如何在iphone的Tabbar中更改选中的tab项?

我在视图控制器中以编程方式创建了选项卡栏。在我的应用程序中,最初显示选项卡栏,视图中有五个选项卡栏项。使用选项卡栏,最初选择第一个选项卡项。现在我想更改所选的项目,例如,第四个项目最初被选中
我如何做到这一点?

这是我的密码,

    tBar = [[UITabBarController alloc] init];

    first = [[first alloc] initWithNibName:@"first" bundle:nil];

    UINavigationController *navFirst = [[[UINavigationController alloc] initWithRootViewController:first] autorelease];

    second = [[second alloc] initWithNibName:@"second" bundle:nil];

    UINavigationController *navsecond = [[[UINavigationController alloc] initWithRootViewController:second] autorelease];

    third = [[third alloc] initWithNibName:@"third" bundle:nil];

   UINavigationController *navthird = [[[UINavigationController alloc] initWithRootViewController:third] autorelease];

   fourth = [[fourth alloc] initWithNibName:@"fourth" bundle:nil];

    UINavigationController *navfourth = [[[UINavigationController alloc] initWithRootViewController:fourth] autorelease];

   fifth = [[fifth alloc] initWithNibName:@"fifth" bundle:nil];

   UINavigationController *navfifth= [[[UINavigationController alloc] initWithRootViewController:fifth] autorelease];

   tBar.viewControllers = [NSArray arrayWithObjects:navFirst,navsecond, navthird, navfourth, navfifth,nil]; 

   [self.view addSubview:tBar.view];

   [self.navigationController pushViewController:tBar animated:NO];

请参见下图。我要的就是这样,

image http://www.freeimagehosting.net/uploads/2798661c55.png

请引导我。

谢谢。