java的面向?qū)ο缶幊掏馕馁Y料翻譯-其他專(zhuān)業(yè)-文庫(kù)吧資料
2025-01-27 06:30本頁(yè)面
【正文】 s and methods of other classes in the same package as the superclass can access protected superclass members. Experience in building software systems indicates that significant portions of the code deal with closely related special cases. It bees difficult in such systems to see the “big picture”because the designer and the programmer bee preoccupied with the special cases. Objectoriented programming provides several ways of “seeing the forest through the trees.” The programmer and designer concentrate on the big picture—the monality among objects in the system—rather than the special cases. This process is called abstraction. If a procedural program has many closely related special cases, then it is mon to see switch structures or nested if/else structures that distinguish among the special cases and provide the processing logic to deal with each case individually. We will show how to use inheritance and polymorphism to replace such switch logic with much simpler logic. We distinguish between the “is a” relationship and the “has a” relationship. “Is a” is inheritance. In an “is a” relationship, an object of a subclass type may also be treated as an object of its superclass type. “Has a” is position. In a“has a” relationship, a class object has one or more objects of other classes as members. For example, a car has a steering wheel. A subclass’s methods might need to access certain of its superclass’s instance variables and methods. A crucial aspect of software engineering in Java is that a subclass cannot access the private members of its superclass. If a subclass could access the superclass’s private members, this would violate information hiding in the superclass. However, a subclass can access the public and protected members of its superclass. A subclass also can use the package access members of its superclass if the subclass and superclass are in the same package. Superclass members that should not be accessible to a subclass via inheritance are declared private in the superclass. A subclass can effect state changes in superclass private members only through public, protected and package access methods provided in the superclass and inherited into the subclass. A problem with inheritance is that a subclass can inherit methods that it does not need or should not have. It is the class designer’s responsibility to ensure that the capabilities provided by a class are appropriate for future subclasses. Even when the superclass methods are appropriate for the subclasses, it is mon for a subclass to require the method to perform a task in a manner that is specific to the subclass. In such cases, the superclass method can be overridden (redefined) in the subclass with an appropriate implementation. Perhaps most exciting is the notion that new classes can inherit from abundant class libraries, such as those provided with the Java API. Organizations develop their own class libraries and can take advantage of other libraries available worldwide. Someday, most software might be constructed from standardized reusable ponents, just as hardware is often constructed today. This will help meet the challenges of developing the ever more powerful software we will need in the future. Often an object of one class “is an” object of another class as well. A rectangle certainly is a quadrilateral (as are squares, parallelograms and tr
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