OBC 判断对象、元素是否为空

//判断对象不为空
if(object){}
//判断对象为空
if(object==nil){}
//数组初始化，空值结束
NSArray *pageNames = [[NSArray alloc] initWithObjects:@"DocumentList",@"AdvancedSearch",@"Statistics",nil];
//判断数组元素是否为空
UIViewController *controller=[NSArray objectAtIndex:i];
if((NSNull *)controller == [NSNull null])){}
//判断字典对象的元素是否为空
NSString *userId = [NSDictionary objectForKey:@"UserId"];
if(userId==[NSNull null]){}