面向对象分析与设计概述.ppt

This example demonstrates how objects interact to achieve the desired business functionality. The example is from an order-entry system where the order-entry clerk enters all sales information on the OrderEntryForm, stereotyped as a boundary class. After all information has been entered into the form, the system must calculate the total dollar value for the sale. Calculating the total dollar value for the sale involves knowledge of the list price of the items in the sale, tax, and any discounts that may apply. It does not make sense for the OrderEntryForm to calculate the total dollar amount because it does not own the attribute that stores this value. The object that owns this attribute is the Order object. Therefore, the Order object has an operation, calculateOrderTotal, that knows how to calculate the sales total. Also, in keeping with the principle of encapsulation, no other object should be allowed to change the value of sales total. The OrderEntryForm knows that the sales total needs to be calculated. Therefore, it sends a message to Order to carry out this behavior. The Order object will calculate the total and send a return value back to the OrderEntryForm where the total can be displayed. Point out how messages support the principle of encapsulation. As a rule of thumb, if an attribute is owned by an object, that object needs to know how to calculate the values for that attribute. No other object should be able to do so. Remember, class responsibilities are realized by its operations and attributes. There are four basic principles of object orientation. They are: Abstraction Encapsulation Inheritance Polymorphism Be sure the students understand objects before you begin this next section. We’ve introduced objects first to help students better apply each of these principles. Abstraction can be defined as: Any model that includes the most important, essential, or distinguishing aspects of something while suppressing or ignoring less important, immat