AppletTalk.com Java discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  static method and private class          AppletTalk.com Forum Index -> Java Help View previous topic :: View next topic   Author Message Martin Gerner Guest Posted: Thu Apr 20, 2006 2:12 pm    Post subject: static method and private class Hi, I've run into an error that I don't understand how to correct... public class Test{ private class PrivateClass{ public int somedata; } static public void DoSomething(){ String astring = new String(); PrivateClass anobject = new PrivateClass(); } } When compiling above code, it gives me the following error: Test.java:8: non-static variable this cannot be referenced from a static context. Commenting out the PrivateClass anobject.. line makes it compile fine. I can obviously create objects, but why not objects of a private class? It has to do with the method being static, of course, but I can't see how. Back to top Martin Gerner Guest Posted: Thu Apr 20, 2006 3:03 pm    Post subject: Re: static method and private class Simon <count.numbers (AT) web (DOT) de> wrote in news:4apl3bFtjsvaU1 (AT) news (DOT) dfncis.de: Quote: Hi, public class Test{ private class PrivateClass{ public int somedata; } static public void DoSomething(){ String astring = new String(); PrivateClass anobject = new PrivateClass(); } } When compiling above code, it gives me the following error: Test.java:8: non-static variable this cannot be referenced from a static context. Commenting out the PrivateClass anobject.. line makes it compile fine. I can obviously create objects, but why not objects of a private class? It has to do with the method being static, of course, but I can't see how. It has nothing to do with the class being private, but rather with the class being non-static. You have two options: 1. Declare PrivateClass static ("private static class PrivateClass") 2. Call the constructor from a non-static context. Test test = new Test(); PrivateClass anobject = test.new PrivateClass(); This may look weird and I believe it isn't used much. If you declare DoSomething() to be non-static, the "new PrivateClass()" automatically becomes "this.new PrivateClass()" which gives you the non-static context for the construcor. The reason for this behaviour is that from within a non-static inner class you can always reference an instance of the outer class (in this case by Test.this), which is only possible if such a reference exists when the instance of the inner class is created. Hm, that's crap. :/ Actually, I wasn't planning to create objects of the 'Test'class, but am rather using it to hold a bunch of static methods. These need to send data between themselves, which I was going to place in the inner private class. I could, of course, make everything non-static and create an instance of the Test class, but it seems a bit awkward.. doesn't seem to be much else I can do, though. (Except put the private class as public in a new java file, but that is even more awkward :p) Back to top Patricia Shanahan Guest Posted: Thu Apr 20, 2006 3:12 pm    Post subject: Re: static method and private class Martin Gerner wrote: Quote: Hi, I've run into an error that I don't understand how to correct... public class Test{ private class PrivateClass{ public int somedata; } static public void DoSomething(){ String astring = new String(); PrivateClass anobject = new PrivateClass(); } } When compiling above code, it gives me the following error: Test.java:8: non-static variable this cannot be referenced from a static context. Commenting out the PrivateClass anobject.. line makes it compile fine. I can obviously create objects, but why not objects of a private class? It has to do with the method being static, of course, but I can't see how. The problem is that PrivateClass, having not been declared static, needs a reference to an object of the surrounding class, Test. There are two solutions. If you want each PrivateClass to belong to a particular Test, you need to create an instance of Test and use it to create a PrivateClass. If you do not need that association you can declare PrivateClass static. Patricia Back to top Simon Guest Posted: Thu Apr 20, 2006 3:12 pm    Post subject: Re: static method and private class Hi, Quote: public class Test{ private class PrivateClass{ public int somedata; } static public void DoSomething(){ String astring = new String(); PrivateClass anobject = new PrivateClass(); } } When compiling above code, it gives me the following error: Test.java:8: non-static variable this cannot be referenced from a static context. Commenting out the PrivateClass anobject.. line makes it compile fine. I can obviously create objects, but why not objects of a private class? It has to do with the method being static, of course, but I can't see how. It has nothing to do with the class being private, but rather with the class being non-static. You have two options: 1. Declare PrivateClass static ("private static class PrivateClass") 2. Call the constructor from a non-static context. Test test = new Test(); PrivateClass anobject = test.new PrivateClass(); This may look weird and I believe it isn't used much. If you declare DoSomething() to be non-static, the "new PrivateClass()" automatically becomes "this.new PrivateClass()" which gives you the non-static context for the construcor. The reason for this behaviour is that from within a non-static inner class you can always reference an instance of the outer class (in this case by Test.this), which is only possible if such a reference exists when the instance of the inner class is created. Cheers, Simon BTW: What's the term for "non-static" once again? Back to top Philipp Leitner Guest Posted: Thu Apr 20, 2006 4:12 pm    Post subject: Re: static method and private class Well, if you want to send "data" between those static objects they clearly should not be static at all, but standard objects (singletons perhaps, but still actual concrete objects)! /philipp Martin Gerner schrieb: Quote: Simon <count.numbers (AT) web (DOT) de> wrote in news:4apl3bFtjsvaU1 (AT) news (DOT) dfncis.de: Hi, public class Test{ private class PrivateClass{ public int somedata; } static public void DoSomething(){ String astring = new String(); PrivateClass anobject = new PrivateClass(); } } When compiling above code, it gives me the following error: Test.java:8: non-static variable this cannot be referenced from a static context. Commenting out the PrivateClass anobject.. line makes it compile fine. I can obviously create objects, but why not objects of a private class? It has to do with the method being static, of course, but I can't see how. It has nothing to do with the class being private, but rather with the class being non-static. You have two options: 1. Declare PrivateClass static ("private static class PrivateClass") 2. Call the constructor from a non-static context. Test test = new Test(); PrivateClass anobject = test.new PrivateClass(); This may look weird and I believe it isn't used much. If you declare DoSomething() to be non-static, the "new PrivateClass()" automatically becomes "this.new PrivateClass()" which gives you the non-static context for the construcor. The reason for this behaviour is that from within a non-static inner class you can always reference an instance of the outer class (in this case by Test.this), which is only possible if such a reference exists when the instance of the inner class is created. Hm, that's crap. :/ Actually, I wasn't planning to create objects of the 'Test'class, but am rather using it to hold a bunch of static methods. These need to send data between themselves, which I was going to place in the inner private class. I could, of course, make everything non-static and create an instance of the Test class, but it seems a bit awkward.. doesn't seem to be much else I can do, though. (Except put the private class as public in a new java file, but that is even more awkward :p) Back to top Simon Guest Posted: Thu Apr 20, 2006 4:12 pm    Post subject: Re: static method and private class Martin Gerner wrote: Quote: Hm, that's crap. :/ What is crap, my answer or Java? If you mean Java is crap, then my answer is: This is the only way it makes sense. Instances of inner classes have a reference to the outer class -> you cannot create them from a static context. Static inner classes don't have such a reference -> you can create them from everywhere. Quote: Actually, I wasn't planning to create objects of the 'Test'class, but am rather using it to hold a bunch of static methods. These need to send data between themselves, which I was going to place in the inner private class. I could, of course, make everything non-static and create an instance of the Test class, but it seems a bit awkward.. doesn't seem to be much else I can do, though. (Except put the private class as public in a new java file, but that is even more awkward :p) I'm not sure I understand. Especially, I don't see why making the private class public and putting it into a file of its own solves the problem. What do you mean by "send data"? If you want to maintain some state of "Test", then you can use static fields in Test. If you want to pass parameters between static methods calling each other then you can declare your inner class static. If Test really only has static methods, then certainly there is no point in making the inner class non-static because there is nothing useful the inner class can access in the instance of the outer class. I'm not sure I'm getting the point, though. Back to top Martin Gerner Guest Posted: Thu Apr 20, 2006 4:25 pm    Post subject: Re: static method and private class Simon <count.numbers (AT) web (DOT) de> wrote in news:4appraFu38l7U1 (AT) news (DOT) dfncis.de: Quote: Martin Gerner wrote: Hm, that's crap. :/ What is crap, my answer or Java? If you mean Java is crap, then my answer is: This is the only way it makes sense. Instances of inner classes have a reference to the outer class -> you cannot create them from a static context. What I meant was that it was crap that I couldn't get it to work as I would have liked it to. No disrespect intended - on the contrary, you gave a quick and good response. (both you and Patricia) I am using the methods as non-static now, anyways. Perhaps that is better, I don't know, but the main point is that it works, and how I would have liked it wouldn't have worked. /Martin Gerner Back to top Hendrik Maryns Guest Posted: Thu Apr 20, 2006 5:12 pm    Post subject: Re: static method and private class -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Martin Gerner schreef: Quote: Simon <count.numbers (AT) web (DOT) de> wrote in news:4apl3bFtjsvaU1 (AT) news (DOT) dfncis.de: Hi, public class Test{ private class PrivateClass{ public int somedata; } static public void DoSomething(){ String astring = new String(); PrivateClass anobject = new PrivateClass(); } } When compiling above code, it gives me the following error: Test.java:8: non-static variable this cannot be referenced from a static context. Commenting out the PrivateClass anobject.. line makes it compile fine. I can obviously create objects, but why not objects of a private class? It has to do with the method being static, of course, but I can't see how. It has nothing to do with the class being private, but rather with the class being non-static. You have two options: 1. Declare PrivateClass static ("private static class PrivateClass") 2. Call the constructor from a non-static context. Test test = new Test(); PrivateClass anobject = test.new PrivateClass(); This may look weird and I believe it isn't used much. If you declare DoSomething() to be non-static, the "new PrivateClass()" automatically becomes "this.new PrivateClass()" which gives you the non-static context for the construcor. The reason for this behaviour is that from within a non-static inner class you can always reference an instance of the outer class (in this case by Test.this), which is only possible if such a reference exists when the instance of the inner class is created. Hm, that's crap. :/ It isn’t. Quote: Actually, I wasn't planning to create objects of the 'Test'class, but am rather using it to hold a bunch of static methods. These need to send data between themselves, which I was going to place in the inner private class. I could, of course, make everything non-static and create an instance of the Test class, but it seems a bit awkward.. doesn't seem to be much else I can do, though. (Except put the private class as public in a new java file, but that is even more awkward :p) I think you just want to declare PrivateClass as being static. H. - -- Hendrik Maryns ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (GNU/Linux) iD8DBQFER7KAe+7xMGD3itQRAsbEAJ0aZ6EOplwlUu9cFtUwD2PbYN60EwCfanP8 ueMNDDfV2Y4sIsOaZSfxC2Y= =c6Ez -----END PGP SIGNATURE----- Back to top Roedy Green Guest Posted: Thu Apr 20, 2006 5:12 pm    Post subject: Re: static method and private class On Thu, 20 Apr 2006 14:12:05 +0000 (UTC), Martin Gerner <martin.gerner (AT) nospam (DOT) com> wrote, quoted or indirectly quoted someone who said : Quote: non-static variable this cannot be referenced from a static context. see http://mindprod.com/jgloss/compileerrormessages.html#NONSTATICCANTBEREF -- Canadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Back to top Andrew McDonagh Guest Posted: Thu Apr 20, 2006 5:12 pm    Post subject: Re: static method and private class Martin Gerner wrote: Quote: Simon <count.numbers (AT) web (DOT) de> wrote in news:4apl3bFtjsvaU1 (AT) news (DOT) dfncis.de: Hi, public class Test{ private class PrivateClass{ public int somedata; } static public void DoSomething(){ String astring = new String(); PrivateClass anobject = new PrivateClass(); } } When compiling above code, it gives me the following error: Test.java:8: non-static variable this cannot be referenced from a static context. Commenting out the PrivateClass anobject.. line makes it compile fine. I can obviously create objects, but why not objects of a private class? It has to do with the method being static, of course, but I can't see how. It has nothing to do with the class being private, but rather with the class being non-static. You have two options: 1. Declare PrivateClass static ("private static class PrivateClass") 2. Call the constructor from a non-static context. Test test = new Test(); PrivateClass anobject = test.new PrivateClass(); This may look weird and I believe it isn't used much. If you declare DoSomething() to be non-static, the "new PrivateClass()" automatically becomes "this.new PrivateClass()" which gives you the non-static context for the construcor. The reason for this behaviour is that from within a non-static inner class you can always reference an instance of the outer class (in this case by Test.this), which is only possible if such a reference exists when the instance of the inner class is created. Hm, that's crap. :/ no its intentional for many good reasons. Quote: Actually, I wasn't planning to create objects of the 'Test'class, but am rather using it to hold a bunch of static methods. These need to send data between themselves, which I was going to place in the inner private class. I could, of course, make everything non-static and create an instance of the Test class, but it seems a bit awkward.. doesn't seem to be much else I can do, though. (Except put the private class as public in a new java file, but that is even more awkward :p) The inner class you have written is called a Inner class If you use the static keyword you have a Static Nested class - these do not have the reference to their enclosing class. class EnclosingClass { ... static class StaticNestedClass { ... } class InnerClass { ... } } http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html Back to top Martin Gerner Guest Posted: Thu Apr 20, 2006 5:57 pm    Post subject: Re: static method and private class Patricia Shanahan <pats (AT) acm (DOT) org> wrote in news:wlP1g.253$Vn.42 (AT) newsread4 (DOT) news.pas.earthlink.net: Quote: I don't understand why you didn't like declaring the private class "static". It seems to me that that would have done exactly what you expected. Hm. Yes, I tried that now, and it actually worked perfectly, just as I wanted. What I thought would happen when declaring the private class static was that the fields contained would have become static as well. Since I needed multiple 'PrivateClass' objects with different data, that wouldn't have worked. But ok, thank you, I believe that I've learned something useful now (Not to mention that my code became a bit nicer.) -- Martin Gerner Back to top Patricia Shanahan Guest Posted: Thu Apr 20, 2006 6:12 pm    Post subject: Re: static method and private class Martin Gerner wrote: Quote: Simon <count.numbers (AT) web (DOT) de> wrote in news:4appraFu38l7U1 (AT) news (DOT) dfncis.de: Martin Gerner wrote: Hm, that's crap. :/ What is crap, my answer or Java? If you mean Java is crap, then my answer is: This is the only way it makes sense. Instances of inner classes have a reference to the outer class -> you cannot create them from a static context. What I meant was that it was crap that I couldn't get it to work as I would have liked it to. No disrespect intended - on the contrary, you gave a quick and good response. (both you and Patricia) I am using the methods as non-static now, anyways. Perhaps that is better, I don't know, but the main point is that it works, and how I would have liked it wouldn't have worked. /Martin Gerner I don't understand why you didn't like declaring the private class "static". It seems to me that that would have done exactly what you expected. Personally, I tend to avoid extensive use of static, but if you are planning to have a lot of static methods, I don't see the objection to a static class. Patricia Back to top Oliver Wong Guest Posted: Thu Apr 20, 2006 7:12 pm    Post subject: Re: static method and private class "Simon" <count.numbers (AT) web (DOT) de> wrote in message news:4apl3bFtjsvaU1 (AT) news (DOT) dfncis.de... Quote: BTW: What's the term for "non-static" once again? Instance. - Oliver Back to top Hendrik Maryns Guest Posted: Fri Apr 21, 2006 1:12 pm    Post subject: Re: static method and private class -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Martin Gerner schreef: Quote: Patricia Shanahan <pats (AT) acm (DOT) org> wrote in news:wlP1g.253$Vn.42 (AT) newsread4 (DOT) news.pas.earthlink.net: I don't understand why you didn't like declaring the private class "static". It seems to me that that would have done exactly what you expected. Hm. Yes, I tried that now, and it actually worked perfectly, just as I wanted. What I thought would happen when declaring the private class static was that the fields contained would have become static as well. Since I needed multiple 'PrivateClass' objects with different data, that wouldn't have worked. You misunderstood. It still is a perfectly normal class, with instance methods and objects. The static just means it is not bound to an object of the enclosing class. A non-static inner class always has a reference to an enclosing object of the outer class through OuterClass.this. I found a nice pdf on the net that explains this all very thoroughly: http://java.sun.com/developer/Books/certification/certbook.pdf. Quote: But ok, thank you, I believe that I've learned something useful now (Not to mention that my code became a bit nicer.) I hope you did :-) H. - -- Hendrik Maryns ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (GNU/Linux) iD8DBQFESM7me+7xMGD3itQRAgi9AJ4zaUB3JM1EaF8UG8tXFDe+NJG85gCfZJAG GZvGbjWgKmr1G5SKG8/193I= =1CEZ -----END PGP SIGNATURE----- Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AppletTalk.com Forum Index -> Java Help All times are GMT Page 1 of 1   Jump to: Select a forum Java Newsgroups----------------Java DiscussionsWeb-based java games comp.lang.java 3D Graphics API's for Java Java: Support and criticism Java System Announcements comp.lang.java.api JavaBeans Java and CORBA Java and Databases comp.lang.java.developer Java GUI Toolkits Java Help comp.lang.java.javascript JVM, native methods and hardware comp.lang.java.misc Java Language Programming Security and Java comp.lang.java.setup Java Software Tools comp.lang.java.tech ibm.software.java.linux  Non-English Java Newsgroups----------------Java Language (German)Language Java (Français)Java (Italian)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.