Class name's as Identifiers

[Mohan Mani]

Hi all

Im just having a query on the below post.

Which of the following are not legal Java identifiers?
Select 1 correct option.
a goto
Although not used, it's a reserved word.
b unsigned
It's not a reserved or keyword.
c String
Yes, it is valid. This is a valid statement: String String = "String";
d _xyz
Can start with _ or $
e $_abc
Can have _ or $.

Ans a.
why not b??

In the option C, a class name "String" is used as an identifer. I had tried with others like Integer and code compiles. How java allows this.

Any specific reason why class name are been allowed to be used as identifiers.

Regards
Mohan

[marc weber]

Essentially, identifiers (e.g., class names) are up to the programmer, while keywords are fixed as part of the Language Specification.

6.1: Declarations...
http://java.sun.com/docs/books/jls/second_edition/html/names.doc.html#33757

3.8: Identifiers...
http://java.sun.com/docs/books/jls/second_edition/html/lexical.doc.html#40625

3.9 Keywords...
http://java.sun.com/docs/books/jls/second_edition/html/lexical.doc.html#229308

[Simon Cockayne]

Marc,

So does the JVM treat an indentifier that has the same name as a class as an indentifier or a class?

package javaranch;
 
class AClass {
   static void aMethod() {
   System.out.println("AClass.aMethod()");
   }
}
 
class BClass extends AClass{
    static void aMethod() {
System.out.println("BClass.aMethod()");
   }
}
 
public class CClass {
public static void main (String args[]) {
AClass BClass = new BClass();
BClass.aMethod(); //
}
}

Output:

AClass.aMethod()

But why does it choose that (selecting the method by using the ACLass type reference that BCLass identifier is assigned to) rather than the static method on BClass?

Oooh - my brain aches now...

Cheers,

Si.

[marc weber]

Originally posted by Simon Cockayne:
...does the JVM treat an indentifier that has the same name as a class as an indentifier or a class? ...

Good question!

See JLS 6.5: Determining the Meaning of a Name...
http://java.sun.com/docs/books/jls/second_edition/html/names.doc.html#106941

Basically, the qualified name "BClass.aMethod()" is identified by its context as a MethodName, and is initially classified as AmbiguousName.Identifier.

The AmbiguousName (as a simple identifier) is then reclassified as an ExpressionName (rather than a TypeName) per section 6.5.2.

But I think the point here -- as above -- is that it's up to the programmer to avoid this type of confusion.
[ March 30, 2005: Message edited by: marc weber ]

[Mike Gershman]

Another way to think about this is that Java has several namespaces and the same isentifier can have a different meaning in each namespace.

For example, C can be a class name, a variable name, and a label name (used in break and continue), all at the same time. The Java compiler always knows which identifier dictionary to look in depending on the statement syntax.

[marc weber]

Originally posted by Mike Gershman:
Another way to think about this is that Java has several namespaces and the same identifier can have a different meaning in each namespace...

Exactly. In the above example, the AmbiguousName is reclassified as an ExpressionName because that variable is in scope.

Specifically (per 6.5.2), "the AmbiguousName is a simple name, consisting of a single Identifier, [and] ... the Identifier appears within the scope ... of a local variable declaration..."