Reviews : Java Books :
Beginning Java Objects : Chapters 2 and 3

Buy this book Title: Beginning Java Objects ISBN: 1861004176 US Price: $ 39.99 Canadian Price: C$ 59.95 UK Price: £ 28.99 Publication Date: November 2000 Pages: 800 © Wrox Press Limited, US and UK.

Objects as Attributes

When we first discussed the attributes and methods associated with the Student class, we stated that some of the attributes could be represented by built-in data types provided by the Java language, whereas the types of a few others (advisor, courseLoad, and transcript) were left undefined. Let's now put what we learned about abstract data types to good use. Rather than declaring the Student class's advisor attribute as simply a String representing the advisor's name, we'll declare it to be of an abstract data type — namely, type Professor, another user-defined class/type:

(We'll still leave the courseLoad and transcript attributes' data types undefined for the time being; we'll see how to handle these attributes a bit later.) By having declared the advisor attribute to be of type Professor, we've just enabled a Student object to maintain a handle on its actual Professor advisor object. The Professor class, in turn, might be defined to have attributes as follows:

(We will leave the type of teachingAssignments open for the time being.) Again, by having declared the studentAdvisee attribute to be of type Student, we've just given a Professor object a way to hold onto its actual Student advisee object.

The methods of the Professor class might be as follows:

• transferToDepartment
• adviseStudent
• agreeToTeachCourse
• assignGrades

A few noteworthy points about the Professor class:

• It is likely that a professor will be advising several students simultaneously, so having an attribute like studentAdvisee that can only track a single Student object is not terribly useful. We'll discuss techniques for handling this in Chapter 6, when we talk about collections, which we'll also see as being useful for defining the teachingAssignments attribute of Professor and the courseLoad and transcript attributes of Student.
• The worksFor attribute represents the department to which a professor is assigned. We can choose to represent this as either a simple String representing the department name — for example, 'MATH' — or as a reference to a Department object — specifically, the Department object representing the 'real-world' Math Department. As we'll see in Part 2 of this book, the decision of whether or not we need to invent an abstract data type to represent a particular real-world concept is not always clear- cut.

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