How to use a variable of one class, in another in Java?

Question

I\'m just working through a few things as practice for an exam I have coming up, but one thing I cannot get my head round, is using a variable that belongs to one class, in

Answer 1

Am I correct in using 'extends' within Course? Or is this unnecessary?

Unfortunately not, if you want to know whether your inheritance is correct or not, replace extends with is-a. A course is a student? The answer is no. Which means your Course should not extend Student

A student can attend a Course, hence the Student class can have a member variable of type Course. You can define a list of courses if your model specifies that (a student can attend several courses).

Here is a sample code:

public class Student{
    //....
    private Course course;
    //...
    public void attendCourse(Course course){
       this.course = course;
    }
    public Course getCourse(){
       return course;
    }
}

Now, you can have the following:

Student bob = new Student(...);
Course course = new Course(...);
bob.attendCourse(course);

Answer 2

Course should not extend Student. If you want to access the courseTitle field of Course, you need to pass a reference to a Course object to the Student and then do course.CourseTitle.

Answer 3

I assume a Course is not a Student, so inheritance between those classes is probably a bad idea.

Answer 4

Maybe you do not need to add the course name to student. What I would do is add Students to some datastructure in Course. This is cleaner and reduces the coupling between Course and Student. This would also allow you to have Students being in more than one course. For example:

public class Course extends Student{
    private Award courseAward;
    private String courseCode;
    public String courseTitle;
    private Student courseLeader;//change to a student Object
    private int courseDuration;
    private boolean courseSandwich;
    private Set<Student> students;//have course hold a collection of students

/**
 * Constructor for objects of class Course
 */
public Course(String code, String title, Award award, Student leader, int duration, boolean sandwich){
    courseCode = code;
    courseTitle = title;
    courseAward = award;
    courseLeader = leader;
    courseDuration = duration;
    courseSandwich = sandwich;
    this.students=new HashSet<Student>();
}

public boolean addStudent(Student student){
    return students.add(student);
}

public Set<Student> getStudents(){
    return students;
} 

}

Answer 5

Extending Student with Couse because they are not of the same kind. Extending one class with another happens when specializing a more general (in a sense) one.
The solution would be to pass courseTitle as an argument of the Student constructor

Answer 6

As mentioned, stay away from the "extends" for this. In general, you shouldn't use it unless the "is-a" relationship makes sense.

You should probably provide getters for the methods on the Course class:

public class Course {
   ...
   public String getTitle() {
       return title;
   }
}

And then if the Student class needs that, it would somehow get a hold of the course (which is up to you in your design), and call the getter:

public class Student {
   private Set<Course> courses = new HashSet<Course>();

   public void attendCourse(Course course) {
       courses.add(course);
   }

   public void printCourses(PrintStream stream) {
       for (Course course : courses) {
           stream.println(course.getTitle());
       }
   }
}