_Nullable -can have a nil value
-Nonnull -not expected to be nil
@interface Book : NSObject @property (nonatomic, copy) NSString *title; @property (nonatomic) Person *author; @property (nonatomic) Person *editor; @property (nonatomic) int yearOfPublication; -(instancetype)initWithTitle:(NSString*)title author:(Person*)author year:(int)year; @end
