You might want to see more of your problem:
try{
Connect ob=new Connect();
con=ob.ctest();
PreparedStatement pst=con.prepareStatement("delete from course_join where student_mail_add=?");
pst.setString(1,student_mail_add);
t=pst.executeUpdate();
con.close();
}
catch(Exception e){
e.printStackTrace();
}
And do you code manually or do you use a proper IDE?
The imports are manual. Please use an IDE, eclipse as also netbeans are free. They will guide you through that problem of not having the correct reference.
You might want to check on how to return an Array from an ArrayList:
public static ArrayList<string> convertStringArrayToArraylist(String[] strArr){
ArrayList<string> stringList = new ArrayList<string>();
for (String s : strArr) {
stringList.add(s);
}
return stringList;
} </string></string></string>
http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#toArray()[
^]