Question

I’ve got an API returning a JSON encoded string of data that returns a real number or “null” as a value. As long as the JSON contains a numeric or string value, everything works as expected. If the key:value pair value is null, the code below crashes.

How do I properly test NSDictionary objectForKey when it’s getting a NULL from SBJSON?

When the API returns a null for filetype, the code below crashes at the if() line.

My Objective-C code attempts to test for expected values:

if (1 == [[task valueForKey:@"filetype"] integerValue]) {
    // do something
} else {
    // do something else
}

The API JSON output:

{"tclid":"3","filename":null,"filetype":null}

The NSLog() output of the NSDictionary is:

task {
    filename = "<null>";
    filetype = "<null>";
    tclid = 3;
}

Answer 1

When transferring data from JSON to a Cocoa collection, the NSNull class is used to represent “no value”, since Cocoa collections can’t have empty slots. <null> is how NSNull prints itself.

To test for this, you can use someObject == [NSNull null]. It’s a singleton — there’s only one instance of NSNull per process — so pointer comparison works, although you may prefer to follow the usual Cocoa comparison convention and use [someObject isKindOfClass:[NSNull class]].

You’re getting the crash because you’re sending integerValue to that NSNull object. NSNull doesn’t respond to integerValue and raises an exception.