tailieunhanh - php objects patterns and practice 3rd edition phần 2

ghi âm, hoặc bằng bất kỳ lưu trữ thông tin hoặc hệ thống thu hồi, mà không có sự cho phép trước bằng văn bản của chủ sở hữu quyền tác giả và nhà xuất cho bạn biết về mối quan hệ kế thừa lớp. Nó sẽ không cho bạn biết rằng một lớp học thực hiện một giao diện. | CHAPTER 3 OBJECT BASICS getSummaryLine method. Later on you will see how it is possible to make this promise in a base class without providing any implementation at all. Each child ShopProduct class inherits its parent s properties. Both BookProduct and CdProduct access the title property in their versions of getSummaryLine . Inheritance can be a difficult concept to grasp at first. By defining a class that extends another you ensure that an object instantiated from it is defined by the characteristics of first the child and then the parent class. Another way of thinking about this is in terms of searching. When I invoke product2- getProducer there is no such method to be found in the CdProduct class and the invocation falls through to the default implementation in ShopProduct. When I invoke product2- getSummaryLine on the other hand the getSummaryLine method is found in CdProduct and invoked. The same is true of property accesses. When I access title in the BookProduct class s getSummaryLine method the property is not found in the BookProduct class. It is acquired instead from the parent class from ShopProduct. The title property applies equally to both subclasses and therefore it belongs in the superclass. A quick look at the ShopProduct constructor however shows that I am still managing data in the base class that should be handled by its children. The BookProduct class should handle the numPages argument and property and the CdProduct class should handle the playLength argument and property. To make this work I will define constructor methods in each of the child classes. Constructors and Inheritance When you define a constructor in a child class you become responsible for passing any arguments on to the parent. If you fail to do this you can end up with a partially constructed object. To invoke a method in a parent class you must first find a way of referring to the class itself a handle. PHP provides us with the parent keyword for this purpose. To refer to a