其他面向对象设计原则2
能用组合的地方，不要用继承 Favor object composition over class inheritance
6.1 代码重用的两种方式
能用组合的地方不要用继承 Favor object composition over class inheritance
这句话是说，在OO技术里，有两种基本类型的代码重用: 组合、继承
What this statement says is that there are basically two ways to reuse code in OO
6.2 继承的特点
类的继承 Class inheritance
子类获得父系“全部功能”，“稍微”调整一下，比如覆盖实现几个方法
You get the “whole packet” and “tweak a bit” by overriding a single or few methods
既快又容易 Fast and easy
代码实现时明确展示，由编程语言加以支持
Explicit in the code, supported by language
(you can directly write “extends”)
继承存在的副作用...
“继承打破了封装性 inheritance breaks encapsulation”
导致父类与子类之间高度耦合 leading to high coupling between super-and subclass
1) 继承的代码是静态/编译-时绑定的 Inheritance is a static/compile-time binding
今后改变行为的唯一方法是 edit-compile-debug-edit-compile-debug
2) 客户需要购买整个软件包 You have to buy the full package !
you cannot access only part of the behaviour of the superclass – you get it all
For example
A nice C++ String implementation in a Library was compiled to have dependencies
with 2MB other parts of the library
3)父类定义了许多硬性的规定 Superclasses define (part) of physical representation 例如
父类用了列表数据结构，而子类最好是用树结构
superclass uses a List data structure, but subclass would be much more efficient using a
tree structure
子类经常直接使用Public和Protected数据成员，一旦父类修改了这些数据，将会影 响全体子类
subclasses usually simple refer to public and protected data members, which means, changing implementation details
in superclass effect all subclasses !!!
yes – we know :
review all subclasses
retest all subclasses
不得不了解父类的代码以求真正理解（相比组合，只要了解接口）
关于类的继承
可以使用继承，但设计师 必须确信使用继承是很好 的 解 决 方 案 时 才 使 用
Inheritance is OK but you must use it for what it handles really nice
以泛化层次的方式，对概 念进行建模 Modelling of concepts in a generalisation hierarchy
不是角色
不是行为
不能随意
6.3 组合的特点
对象组合 Object composition
没有打破封装性 No breaking encapsulation !!!
A) 对象组合是一种动态/运行时绑定 object composition is a dynamic/run-time binding
在运行时切换对象引用，就可以改变行为 you may change behaviour at run-time by “switching” an object reference
B) 整体与部分之间只有接口边界关联，耦合较低 Loose coupling by respecting the interface borders
对私有数据不存在“窥视与拨弄”No peeking and poking in semiprivate data members!
不存在大量无用代码一类的负担 No heavy luggage in the form of data structures that are useless
C) 各部分的职责明确 Clear division of responsibilities !!!
每个对象清晰地集中在少量的任务上 each object is clearly focused on a single/few tasks
容易理解 easier to understand、容易维护 easier to maintain
只要阅读接口，就可以了解系统（至少理论上是如此）understanding by reading interfaces only
容易独立测试 easier to test in isolation 使用得当，将导致更可靠的设计
每个类依然“苗条”each class remains small
避免的“巨无霸”这种反模式的现象，即一个类似乎完成了几乎全部的功能…
avoiding the “Blob” anti-pattern where one class seems to “suck in” all functionality...
重用的概率大增！larger potential for reuse in other contexts