sample code of category in objective-c

Appdelegate.h:

//

//  CategoriesExampleAppDelegate.h

//  CategoriesExample

//

//  Created by santoshkumar on 27/11/10.

//  Copyright __MyCompanyName__ 2010. All rights reserved.

//

#import <UIKit/UIKit.h>

@class CategoriesExampleViewController;

@interface CategoriesExampleAppDelegate : NSObject <UIApplicationDelegate> {

    UIWindow *window;

    CategoriesExampleViewController *viewController;

}

@property (nonatomic, retain) IBOutlet UIWindow *window;

@property (nonatomic, retain) IBOutlet CategoriesExampleViewController *viewController;

@end

.m:

//

//  CategoriesExampleAppDelegate.m

//  CategoriesExample

//

//  Created by santoshkumar on 27/11/10.

//  Copyright __MyCompanyName__ 2010. All rights reserved.

//

#import "CategoriesExampleAppDelegate.h"

#import "CategoriesExampleViewController.h"

#import "MyString.h"

@implementation CategoriesExampleAppDelegate

@synthesize window;

@synthesize viewController;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {   

   

    NSString *str = [[NSString alloc]init];

    NSLog(@"result is %@",[str defaultString]);

  

    // Override point for customization after app launch   

    [window addSubview:viewController.view];

    [window makeKeyAndVisible];

   

    return YES;

}

- (void)dealloc {

    [viewController release];

    [window release];

    [super dealloc];

}

@end

controllercategory.h:

//

//  MyString.h

//  CategoriesExample

//

//  Created by santoshkumar on 27/11/10.

//  Copyright 2010 __MyCompanyName__. All rights reserved.

//

#import <Foundation/Foundation.h>

@interface NSString(MyOwnMethods)

-(NSString*)defaultString;

@end

.m:                                  

//

//  MyString.m

//  CategoriesExample

//

//  Created by santoshkumar on 27/11/10.

//  Copyright 2010 __MyCompanyName__. All rights reserved.

//

#import "MyString.h"

@implementation NSString(MyOwnMethods)

-(NSString*)defaultString{

    return @"My Own String";

}

@end