How to access a string in a class stored in an array?

I have a string (titleName) stored in a class (newNoteBook) stored in an array (myLibrary). I was trying to access it, but I only get a (null) printed in the log.

What am I doing wrong?

-(void) setupLibrary {

    NoteBook *newNoteBook = [[NoteBook alloc] init];

    newNoteBook.titleName = @"TEST";
    NSLog(@"titleName:%@", newNoteBook.titleName); // this prints TEST in the log
    [myLibrary addObject:newNoteBook];
    NSLog(@"开发者_开发技巧titleName:%@", [[self.myLibrary objectAtIndex:0] titleName]); // this prints (null) in the log)
}

There is nothing fancy in my class... simply:

@interface NoteBook : NSObject {

NSString *titleName; }


@property (nonatomic, retain) NSString *titleName;
@end

@implementation NoteBook
@synthesize titleName;

Try this

NSLog(@"titleName:%@", ((NoteBook *)[self.myLibrary objectAtIndex:0]).titleName);

Possible reasons:

• myLibrary (the instance variable) is nil;
• self.myLibrary is nil or its backing instance variable isn’t myLibrary;
• [self.myLibrary objectAtIndex:0] is not the same object as newNoteBook because there was at least one other element in self.myLibrary.

Edit: you need to create a new mutable array and assign it to your property/instance variable myLibrary:

self.myLibrary = [NSMutableArray array];

or

myLibrary = [[NSMutableArray alloc] init];

Where you should this depend on how your class is used. If an instance of your class should always have valid myLibrary, a good place to do that is in -init:

- (id)init {
    self = [super init];
    if (self) {
        myLibrary = [[NSMutableArray alloc] init];
    }
    return self;
}

Alternatively, if you want to lazily create myLibrary only when -setupLibrary is executed, create it in that method:

-(void) setupLibrary {
    self.myLibrary = [NSMutableArray array];

    NoteBook *newNoteBook = [[NoteBook alloc] init];
    …
}

Don’t forget to release it in your -dealloc method:

- (void)dealloc {
    [myLibrary release];
    [super dealloc];
}

I think you are not type casting object from array -

NSLog(@"titleName:%@", [(NoteBook*)[self.myLibrary objectAtIndex:0] titleName]);

and you should alloc your array before adding object to it -

myLibrary = [[NSMutableArray alloc] init];

NSLog(@"titleName:%@", [self.myLibrary objectAtIndex:0].titleName);

Is correct as they said before you don't need to cast.