stringValue of NSDictionary objectForKey runtime error - casting

Question

So I have the following code that causes a runtime error in XCode
NSString* temp = [[param objectforkey#"firstParam"] stringValue];
int tempNum = [[param objectforkey#"secondParam"] intValue];
param loads from a plist. firstParam is as string, secondParam is a number
First line crashes the program.
Now what's interesting is it works if I do a hard caste i.e:
NSString* temp = (NSString*)[param objectforkey#"firstParam"];
int tempNum = [[param objectforkey#"secondParam"] intValue];
Just wondering why the id would have inconsistent implementation in that I have to use intValue to cast to int, but have to do hard cast to get NSString? Why not stringValue?

Answer 1

Your first call to objectForKey returns an object which is an NSString. NSString doesn't have a stringValue method which is why a runtime error is generated. It doesn't make sense to get the stringValue of something that is already a string.
The second call to objectForKey returns an NSNumber. NSNumber does have an intValue method so it doesn't cause an error.
In the second case you are changing the type of the returned value from NSNumber to int. In the first case the object returned is already an NSString and there is no point trying to get the stringValue of a string.

Answer 2

The call [param objectforkey#"firstParam"] DOES return a NSString, so there's no need to call stringValue. Furthermore, NSString does not have a method called "stringValue", so that's the source of your problem.
Just call this: NSString *temp = (NSString *)[param objectForKey:#"firstParam"];
Also note that the selector is objectForKey:, not objectforkey: (capitalization matters).
And, answering your question, NSNumber is a class, so any object is not an "int", but a NSNumber pointer. intValue returns an actual "int", as floatValue returns a "float" (NSNumber can represent any type of number). In the case of the NSString that's not necessary, because there is only one type of NSString (there is, though, a method for returning a const char *, and that would be used like char *string = [[param objectForKey:#"firstParam"] cString];... not very useful for Objective-C apps anyway).

Answer 3

Lets say you call a web service that returns JSON
Then you process data
NSDictionary *json_response = [NSJSONSerialization JSONObjectWithData:data
options:0
error:NULL];
now you try something like this
NSString *name = [[object objectForKey:#"Name"] stringValue];
then you get the error....
As mttrb mentions you get the error because of stringValue, objectForKey default return value is String so a potential fix in this case would be:
NSString *name = [object objectForKey:#"Name"];
Hope this hints some one!

Answer 4

If you can not be sure that the returned value is a NSString you should do this:
id value = [param objectForKey#"firstParam"];
if ([value isKindOfClass:[NSString class]]) {
NSString *temp = (NSString *)id;
//Handle string
}
else {
//Handle other case
}

Answer 5

Completing the answer of #Torge, I add my solution to get always an NSString from objectForKey method:
NSObject * value = [param objectForKey#"firstParam"];
NSString *stringValue;
if ([value isKindOfClass:[NSString class]]) {
//Handle string
stringValue = (NSString *)value;
}
else if([value isKindOfClass:[NSNumber class]]){
//Handle NSNumber
stringValue = [((NSNumber *) value) stringValue];
}
else {
//Handle other case
}