public class Student{
    ....
    private StudentUnion;
    ....
}

public class StudentUnionComparator implements Comparator<StudentUnion>{
    public int compare(StudentUnion s1, StudentUnion s2){
        ....
        return result // 1 or 0;
    }
}

public class StudentUnion implements Comparable<StudentUnion> {

    public int compareTo(StudentUnion s) {
        ....
        return result // 1 or 0;
    }
}

public class Student implements Comparable<Student> {
    public int compareTo(Student s) {
        int result = studentUnion.compareTo(s.studentUnion);
        if (result != 0)
            return result;
        // use another field like name to break ties on StudentUnion
        return name.compareTo(name);
    }
}

public class StudentComparator implements Comparator<Student> {
  public int compare(Student s1, Student s2) {
    return s1.getStudentUnion().compareTo(s2.getStudentUnion();
    }
  }

public class CommonComparator implements Comparator<Object> {
    public int compare(Object s1, Object s2){
        ....
        return result // 1 or 0;
    }
}