Created a TabBar Controller programmatically - but where is the xib file? - objective-c

Question

I implemented a TabBar Controller, it is initalised like this
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
firstView = [[FirstView alloc]init];
secondView = [[SecondView alloc]init];
firstItem = [[UITabBarItem alloc]initWithTitle:#"First" image:nil tag:0];
secondItem = [[UITabBarItem alloc]initWithTitle:#"Second" image:nil tag:1];
firstView.tabBarItem = firstItem;
secondView.tabBarItem = secondItem;
NSArray *viewArray = [[NSArray alloc]initWithObjects:firstView, secondView, nil];
[self setViewControllers:viewArray];
}
return self;
}
In the AppDidFinishedLoading I just did this:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
mainbarController = [[MainBarController alloc]initWithNibName:#"MainBarController" bundle:nil];
[self.window setRootViewController:mainbarController];
[self.window makeKeyAndVisible];
return YES;
}
My problem is, I want to place the tabBar somewhere else, not at the bottom, as it is when I run the project. But I can´t find a xib File to my code. The mainwindow.xib is just an empty window. I know it´s very basic but I´m confused about this.
thanks anyway