关于objective-c 类目（Category）,遇到的有关问题新手

#import <Cocoa/Cocoa.h>
#import <NSString.h>

@interface NSString(Utilities)
    -(BOOL) isURL;
@end

#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

#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];
}