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];}