how to check for null object returned in xcode

Question

I have a NULL object returned from a JSON query string and I don't know how to check for it in an If statement. My syntax is below but I still don't seem to be able to trap for the NULL class (i.e., if nothing is returned then no text variable can be set therefore it MUST be a NULL class?), anyway, I need to check that the @"BillingStreet" has something in it and if not to avoid processing it (else the app crashes as it tries to set nothing to the text value of one of the fields in the VC):

- (void) tableView: (UITableView *)itemTableView didSelectRowAtIndexPath: (NSIndexPath *)indexPath{
    NSDictionary *obj = [self.dataCustomerDetailRows objectAtIndex:indexPath.row];
    NSString *text = [obj objectForKey:@"BillingStreet"] == nil ? @"0" : [obj objectForKey:@"BillingStreet"];
    NSLog(@"%@",text);
    if (text.class == NULL){
    } else {
        NSLog(@"no street");
        self.labelCustomerAddress.text = [obj objectForKey:@"BillingStreet"];
    }
}

Martin R · Accepted Answer · 2013-09-20 14:11:12Z

8

A JSON "null" value is converted to [NSNull null], which you can check for with

if (text == [NSNull null]) ...

because it is a singleton.

Alternatively, you can check if the object contains the expected type, i.e. a string:

NSString *text = [obj objectForKey:@"BillingStreet"];
if ([text isKindOfClass:[NSString class]]) {
    self.labelCustomerAddress.text = text;
} else {
    self.labelCustomerAddress.text = @"no street";
}

This is more robust in the case that a server sends bad data, e.g. a number or an array instead of a string.

answered Sep 20, 2013 at 14:11

Martin R

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3 Comments

Bartley Over a year ago

Thanks Martin, tested and that works (I put this in: ' if (text == (id)[NSNull null] || text.length ==0 || [text isEqualToString:@"null"])' I can across the answeer as well in this question, so I may have duplicated my question: stackoverflow.com/questions/8785124/exception-if-nil-or-null

Bartley Over a year ago

I think this is more efficient as it removes the need for creating a string variable : 'if ([obj objectForKey:@"BillingStreet"] == (id)[NSNull null])'

Martin R Over a year ago

@Bartley: No, assigning text = [obj objectForKey:@"BillingStreet"] is more efficient because it retrives the dictionary value only once.

Ildar Sh · Accepted Answer · 2013-09-20 14:11:38Z

6

text == nil ? @"0" : text

or

text ? text : @"0"

But if you get it from JSON then you may get instance of NSNull class. In this case you should check

text && ![text isKindOfClass:[NSNull class]] ? text : @"0"

answered Sep 20, 2013 at 14:11

Ildar Sh

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