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Java Forum / General / June 2006two public class in a single file
i have two public class in a single file.it is giving compilation error so can any one tell what is the resion behind this. i know it will give but i dont know what is the think on this. AFAIK it is simply a convention - but i believe it is a good one, since it actually makes you seperate different aspects of your program (seperate public classes) into seperate files > i have two public class in a single file.it is giving compilation error > so can any one tell what is the resion behind this. > i know it will give but i dont know what is the think on this. The java compiler requires a public class to be in a file with the same name (i.e. MyClass should be in MyClass.java which will be compiled to MyClass.class). This is to enable the classloader to find the class when it is needed. As a result you can have only one public class in a file. Axel Hallez >> i have two public class in a single file.it is giving compilation error >> so can any one tell what is the resion behind this. [quoted text clipped - 5 lines] > it is needed. > As a result you can have only one public class in a file. I think the OP is asking why you can have only one class per *source* file. (Compiling a single source file with several public classes could still create several .class files such the classloader could find all of them.) Here is another reason: Assume you compile A.java which depends on class B. Having only one public class per source file, the compiler can find the source B.java, compile that first, and then compile A.java. (Try javac -sourcepath.) Cheers, Simon > The java compiler requires a public class to be in a file with the same > name Filenames are ASCII, class names are Unicode. There is a mapping between classnames and how to find them. Most of the time though you're right: class names and file names match. Cheers Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ <opalpa@gmail.com> wrote in message news:1147352034.811134.29300@u72g2000cwu.googlegroups.com... >> The java compiler requires a public class to be in a file with the same >> name > > Filenames are ASCII, class names are Unicode. There is a mapping > between classnames and how to find them. Most of the time though > you're right: class names and file names match. Depending on the OS, why wouldn't filenames "be unicode"[*] as well? - Oliver [*] the phrase is in quotes because comparing ASCII and Unicode doesn't make sense, as ASCII both an encoding system and a character set, while Unicode is a character set without an encoding system. > Depending on the OS, why wouldn't filenames "be unicode"[*] as well? Yes, in some environments. My statements were not careful enough. > [*] the phrase is in quotes because comparing ASCII and Unicode doesn't make > sense, as ASCII both an encoding system and a character set, while Unicode > is a character set without an encoding system. After spending some time reviewing defitiontion of unicode I see that many, including one from unicode.org (http://www.unicode.org/faq/basic_q.html#a) talk about unicode being an encoding as well as character set. Unicode.org technical introductions tarts with: "The Unicode Standard is the universal character encoding standard used for representation of text for computer processing." Perhaps I am completely misunderstanding your point. There is plenty of content on the internet where ASCII and Unicode are compared. Even if that comparison is not between two things of the same type, isn't it fairly clear that when comparing a filename's ASCII bytes equence to a classname's Unicode byte sequence one cannot perform a byte by byte match? Regards, Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ <opalpa@gmail.com> wrote in message news:1147538377.052199.15000@d71g2000cwd.googlegroups.com... >> [*] the phrase is in quotes because comparing ASCII and Unicode doesn't >> make [quoted text clipped - 6 lines] > (http://www.unicode.org/faq/basic_q.html#a) talk about unicode being an > encoding as well as character set. Hmm, maybe my terminology is a bit loose. I meant that ASCII encodes characters directly to byte sequences, whereas Unicode is a mapping from characters to natural numbers (of arbitrary size; e.g. not merely 0 to 2^32), and then you need a seperate encoding, like UTF-8, to map from those natural numbers to byte-sequences of finite size. > Unicode.org technical introductions tarts with: > "The Unicode Standard is the universal character encoding standard used [quoted text clipped - 6 lines] > classname's Unicode byte sequence one cannot perform a byte by byte > match? A lot of people (subconciously?) think Unicode maps directly from characters to byte sequences; it's a common misconception, so it wouldn't surprise me that there would be a large amount of content on the Internet which makes this mistake, or gloss over it. AFAIK, there's no such thing as a "Unicode byte sequence". You could talk about comparing ASCII byte sequences to UTF-8 byte sequences, but not to "Unicode byte sequences". - Oliver > A lot of people (subconciously?) think Unicode maps directly from >characters to byte sequences if you just use 16 bit Unicode, it does.  SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. >> A lot of people (subconciously?) think Unicode maps directly from >>characters to byte sequences > > if you just use 16 bit Unicode, it does. Only in UTF-16, AFAIK. In UTF-8 (which is one of the few encodings which is backwards compatible with ASCII when using only characters from the ASCII character set, and so one of the most commonly used encodings), the unicode character \u00A9 (the copyright symbol), for example, maps onto the byte sequence C2A9, and not 00A9. - Oliver > A lot of people (subconciously?) think Unicode maps directly from >>>characters to byte sequences >> >> if you just use 16 bit Unicode, it does. Let me restate that in a more lawyerly way. If you use only the codepoints 0..0xffff and you use UTF-16BE encoding, you can think of Unicode as mapping directly to byte sequences. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. One public class per file is an implementation detail of most java compilers. http://java.sun.com/docs/books/jls/third_edition/html/packages.html#7.2 Specifically, an alternative is proposed: " 7.2.2 Storing Packages in a Database A host system may store packages and their compilation units and subpackages in a database. Such a database must not impose the optional restrictions (§7.6) on compilation units in file-based implementations. For example, a system that uses a database to store packages may not enforce a maximum of one public class or interface per compilation unit. Systems that use a database must, however, provide an option to convert a program to a form that obeys the restrictions, for purposes of export to file-based implementations. " Opalinski opalpa@gmail.com http://www.geocities.com/opalpaweb/ > i have two public class in a single file.it is giving compilation error > so can any one tell what is the resion behind this. > i know it will give but i dont know what is the think on this. You may have only one public class defined in a class file. You may have multiple package-private classes (default visibility) in a single file. All other multiple class-per-file constructs involve inner classes. 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 ...
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