iOS / Objective-C：将NSString对象添加到NSMutableArray，NSMutableArray为（空）

Question

- (void)webViewDidFinishLoad:(UIWebView *)webView {
if (urlIndex == maxIndex) { 
    maxIndex = maxIndex + 1;

    NSString* sURL = webpage.request.URL.absoluteString;

    [urlHistory addObject:sURL];
    NSLog(@"%d: %@", urlIndex, [urlHistory objectAtIndex:urlIndex]);
    NSLog(@"%d: %@", urlIndex, webpage.request.URL.absoluteString);


    [sURL release];

    urlIndex = urlIndex + 1;
}
else {
    [urlHistory insertObject:webView.request.URL.absoluteString atIndex:(urlIndex - 1)];
}
}

This line

    NSLog(@"%d: %@", urlIndex, [urlHistory objectAtIndex:urlIndex]);

prints (null), while as this line

    NSLog(@"%d: %@", urlIndex, webpage.request.URL.absoluteString);

prints the actual URL.

On my initWithNibName I have:

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
self = [super initWithNibName:@"Tab1ViewController" bundle:nil];

if (self) {
    urlHistory = [[NSMutableArray alloc] init];
    urlIndex = 0;
    maxIndex = 0;
}

return self;
}

But I still keep getting (null) when I access my array. Why is that?

Answer 1

It sounds like your urlHistory object is nil . Calling -objectAtIndex: on a nil object will return nil . You probably forgot to initialize urlHistory in your -init , or you're not actually calling the -init method you think you are (eg if your VC is loaded from a nib it will be using -initWithCoder: instead of -initWithNibName:bundle: ).

For the record, if -objectAtIndex: on an NSArray ever returns nil , it means the array itself is nil . Since NSArray cannot store nil , -objectAtIndex: with a valid index will never return nil , and -objectAtIndex: with an invalid index will throw an exception. So the only way for -objectAtIndex: to return nil is if the method itself is never actually called.