cant convert String to integer

cant convert String to integer

I am a beginner. I know this is basic. In my project i am using java and
mysql workbench. I am reading a data from user by using select box which
is coming from database by using ajax The code for select box is mentioned
below
<%
String a =request.getParameter("course");
if(a!=null)
{
ResultSet rs=s.selectsub(a);
String Query="select * from subject where course_id='"+a+"'";
%>
<select name="subject" id="subject">
<option>Select Subject</option>
<%
while(rs.next())
{
%>
<option value="<% out.println(rs.getString("subject_id")); %>">
<% out.println(rs.getString("subject")); %></option>
<% } %>
</select>
<%
}
%>
and these subject id is passed to another page using post method and
trying this code
String subject=request.getParameter("subject");
int subjectid=Integer.parseInt(subject);
But the line Integer conversion is not working. Error is showing. The
error is
org.apache.jasper.JasperException: An exception occurred processing JSP
page /saveuser.jsp at line 29
26: String email=request.getParameter("email");
27: String designation=request.getParameter("designation");
28: String subject=request.getParameter("subject");
29: int subjectid=Integer.parseInt(subject);
30: String institute=request.getParameter("institute");
31: String inemail=request.getParameter("inemail");
32: String uname=request.getParameter("uname");
Please somebody help me.