Question

I am working with objective c for an iphone app.

I see that [dictionary objectForKey:@"key"] return <null>. Doing a if([dictionary objectForKey:@"key"] == nil || [dictionary objectForKey:@"key"] == null) does not seem to catch this case.

Doing a if([[dictionary objectForKey:@"key"] isEqualToString:@"<null>"]) causes my program to crash.

What is the correct expression to catch <null>?

More Details
An if statement for nil still isn’t catching the case… Maybe i’m just too tired to see something, but here’s additional info:

Dictionary is populated via a url that contains json data like so:

NSURL *url = [NSURL URLWithString:"http://site.com/"];

dataresult = [NSData dataWithContentsOfURL:url];

NSError *error;
NSMutableDictionary *dictionary = [NSJSONSerialization JSONObjectWithData:dataresult options:kNilOptions error:&    error];

doing an NSLog on the dictionary gives this output:

{
    key = "<null>";
    responseMessage = "The email / registration code combination is incorrect";
}

Answer 1

You have an instance of NSNull. Actually, the instance, since it’s a singleton.

Cocoa collections can’t contain nil, although they may return nil if you try to access something which isn’t present.

However, sometimes it’s valuable for a program to store a thing meaning “nothing” in a collection. That’s what NSNull is for.

As it happens, JSON can represent null objects. So, when converting JSON to a Cocoa collection, those null objects get translated into the NSNull object.

When Cocoa formats a string with a “%@” specifier, a nil value will get formatted as “(null)” with parentheses. An NSNull value will get formatted as “<null>” with angle brackets.