objectForKey stringValue crashing my app?

Question

I have a class that I use to setup objects in an array. In this class I have a custom "initWithDictionary", where I parse a JSON dictionary. However, as I am running into NSNull, this crashes my app. To get around this, I set up a class that handles exceptions, so when a string is NSNull, it's replace it with @"". or -1 for integers.

This is my NullExtensions class:

@interface NSNull (valueExtensions)

-(int)intValue;
-(NSString *)stringValue;

@end

@implementation NSNull (valueExtensions)

-(int)intValue {
    return -1;
}

-(NSString*)stringValue {
    return @"";
}

@end

However, in my initWithDictionary method, the following code crashes my app:

self.bookTitle = [[parsedDictionary objectForKey:@"book_title"] stringValue];

It doesn't work regardless of the object in the parsed dictionary being NSNull or containing a valid string. Only if I do the following (and the string is not null):

self.bookTitle = [parsedDictionary objectForKey:@"book_title"];

Is stringValue incorrect in this case? And if so, how do I use it properly in order to setup proper NSNull replacements?

Thx

Answer 1

You are asking an NSString for its stringValue. No need to convert a string to a string.

Try this:

if (![[parsedDictionary objectForKey:@"book_title"] isKindOfClass:[NSNull class]]) {
    self.bookTitle = [parsedDictionary objectForKey:@"book_title"];
} else {
    self.bookTitle = @"";
}

Edit: You should not use the category on NSNull you created. You don't need it, nor should you want it. If the source for the dictionary inserts NSNull instances, go ahead and use my code above. Normally you would expect to simple have no value inserted for the key, at which time you can simple see if [parsedDictionary objectForKey:@"book_title"] returns anything.

Answer 2

The NSNull extensions you have written look ok to me but using a method like stringValue may be confusing since other classes like NSNumber use this.

Personally though, I think NSNull replacement in this instance is unnecessary. If you just made a quick test you can replace the NSNull where you need to. e.g.

id testObject = [parsedDictionary objectForKey:@"book_title"];
self.bookTitle = testObject==[NSNull null] ? @"" : testObject;

Answer 3

You really really don't want to add a category to NSNull that adds such common methods. That will change the behavior of NSNull for all instances in the application, including ones created by the underlying frameworks solely for their private use.

If you need a value class that represents the notion of "value doesn't exist and therefore I'm going to return these default values instead", create a class or instance that represents exactly that.

As for why it crashes, I couldn't tell you without seeing the actual details of the crash.

---

And, yes, it really is THAT bad to add a category to a class that adds such a common method. All it takes is one bit of code in a plug-in or framework that does:

if ([fooMaybeNull respondsToSelector: @selector(intValue)] bar = [fooMaybeNull intValue];

Not terribly farfetched -- I have had to debug nasty crashers or misbehaviors due to exactly this kind of willy-nilly category addition.

If you are going to add methods to a class via categories, prefix your method names so as to isolate them from existing functionality. It is still fragile, but manageably so.

Answer 4

Are you sure that the dictionary is returning [NSNull null]? By default, dictionaries return nil, not [NSNull null], when an value isn't found for a key.

Answer 5

Instead of creating categories on NSNull, for which you would also have to add a similar category to NSString (that's why it crashes, because real strings do not respond to stringValue) - instead try creating a helper category on NSDictionary like "stringForKey" that uses the code Johan posted and returns an NSString, probably also should enforce all other types get mapped to empty strings as well.

Answer 6

However, in my initWithDictionary method, the following code crashes my app:

self.bookTitle = [[parsedDictionary objectForKey:@"book_title"] stringValue];

It doesn't work regardless of the object in the parsed dictionary being NSNull or containing a valid string.

That makes sense, since stringValue is not a valid method on NSString. It will work for NSValue and its subclasses, but not NSString.