java的面向?qū)ο缶幊掏馕馁Y料翻譯-其他專業(yè)-展示頁
2025-01-31 06:30本頁面
【正文】 apezoids). Thus, class Rectangle can be said to inherit from class Quadrilateral. In this context, class a superclass, and class Rectangle is a subclass. A rectangle is a specific type of quadrilateral, but it is incorrect to claim that a quadrilateral is a rectangle (the quadrilateral could be a parallelogram). Inheritance normally produces subclasses with more features than their superclasses, so the terms superclass and subclass can be confusing. There is another way, however, to view these terms that makes perfectly good sense. Because every subclass object “is an”object of its superclass, and because one superclass can have many subclasses, the set of objects represented by a superclass is normally larger than the set of objects represented by any of that superclass’s subclasses. For example, the superclass Vehicle represents in a generic manner all vehicles, such as cars, trucks, boats, bicycles and so on. However, subclass Car represents only a small subset of all the Vehicles in the world. Inheritance relationships form treelike hierarchical structures. A superclass exists in a hierarchical relationship with its subclasses. A class can certainly exist by itself, but it is when a class is used with the mechanism of inheritance that the class bees either a superclass that supplies attributes and behaviors to other classes or a subclass that inherits those attributes and behaviors. Frequently, one class is both a subclass and a superclass. An object of a subclass can be treated as an object of its superclass. This makes possible some interesting manipulations. For example, despite the fact that objects of a variety of classes derived from a particular superclass might be quite different from one another, we can create an array of references to them—as long as we treat them as superclass objects. But the reverse is not true: A superclass object cannot always be treated a subclass object. For example, a Shape is not always a Circle. Despite the fact that a subclass object also “is a” superclass object, the subclass type and the superclass type are different. Subclass objects can be treated as superclass objects. This makes sense because the subclass has members corresponding to each of the superclass members—remember that the subclass normally has more members than the superclass has. Assignment in the other direction is not allowed because assigning a superclass object to a subclass reference would leave the additional subclass members undefined. A reference to a subclass object could be implicitly converted into a reference to a superclass object because a subclass object is a superclass object through inheritance. There are four possible ways to mix and match superclass references and subclass references with superclass objects and subclass objects: 1. Referring to a superclass object with a superclass reference is straightforward. 2. Referring to a subclass object with a subclass reference is straightforward. 3. Referring to a subclass object with a superclass reference is safe, because the subclass object is an object of its superclass as well. Such code can refer only to superclass members. If this code refers to subclassonly members through the superclass reference, the piler will report a syntax error. 4. Referring to a superclass object with a subclass reference is a syntax error
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