當定義類之間的合一行為，重要的是要記住合一是方向性的：我們可以分配一個更特別的類（例如一個子類）到一個通用類型（例如一個父類），但是反過來是不允許的。 > When de?ning uni?cation behavior between classes,it is important to remember that uni?cation is directional: We can assign a more specialized class (e.g. a child class) to a generic class (e.g. a parent class) but the reverse is not valid. 如下的賦值是允許的： * 子類分配到父類 * 類分配到實現接口 * 接口到基礎接口 > The following assignments are allowed: > * child class to parent class > * class to implementing interface > * interface to base interface 這些規則是可傳遞的，意味著一個子類也可以被分配到它的基礎類的基礎類，它的基礎類實現的一個接口，實現類的基礎接口等等。 > These rules are transitive, meaning that a child class can also be assigned to the base class of its base class,an interface its base class implements,the base interface of an implementing interface and so on.