public abstract class Pizza {
    public static final Pizza.NULL = new Pizza() {
        /* "null" implementations */
    }

    /* actual/abstract implmentations */
}

@interface NullPizza : NSObject
{
}
- (BOOL)areYouANullPizza;
@end

@interface Pizza : NullPizza
{
}

+ (Pizza*)Null;
@end

@implementation Pizza
+ (Pizza*)newNullPizza
{
    return [[NullPizza]alloc init]; // Singleton code left as an exercise.
}

- (BOOL)areYouANullPizza;
{
    return NO;
}
@end