Moving Toward Object-Oriented Programming - Beginning Java Programming: The Object-Oriented Approach - page 105

How It Works

Now take a look at how it works.
1.

You're creating two new Student and Course classes in a new Eclipse project.

2.

For the Course class, note the use of the nextId variable to automatically use an incrementing coun-
ter to provide IDs for courses. This is a well‐known pattern that you will see show up commonly in
Java code. Also note the use of the HashSet object. A HashSet is a set, storing a bunch of objects
( Student objects, in this case). The HashSet class is built‐in by default in Java, but to enable its use,
you first have to import it, hence the import statement before defining the class itself. Don't concern
yourself too much with its usage for now, but make sure you understand the registerStudent and
unregisterStudent methods, which add and remove students to and from the set.

3.

For the Student class, you're using the keyword this to pass the current student object (meaning
the object the method was called on) to the course object to register or unregister a student, as it is
the course object that keeps a list of registered students.

4.

The Program class contains the main method, and it creates two courses, gives some information
about them, creates two students, and registers them in the courses. The p method just serves as a
shorthand to avoid having to write System.out.println all the time.

5.

You then run this program from Eclipse by invoking the main method in the Program class. Note
that it is possible at this point to create another class containing a main method and run that one in
Eclipse by just making sure the main method you want to run is open in the code editor.

access modifiers

You might have noticed that this code uses courseA.id (directly accessing a vari-
able) and courseA.getName() (accessing a variable through a method) in the Try
It Out. In other examples, you read that it's generally better to access variables
through methods whenever possible instead of directly accessing variables.

If it is generally recommended to go through methods, then why does Java allow
you to access courseA.id directly? The reason for this is due to the access modifier
being used. Classes, variables, and methods can all take access modifiers. In Java,
four access modifiers exist:
public : For classes, methods, and member variables (class or instance)
protected : Methods and member variables (class or instance) (not for classes)
no modifier: : For classes, methods, and member variables (class or instance)

private : Methods and member variables (class or instance) (not for classes)
You have seen one of these ( public ) already, namely in the main method, where the
inclusion of this access modifier was mandatory:

public static void main(String[] args)

continues