how to show alert when values is null - iphone

Question

I want to show alert when values is null i have titleCategory if it has value null then it should show alert
NSString*test=titleCategory;
if ([titleCategory isEqualToString:nil])
{
UIAlertView * alert = [[UIAlertView alloc] initWithTitle:#"Alert" message:#"Select Category " delegate:nil cancelButtonTitle:#"OK" otherButtonTitles:nil];
[alert show];
[alert release];
}
else
{
FirstViewController*targetController=[[FirstViewController alloc]init];
[self.navigationController pushViewController:targetController animated:YES];
}

Answer 1

if(test == nil)
{
UIAlertView* alert = [[[UIAlertView alloc] initWithTitle:#"" message:#"Test is null" delegate:self cancelButtonTitle:#"OK" otherButtonTitles:nil] autorelease];
[alert show];
}

Answer 2

Do this:
if (!(titleCategory.length > 0) || titleCategory == nil )
{
UIAlertView * alert = [[UIAlertView alloc] initWithTitle:#"Alert" message:#"Select Category " delegate:nil cancelButtonTitle:#"OK" otherButtonTitles:nil];
[alert show];
[alert release];
}
else
{
FirstViewController*targetController=[[FirstViewController alloc]init];
[self.navigationController pushViewController:targetController animated:YES];
[targetController release]; // if not ARC
}

Answer 3

Please Use this code:
if([titleCategory.text isKindOfClass:[NSNull class]]){
UIAlertView * alert = [[UIAlertView alloc] initWithTitle:#"Alert" message:#"Select
Category " delegate:nil cancelButtonTitle:#"OK" otherButtonTitles:nil];
[alert show];
[alert release];
}
else{
FirstViewController*targetController=[[FirstViewController alloc]init];
[self.navigationController pushViewController:targetController animated:YES];
[targetController release]; // if not ARC
}