Олег

/**
 * Created by Labunsky on 24.03.2017.
 */
public class MultiListEntry<S, C> {
    private MultiListEntry<S, C> nextStudentEntry = null;
    private MultiListEntry<S, C> nextCourseEntry = null;

    private S student;
    private C course;

    MultiListEntry(S student, C course) {
        this.student = student;
        this.course = course;
    }

    public S getStudent() {
        return student;
    }

    public C getCourse() {
        return course;
    }

    public void setNextStudentEntry(MultiListEntry<S, C> next) {
        nextStudentEntry = next;
    }

    public void setNextCourseEntry(MultiListEntry<S, C> next) {
        this.nextCourseEntry = next;
    }

    public MultiListEntry<S, C> getNextStudentEntry() {
        return nextStudentEntry;
    }

    public MultiListEntry<S, C> getNextCourseEntry() {
        return nextCourseEntry;
    }
}

/**
 * Created by Labunsky on 24.03.2017.
 */
public class MultiList<S, C> {
    private Map<S, MultiListEntry<S, C>> studentsIndex = new HashMap<>();
    private Map<C, MultiListEntry<S, C>> coursesIndex = new HashMap<>();

    public void add(S student, C course) {
        MultiListEntry<S, C> newEntry = new MultiListEntry<>(student, course);
        MultiListEntry<S, C> lastEntry;

        if (studentsIndex.containsKey(student)) {
            lastEntry = studentsIndex.get(student);

            while (lastEntry.getNextStudentEntry() != null)
                lastEntry = lastEntry.getNextStudentEntry();
            lastEntry.setNextStudentEntry(newEntry);
        } studentsIndex.put(student, newEntry);

        if (coursesIndex.containsKey(course)) {
            lastEntry = coursesIndex.get(course);
            
            while (lastEntry.getNextCourseEntry() != null)
                lastEntry = lastEntry.getNextCourseEntry();
            lastEntry.setNextCourseEntry(newEntry);
        } else coursesIndex.put(course, newEntry);
    }

    public MultiListEntry<S, C> getStudentEntries(S student) {
        return studentsIndex.get(student);
    }

    public MultiListEntry<S, C> getCourseEntries(C course) {
        return coursesIndex.get(course);
    }
}