[[[NSURLConnection alloc] initWithRequest:[NSURLRequest requestWithURL:
[NSURL URLWithString:urlStr]] delegate:self startImmediately:YES] autorelease];
//urlStr is NSString ref

- (void)connection:(NSURLConnection *)connection didReceiveResponse:
   (NSURLResponse *)response{
    [receivedData setLength:0];
}
//receivedData is NSMutableData ref

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    [receivedData appendData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection{

    // json data retrival complete    (received data is NSMutableData)
    if(receivedData == nil || [receivedData length] == 0){
      //handle error in getting response, may be no network
        [self showAlertViewWithTitle:@"No network" andMessage:@"Could not open the
        page, because internet connection was not found." andTag:0];
        return;
    }

    NSString *responseBody = [[[NSString alloc] initWithData:receivedData
      encoding:NSUTF8StringEncoding] autorelease];
    ////NSLog(responseBody);
    NSError *jsonParsingError = [[[NSError alloc] init] autorelease];
    NSDictionary * jsonData = [NSDictionary dictionaryWithJSONData:
                                 [responseBody dataUsingEncoding:NSUTF8StringEncoding]
                                     error:&jsonParsingError];

    NSDictionary *serverError = [jsonData objectForKey:@"error"]==[NSNull null]?
                                   nil:[jsonData objectForKey:@"error"];

    if(serverError==nil || [serverError count]==0){
        // no error found else
        NSDictionary *response = [jsonData objectForKey:@"response"]==[NSNull null]?
                                   nil:[jsonData objectForKey:@"response"];
        NSArray *array = [response objectForKey:@"array"]==[NSNull null]?nil:
                              [response objectForKey:@"array"];
// I have left the code incomplete, its just for understanding

NSString *dataValue =  [[[array objectAtIndex:i] objectForKey:@"key"]==[NSNull null]?
                         @"0":[[array objectAtIndex:i] objectForKey:@"key"] intValue]; 

Please let me know if you require further explanation..