Java program to query database error

I'm making a program that uses Java as a front to do some SQL manipulations of data. I've got most of it set up, and I am able to make the connection to my database, however for some reason when I try and run my query, it returns a line saying "Could not execute query" and I cannot figure out why it can't execute the query.

public class application {

   public static void main (String args[]) throws Exception,
                 IOException, SQLException {
      try {
          Class.forName ("oracle.jdbc.driver.OracleDriver");
      } catch (ClassNotFoundException e) {
          System.out.println ("Could not load the driver");
      }
      String user = readEntry("Enter userid: ");
      String pass = readEntry("Enter password: ");
      Connection conn = DriverManager.getConnection (
                  "jdbc:oracle:thin:@serveraddress",user,pass);

      boolean done = false;
      do {
         printMenu();
         System.out.print("Type in your option: ");
         System.out.flush();
         String ch = readLine();
         System.out.println();
         switch (ch.charAt(0)) {
             case '1': studentModules(conn);
                 break;
             //some other cases
             case '0': done = true;
                 break;
             default : System.out.println(" Not a valid option ");
         } //switch
     } while(!done);

  } // main

  private static void studentModules(Connection conn) throws SQLException, IOException {

  String sqlString = null;
  Statement stmt = conn.createStatement();

  sqlString = "select Student_id" +
            "from STUDENT;";

  ResultSet rset;
  ResultSet rset2;

  try {
     rset = stmt.executeQuery(sqlString);
  } catch (SQLException e) {
     System.out.println("Could not execute query");
     stmt.close();
     return;
  }

  String print;

  while (rset.next()) {

      print = rset.getString(1) + ": ";

      sqlString = "select Module_code" +
              "from EXAM" +
              "where Student_id = " + rset.getString(1) + ";";

      try {
         rset2 = stmt.executeQuery(sqlString);
      } catch (SQLException e) {
         System.out.println("Could not execute query");
         stmt.close();
         return;
      }

      while (rset2.next()) {
         print = print + rset.getString(1) + " ";
      }

      System.out.println(print);

   }

   stmt.close();

}

I know it's a long piece of code, but I cannot figure why it goes wrong. I know it is erroring and being caught when trying to query "SELECT Student_id FROM STUDENT", but that query isn't wrong and I can't understand why it would throw an error there?

Thanks for any help.