smeeter#29 WSD#30 sTaRShInE_mOOnBeAm aT HoTmAil dOt CoM NP: "Informer" -- Snow "Our earth is degenerate in these latter days, bribery and corruption are common, children no longer obey their parents and the end of the world is evidently approaching." --Assyrian clay tablet 2800 B.C. Noodles Jefferson skrev: > In article <1137400803.935472.234170@f14g2000cwb.googlegroups.com>, > iamfractal@hotmail.com took the hamburger, threw it on the grill, and I [quoted text clipped - 15 lines] > Noodles Jefferson > mhm31x9 Smeeter#29 WSD#30 Note that, "protected," does not exclusively denote that a method can be overridden by a subclass. The default access modifier also denotes that a method can be overridden by a subclass. (So does, "public.") The following is perfectly acceptable (untested). class A { void sayCheese() { system.out.println("A says cheese"); } } class B { void sayCheese() { system.out.println("B says cheese"); } } .ed -- www.EdmundKirwan.com - Home of The Fractal Class Composition. Ooops, that class B should have extended class A .. class B extends A { void sayCheese() { system.out.println("B says cheese"); } } .ed -- www.EdmundKirwan.com - Home of The Fractal Class Composition. > SMC skrev: > [quoted text clipped - 4 lines] > Don't use, "protected," unless you can't use the default access > modifer. If you use the default access modifier, the only way for subclasses to access the variable/method/whatever is to be in the same package. I would say, be very careful about which modifiers you choose, especially if you're publishing an API. Signaturemonique Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html To find out more about access specifiers you can look here : http://javalessons.com/cgi-bin/fun/java-tutorials-main.cgi?ses=ao789 then lookup 33.Access under Java Fundamentals. --- Paul Hamaker, SEMM http://javalessons.com Free MagazinesGet these publications absolutely FREE for up to 12 months. 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