NSArray construction causing app crash

My app crashed, is this correctly constructed?

NSArray *array = [mapView.annotations  filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"(self isKindOfClass: %@)", [MapLocation class]]];
if (array != nil)
{
    annotation = [array objectAtIndex:0];
}

I see that array isn't nil but it has 0 objects (on debug). Is correctly 开发者_C百科constructed?

NSArray will raise an exception if you access something out of its bound. If the array is empty, accessing element at index 0 is out of its bound. You can check if the array contains elements by calling [array count] eg:

NSArray *array = [mapView.annotations  filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"(self isKindOfClass: %@)", [MapLocation class]]];
if([array count] > 0) // No need to check if the array is != NULL, the runtime won't send messages to NULL
{
    annotation = [array objectAtIndex:0];
}