About objective-c categories (Category), problems encountered novice help!

My code
NSString-Utilities.h:

 
# Import  
# Import  

@ Interface NSString (Utilities) 
- (BOOL) isURL; 
@ End 

NSString-Utilities.m:

 
# Import "NSString-Utilities.h" 

@ Implementation NSString (Utilities) 

- (BOOL) isURL 
{
if (self.length <7) return NO; 

NSRange range = NSMakeRange (0,7); 
NSString * prefix = [self substringWithRange: range]; 

if ([prefix isEqualToString: @ "http://"]) 
return YES; 
else 
return NO; 
} 

@ End 

main.m:

 
# Import "NSString-Utilities.h" 

int main (int argc, const char * argv []) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; 
NSString * string1 = @ "http://sdfing"; 
NSString * string2 = @ "sdfa"; 

if ([string1 isURL]) 
NSLog (@ "string1 is URL"); 
if ([string2 isURL]) 
NSLog (@ "string2 is URL"); 

[Pool release]; 
} 

Compile no problem, NSConstantString (instance) does not recognize isURL
tips runtime
I am a novice. . Help us to see, thank you.

Reply:
I do not know the expression. Did not care for it. .
Reply:
The instance methods to class methods to try, Tip has been relatively clear.
Reply:
The change of the name of the class, not just in the name of Canada - symbols
Reply:
Thank you, upstairs, according to your question is resolved.