Public class name same as filename

Every book I've read stated that if you have a public class declared in the source file, then you have to name the file the same as the class name. However, none of the book explain why, can anyone tell me about this?

Not all public classes need to have the same name as the file that contains them -- only top-level public classes. Public nested classes can have a name that's different from the filename.
Also, not only do top-level public classes need to have the same name as the file that contains them, but the names of the package and the directory structure must match as well.
This is done simply so that if you see a class named com.mycompany.mypackage.MyClass referred to somewhere, you know that the source code is in the file com/mycompany/mypackage/MyClass.java. Isn't that helpful? ;-)

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Public class name same as file name

Why should the public class name be same as the file name in which it is written?
http://forum.java.sun.com/thread.jspa?threadID=198585&messageID=657978
The above thread says it is because it makes the compilation faster. is that the only reason?
Thank you.

That's specified in the JLS. See � 7.6, Top Level Type Declarations.
<i>
When packages are stored in a file system (�7.2.1), the host system may
choose to enforce the restriction that it is a compile-time error if a type is not
found in a file under a name composed of the type name plus an extension (such
as .java or .jav) if either of the following is true:
. The type is referred to by code in other compilation units of the package in
which the type is declared.
. The type is declared public (and therefore is potentially accessible from
code in other packages).
This restriction implies that there must be at most one such type per compilation
unit. <b>This restriction makes it easy for a compiler for the Java programming language or an implementation of the Java virtual machine to find a named class
within a package; for example, the source code for a public type
wet.sprocket.Toad would be found in a file Toad.java in the directory wet/
sprocket, and the corresponding object code would be found in the file
Toad.class in the same directory.</b>
When packages are stored in a database (�7.2.2), the host system must not
impose such restrictions.
</i>

Why it is necessary that public class name should same as filename in java

hi,
why it is necessary that public class name should same as filename in java.
As I know that it is a convention, but I want the information why this convention is made.
thanx.

user13445117 wrote:
but suppose if i am creating a java file with default access modifire then we do not need to save the file name as a class name.So, your question is "Why does the rule about top-level classes being in a file of the same name apply +only+ to public classes"?
If it were applied to all classes, then we could never have two top-level classes in the same file. Doing that is generally not a good idea, but it might be useful from time to time. EJP already told you why it applies to public classes, and, since these are the most common by far, it's a fair compromise that it apply to them but not to other classes.
Was this the reason the original designers did it that way 15 or so years ago? Don't know, don't care, but it's one plausible explanation. Maybe it was just a balance between simplicity and clarity on the one hand, and flexibility on the other.
It's certainly not something I can see caring that much about, unless you're doing an in-depth history on Java, or writing an in-depth treatise on computer language and compiler theory.

Why public class name should be same as the java file name

Hi,
I would like to know, why public class name should be same as the its java file name. Iam in the process of finding the answer to this question. Can someone help me out in finding the explanation.
Thanks in advance,
Manoj

This is a requirement of the Java reference compiler released by Sun. I have used compilers that did not require this, but most seem to follow the reference compiler (which is a very good idea). I am NOT sure if this is specified in the Java Language Specification. Some of the other regulars who are more familiar with the JLS than I may be able to tell you.
? {?

Why File Name is Same Name as The Public Class Name

Hi Friends,
What is the reason for Naming the File Name is puvlic class name??
thanks in advance.

konstkaras wrote:
What could be reason to give it other name?
It's as much better as we have less distinct names.
When I was a schoolboy, I programmed Turbo Pascal. Each program there had a name (declared in file's beginning). The name was never used, but: it has no such restriction of length (as MS-DOS file names had) and it could clash with other identifiers described in code.
So why those names should be distinct?Java was invented after MSDOS was obsolete, so 8.3 limits were not a consideration.
Java is designed for larger programs than were attempted in the MSDOS era, so supports compilation of programs which are created from many source files. Turbo Pascal 4 also supported separate compilation units, and did use the name you mention for exactly that purpose, but I can't remember how it dealt with dependencies between compilation units.
Having the public class' name and package as component in the source file path and the class file path simplifies the dependency problem between the compilation units - all the compiler has to do to determine whether the foo.bar.Baz class needs updating is to look whether foo/bar/Baz.class exists, and if it is less recently modified than foo/bar/Baz.java.

Why a public class name and file name same?

All of us know that, a public class should be declared in a file which has the same name of that class. What is the underlying concept behind this..Can someone give a clear explanation?
Thanx..
Sandeep Joseph
Paragon Solutions
[email protected]

Another issue, it was said that the file must be named
after the class to find the class's code when
compiling dependent classes. Doesn't this apply to
non-public classes too (with the dependent class being
located in the same package)?This applies to all top-level classes ...
You can define multiple classes in one file though:
class A {
class B {
}It's perfectly valid to have both classes defined in the same file (named A.java if A is the first class to be defined in that file).
You will get a compiler warning as soon as you reference a non-top-level class from a class that is defined in another file, though.

Why the name of our source file should be same as the 'public class' name

Hi all,
I am very new in java and have a question in my mind.
I read somewhere , our souce file name should be same as public class in that file.
but i don't know the reason for this.
Can anybody help me out with this .
Thanks,

HMRPanchal wrote:
Thanks,
Can you give me some link or documents from where I can go through this article.
because I am not clear with what you are saying( makefiles , automatic recompilation etc).
I am a new in Java .The fact that you don't have to worry about those things is what makes it A Good Thing.

Public class and the source filename

May be I'm asking a silly question, but why the name of the source file needs to match with the public class's name?
If the rule is that only one public class is allowed per source file, then the compiler can simply flag the error and quit.
Any reasons behind this, other than for the ease of compilation?

I refer you to the Java Language Spec:
When packages are stored in a file system (�7.2.1), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as .java or .jav) if either of the following is true:
- The type is referred to by code in other compilation units of the package in which the type is declared.
- The type is declared public (and therefore is potentially accessible from code in other packages).
This restriction implies that there must be at most one such type per compilation unit. This restriction makes it easy for a compiler for the Java programming language or an implementation of the Java virtual machine to find a named class within a package; for example, the source code for a public type wet.sprocket.Toad would be found in a file Toad.java in the directory wet/sprocket, and the corresponding object code would be found in the file Toad.class in the same directory.

Only one public class - why???

Hi
I have come across this statement many places -
"There can be only one Top-Level 'public' class in
a java source file which should have same name as that
of the fore mentioned class."
I know that the compiler searches for the class
with same name as that of the file that is passed to
the compiler.I am also aware that a Java source compiles
smoothly with One or None Top-Level 'public' class,
but I fail to comprehend this -
Why can there be only one TOP-LEVEL 'public' class
in a Java Source file?
The importance of 'TOP-LEVEL' is as important as
any thing else, as you yourself check that a Nested
class within an Enclosing class can be 'public' along
with someother Top-Level 'public' class in the program
compiles smoothly unlike having TWO Top-Level 'public'
classes in same file.
I have found no reason supporting this statement in any
of the refernces I have checked out.Ppl I have asked told
me that it is so coz Java Spec say so.
Is there an better answer to my question?
Thanx in advance, appretiate it.
Regards
Pradeepto

I have found no reason supporting this statement in
in any
of the refernces I have checked out.Ppl I have asked
told
me that it is so coz Java Spec say so.I can tell you that anybody who says its because the Java spec says so, is wrong - the Java spec does not say so.
It is a limitation of Sun's javac compiler, which many other compilers exhibit as well. I don't know the full formal reasoning behind it (and would be interested in knowing if you find a good answer), but I assume it simply has to do with finding classes without having to load everything on your classpath.
Whether other compilers are purposely acting the same way as javac, or whether it is some sort of performance optimisation that most vendors feel is worthwhile, I couldn't say.
I don't see what you're saying about top-level and nested classes. Although you can declare a public nested class inside a non-public top-level class, the nested class is not actually publicly visible, by virue of its enclosing class not being publicly visible.

Two public classes in one source file

Can anyone please explain what is the exact reason why the java source file name should be same as the only allowed public class name in the source file. Answer only if you know the correct answer. No gusses please. I read the other postings on this topic. None of the answers were correct.

One reason is that some RISC processor architectures
(like those Sun uses mostly) have hierarchical memory
architectures. This hierarchy is divided into global
modules and local (global-accessible) submodules. The
advantage of this is that the processor needs lesser
memory access "points" (handles), as the modules
delegate CPU calls to the submodules.
The java compiler can utilize this and increase
performance by loading the entire source files into
the memory and assign them a CPU handle (the module
ID). It'll be later used for linking, e.g. The public
class in a file will be loaded as a module and gets
the handle, all other non-public classes in that file
will be submodules.
If there is no public class, a generic module will be
used, so that's no problem. But if there are multiple
public classes, you'd end up with several modules and
just one handle to assign, thus having ambiguity. The
CPU won't be able to address the correct module.
This all only applies to RISC CPUs, but for obviuos
cross-platform compatibility reasons it was added to
the standard - it doesn't hurt the other
architectures, but helps those with hierarchical
memory management.Thanks, but the rooster explanation makes more sense to me.
Now could someone please express it a la Majinda?

Compiling a public class

Why Compiler imposes that the source file name should be same as the Public class name ? I want to know what is the rational behind it. This constraint is not there for a class without any access control identifier.

It's probably something to do with making dependencies easier to find out. If you name all the .java files the same as the .class files, you don't need a 5,000-line makefile telling you what derives from what. And Javac is able to figure out basic dependencies and recompile them itself, so this was probably high on their list of requirements.

Where is the Package Name and Class Name for Android to Facebook SSO?

Where is the Package Name and Class Name for Android to Facebook SSO?
I am trying to fill these fields http://developers.facebook.com/docs/mobile/android/build/#ref for Facebook but i can't figure out what to place in Android Package Name and Andorid Class Name,
same thing for the Android Key Hash, I followed this guide http://www.skoobalon.com/blog/2012/06/13/getting-the-android-key-hash-for-facebook-from-an -air-app/ but I am not sure if it is the right thing to do.
I was thinking maybe the Pakage Name is air.com.myname.myappname (where com.myname.myappname is also the id used in the application.xml) and the Class name is air.com.myname.myappname.MyDocumentClass ?
Is there an official guide somewhere?
Also do you guys know if there is somewhere a library we can use within AIR to do SSO on Facebook from Andorid?

For what it's worth, I don't think a WiFi connection would be reliable enough. You wouldn't want your picture to freeze just because a neighbour has turned on their baby monitor. WiFi uses the same frequencies as a myriad of other devices, as well as all your neighbours' WiFi.

Why is the name of java file is same as public class?

hello friends
why we need to assign same name to .java file as the name of public class in .java file? while its not necessary for the class having no modifier?

This question has been asked several times. Serch the forum.
x

Are Multiple Classes with Same name in a single Class valid ?

package inheritance;
interface Foo
     int bar();
public class Inh6 {
     class A implements Foo // THis is the first class
          public int bar()
               return 1;
     public int fubar(Foo foo)
          return foo.bar();
     public void testFoo()
          class A implements Foo // THis is the second class
               public int bar()
                    return 2;
          System.out.println(fubar(new A()));
     public void testFooModified()
          class A implements Foo // THis is the first class
               public int bar()
                    return 4;
          System.out.println(fubar(new A()));
     public static void main(String[] args) {
          new Inh6().testFoo();
          new Inh6().testFooModified();
O/P :
2
4My question is this class "A" which is present in the different methods like testFoo() and testFooModified() different declarations of the same class or altogether different classes local to each method,something like err...Local Class ?
Thanks in Advance.

kajbj wrote:
My question is this class "A" which is present in the different methods like testFoo() and testFooModified() different declarations of the same class Why would it be the same class? They have different scopes.
KajSo you mean to say they are actually different classes with the same name,which will be invisible out of the respective methods in which they are defined ?
Thanks.

File name vs. public class?

Could anyone tell me that why should the file name be the same as the only public class in that file? Is there any reason for this rule? Thanks in advance. I really appreciate it.
David

A java restrication rule and only one class declared public.

Public class name same as filename

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