User-Defined Types and Reference Variables

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

  int x;

is a declaration that variable x is an int(eger), one of several simple, predefined types defined to be part of the C language.

What does this really mean? It means that

• x is a symbolic name that represents an integer value.

• The "thing" that we've 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 type.

• Whenever we want to operate on this particular integer value in our program, we refer to it via its symbolic name x:

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

In an object-oriented language like C#, 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 (an instance of the Student class).

• 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:

    // Pseudocode.
    if (y hasn't chosen an advisor yet) Console.WriteLine ("Uh oh ...");
  

Note the parallels between y as a Student in the preceding example and x as an int earlier. Just as int is a predefined type (in both C and C#), the Student class is a user-defined type. And, because y in the preceding example is a variable that refers to an instance (object) of class Student, y is informally known as a reference variable.

Names for (nonattribute) reference variables follow the same convention as method and attribute names: i.e., they use Pascal casing. Some sample reference variable declarations are as follows:

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