How Do I Fix The "Incompatible Type" Error That I Am Getting On Compiling This Java Program?

package join;
import java.sql.*;
import java.util.*;
import connection.*;
import java.util.ArrayList;

public class Course
{
	private String student_mail_add="",student_password="";
	Connection con=null;

ArrayList<string> course_id= new ArrayList<string>();
ArrayList<string> course_name = new ArrayList<string>();
	
public static ArrayList<string> convertStringArrayToArraylist(String[] strArr){
    ArrayList<string> stringList = new ArrayList<string>();
    for (String s : strArr) {
        stringList.add(s);
    }
    return stringList;
}	
	
	public void setStudent_mail_add(String student_mail_add) {
	this.student_mail_add=student_mail_add;
	}
	public String getStudent_mail_add()
	{
	return student_mail_add;
	}
	

	public void setStudent_password(String student_password) {
	this.student_password=student_password;
	}
	public String getStudent_password()
	{
	return student_password;
	}
	

	public void setCourse_id(String course_id) {
	this.course_id=course_id;
	}
	public String getCourse_id()
	{
	return course_id;
	}


	public void setCourse_name(String course_name) {
	this.course_name = course_name;
	}
	public String getCourse_name()
	{
	return course_name;
	}	

	public void fetch()
	{
		try
		{
		  Connect ob=new Connect();
		  con=ob.ctest();
		  PreparedStatement pst=con.prepareStatement("select * from course_join where course_join.student_mail_add=student_profile.student_mail_add=?");
		  pst.setString(1,student_mail_add);
		  ResultSet rs=pst.executeQuery();

		  while(rs.next()){
		   student_mail_add=rs.getString(1);
		  student_password=rs.getString(2);
		 
		  course_id.add(rs.getString(3));
		course_name.add(rs.getString(4));

			            }
		con.close();
		}
		catch(Exception e){}
	}
	

public int delete()
	{
	int t=0;
	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){}
	return t;
	}

	}