问如何从java代码中绘制序列图

EN

import java.util.List;

public static void main(String[] args) {

        Student student = new Student(2);

        student.addClass(new Class("SE 450"));

        student.addClass(new Class("SE 550"));

        System.out.println(student.getFormattedSchedule());
}

class Student {

        //instance field;

        private final List<Class> classes;

        private final int maxNumberOfClasses;

        //constructor

        Student(int maxNumberOfClasses){

                classes = new ArrayList();

                this.maxNumberOfClasses = maxNumberOfClasses;

        }

        //Mutator Method     

        void addClass(Class newClass){

                if(classes.size() < maxNumberOfClasses)

                        classes.add(newClass);
        }


        List<Class> getClasses(){

                return classes;         

        }

        String getFormattedSchedule(){

                if(classes.size() == 0)

                        return "No classes added.";


                String schedule = "";

                for (Class studentClass : classes) {

                        schedule += studentClass.getName() + "\r\n";

                }

                return schedule;