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Why java file cannot consist two public classes  RSS feed

 
davidson john
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Can anyone please tell me why a java file cannot consist two public calsses with detailed explanation.

Thanks in advace.

regards,
david
[ March 03, 2007: Message edited by: davidson john ]
 
Jan Cumps
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It's your java platform that decides whether this is enforced or not.
But it's a convention among the java community to work this way for java platforms that use a file system to store the sources.

See the java specification:

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.

When packages are stored in a database (�7.2.2), the host system must not impose such restrictions.

In practice, many programmers choose to put each class or interface type in its own compilation unit, whether or not it is public or is referred to by code in other compilation units. A compile-time error occurs if the name of a top level type appears as the name of any other top level class or interface type declared in the same package (�7.6).



The convention is documented in Code Conventions for the JavaTM Programming Language , 3 - File Organization.

Regards, Jan
 
Raj Kumar Bindal
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1.The execution of a file starts from the main() method of the class declared as public,so if two classes are declared public jvm cannot decide from where to start the execution.
2.compiler also doesn't allow two public classes in the same file.
 
Ernest Friedman-Hill
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Originally posted by Raj Kumar Bindal:
1.The execution of a file starts from the main() method of the class declared as public,so if two classes are declared public jvm cannot decide from where to start the execution.


Well, no. You don't execute source files, you execute class files. A class file, by definition, has only one class file in it.

The answer is just as Jan has given it above; it's a convenience for compiler implementors.
 
davidson john
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Thank You Jan Cumps & Ernest Friedman-Hill. Now i have a clear understanding of the reason behind having a single public class in a java source file.
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