public class Classwork {

    public static void main(String[] args){

        Course courseA = new Course("Курс А");
        Course courseB = new Course("Курс В");

        Student S1 = new Student("Иванов", "Иван");
        Student S2 = new Student("Петров", "Петр");

        p("Регистрация на курсы");
        S1.registerForCourse(courseB);
        S2.registerForCourse(courseA);
        courseA.registerStudent(S2);

        p("Студенты курса А");
        courseA.outp();
        p("Студенты курса В");
        courseB.outp();

        p("Количество студентов на курсе А" + courseA.nrOfRegisteredStudent());
        p("Количество студентов на курсе А" + courseA.nrOfRegisteredStudent());
    }

    public static  void p(String s){
        System.out.println(s);
    }
}




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public class Student {

    static int nextId = 0;
    final int id;
    final String firstName, lastName;

    public Student(String fn, String ln){
        nextId++;
        id = nextId;
        firstName = fn;
        lastName=ln;
    }

    public String getFirstName(){
        return firstName;
    }

    public String getLastName(){
        return lastName;
    }

    public void registerForCourse(Course c){
        c.registerStudent(this);
    }

    public void unregisterForCourse(Course c) {
        c.unregisterStudent(this);
    }
}




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import java.util.HashSet;

public class Course {

    static int nextId = 0;
    final int id;
    final String name;

    public Course(String n){
        id = nextId;
        nextId++;
        name = n;
    }

    String getName(){
        return  name;
    }

    final HashSet<Student> registeredStudents = new HashSet<Student>();

    void registerStudent(Student s){
        registeredStudents.add(s);
    }

    void unregisterStudent(Student s){
        registeredStudents.remove(s);
    }

    HashSet<Student> registeredStudents(){
        return registeredStudents;
    }

    int nrOfRegisteredStudent(){
        return  registeredStudents.size();
    }

    void outp(){
        for (Student  s: registeredStudents)
            System.out.println(s.getFirstName());
    }
}