i/o from jsp to a file (via creating an object)

Hello everyone,

I created 3 JSP pages: 1 - login, 2-displays 4 checkboxes with a submit, 3-displays the info submitted and also creates a file to store this information. I have a separate StudentElective.java where I created an object.
page.3 jsp looks like this:

Code Java:

<%  String knum = (String) session.getAttribute("knum");
    String mycourse = (String) session.getAttribute("course");
    String[] id = request.getParameterValues("id");%>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>Page 3</title>
    </head>
    <body>
        <h3>Thank you, <%=knum%>!</h3>
        You have selected
        <%
            int i;
            for (i = 0; i < id.length; i++) {
                out.print(id[i]);
                if (i == 0) {
                    out.print(" and ");
                }
            }
 
        %>!
        <br>
        Your application is now registered!
 
        <%  StudentElective se = new StudentElective();
            se.setKnum(knum);
            se.setCourse(mycourse);
            se.setElchoice(id);
 
 
 
            try {
                File DATAFILE = new File("c:/Users/k000000/Documents/studentData.dat");
                if (!DATAFILE.exists()) {DATAFILE.createNewFile();}
 
                FileOutputStream DATA = new FileOutputStream(DATAFILE);
                ObjectOutputStream SAVE = new ObjectOutputStream(DATA);
 
                SAVE.writeObject(se);
            } catch (Exception X) {
                System.out.print("\n\tFile Access Error. Could not SAVE.");
            }
 
        %>
    </body>
</html>

My object file:

Code Java:

public class StudentElective implements java.io.Serializable {
 
    private String knum,
            course;
    private String[] elchoice;
 
    public StudentElective() {
    }
 
    public StudentElective(String knum, String course, String[] elchoice) {
        this.knum = knum;
        this.course = course;
        this.elchoice = elchoice;
    }
 
    public String getCourse() {
        return course;
    }
 
    public String[] getElchoice() {
        return elchoice;
    }
 
    public String getKnum() {
        return knum;
    }
 
    public void setCourse(String course) {
        this.course = course;
    }
 
    public void setElchoice(String[] elchoice) {
        this.elchoice = elchoice;
    }
 
    public void setKnum(String knum) {
        this.knum = knum;
    }
       public void print() {
        System.out.print(this.knum);
        System.out.print(this.course);
        System.out.print(this.elchoice);
    }
 
}

I have 2 questions:
1. At the moment everytime I write new information into a file, my old info is gone.How can I modify my code so everytime I write a new info into a file it will be saved onto a new line. In other words, how can I open my file for multiple entries everytime new instance of object gets created?
2.I made an error somewhere when I was trying to pass selected checkboxes to an array in my object. Everytime I read the info from file created it shows something like

[Ljava.lang.String;@5456a499

instead of the names of my checkboxes.Where I made this mistake and how to fix it?

Thank you in advance for any help possible.