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.

Classes as Abstract Data Types

In a non-OO programming language such as C, the statement

int x;

is a declaration that variable 'x' is an integer, one of several simple, built-in data types defined to be part of the C (C++, Java) language.

What does this really mean? It means that:

• 'x' is a symbolic name that represents an integer value.
• The 'thing' that we have named 'x' understands how to respond to a number of different operations such as addition (+), subtraction (-), multiplication (*), division (/), logical comparisons (>, <, =), and so on that have been defined for the int data type.
• Whenever we want to operate on this particular integer value in our program, we refer to 'x' instead:

  if (x > 17) x = x + 5;
  

In an object-oriented language like Java, we can define a class such as Student, and then declare a variable as follows:

Student y;

What does this mean? It means that:

'y' is a symbolic name that refers to a Student object/instance.

The 'thing' that we have named 'y' understands how to respond to a number of different
service requests — how to register for a course, drop a course, and so on — that have been
defined by the Student class.

Whenever we want to operate on this particular object, we refer to 'y' instead:

if (y hasn't chosen an advisor yet  )
 System.out.println("Oh oh ...");

[The lines above are one line. They have been split for formatting purposes.]


Note the parallels between x as an int in the previous example and y as a Student above. Just as
int is referred to as a simple, or built-in, data type in a language like C or Java, we can refer to a user-
defined class such as Student as an abstract data type (ADT): that is, a user-defined data type that
specifies structure as well as behavior. It is called 'abstract' because a class is an abstraction of a real-
word object that reflects only its relevant details. And, because 'y' in the above example is a variable
that refers to an instance (object) of the class Student, 'y' may be alternatively referred to as either a
reference variable or sometimes just as a reference — a reference to a Student object, to be precise.

Names for reference variables follow the same convention as method and attribute names: they start
with a lower case letter and use mixed case to separate 'words' within the name. Some sample reference
variable declarations are as follows:

Student x;
Student aStudent;
Course prerequisiteOfThisCourse;
Professor myAdvisor;
























	





    

		

			
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