2. 计算机
  3. In looking at the class structure of an entire system, we may find that its inheritance (1) is either wide and shallow, narrow a

In looking at the class structure of an entire system, we may find that its inheritance (1) is either wide and shallow, narrow a

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问题 In looking at the class structure of an entire system, we may find that its inheritance (1) is either wide and shallow, narrow and deep, or balanced. Class structures that are wide and shallow usually represent forests of (2) classes that can be mixed and matched. Class structures that are narrow and deep represent trees of classes that are related by a common ancestor. There are advantages and disadvantages to each approach. Forests of classes are more loosely (3) , but they may not exploit all the commonality that exists. Trees of classes exploit this commonality, so that individual classes are smaller than in forests. However, to understand a particular class, it is usually necessary to understand the meaning of all the classes it inherits from or uses. The proper shape of a class structure is highly problem-dependent.  We must make similar trade-offs among inheritance, aggregation, and using relation- ships. For example, should the class Car inherit, contain, or use the classes named Engine and Wheel? In this case, we suggest that an (4) relationship is more appropriate than inheritance relationship. Meyer states that between the class A and B," (5) is appropriate if every instance of B may also be viewed as an instance of A. The client relationship is appropriate when every instance of B simply possesses one or more attributes of A". From another perspective, if the behavior of an object is more than the sum of its individual parts, then creating an aggregation relationship rather than an inheritance relationship between the appropriate classes is probably superior.
选项 A、aggregation
B、inheritance
C、association
D、using
答案B
解析 译文如下：在查看整个系统的类结构时，会发现它的继承层次或者是宽而浅，或者是窄而深，或者是均衡的。宽而浅的类结构通常表示一些可被混合和匹配的自由独立的类组成的类森林。窄而深的类结构，表示由公共的祖先相关联的类树。每种方法都有优缺点。类的森林的耦合度较低，但它们可能不能体现出存在的所有共同处。类树体现出这种共同处，所以它的类个体比类森林中的类要小。但是为了理解某个特定的类，通常需要理解它所继承的或使用的类的含义。类结构的形状是否恰当与特定问题紧密相关。我们必须在继承、聚合和使用这三个关系中选择使用合适的。例如，类Car是继承、还是包含、还是使用类Engine和类Wheel呢?在这种情况下，聚合关系比继承关系更合适。Meyer建议在类A和类B之间，“如果B的每个实例也可被看做A的一个实例，那么继承较为合适。如果B的每个实例只处理A的一个或多个属性，则客户关系更为合适一些”。从另外一个角度看，如果一个对象的行为多于它的单个部分的总和，那么在适当的类间创建聚合关系比继承关系更合适。
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