JDBC driver, getTable Method

i am  writing a jdbc driver for one of the application. while writing a code for method getTables in DatabaseMetadata.java, i am getting some weird exception while traversing the resultset. (on further debuging, it is coming from next method in Resultset class.)

this is a part of client application which is using JDBC driver.
--------------------------------------------
DatabaseMetaData md = conn.getMetaData();
      System.out.println("TEST-2");

      ResultSet rs1 = md.getTables(null, null, null, null);
      System.out.println("TEST3");
      //System.out.println(rs1.size());
       while ( rs1.next() )
      {
            System.out.println(rs1.getString(1));
      }
    } catch(Exception ex) {
      System.out.println(ex.getMessage());
     
    }
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this is a getTable Method
--------------------------------------------
public ResultSet getTables(String catalog, String schemaPattern, String tableNamePattern, String[] types) throws SQLException {
    // the field descriptors for the new ResultSet
System.out.println("in gettable-1");
    Field f[] = new Field[4]; // = new Vector();
    Vector field1_ = new Vector();
    ResultSet r;      // ResultSet for the SQL query that we need to do
    Vector v = new Vector();            // The new ResultSet tuple stuff
// FIELD_TYPE_VAR_STRING = 253 in DBQDefs.java
    f[0] = new Field("FILE_ALIAS", 253, 32);
    f[1] = new Field("USER_EXIT", 253 ,32);
    f[2] = new Field("FILE_INDEX", 253, 32);
    f[3] = new Field("FILE_NAME", 253 ,256);

field1_.addElement(f[0]);
field1_.addElement(f[1]);
field1_.addElement(f[2]);
field1_.addElement(f[3]);

    // Now form the query
    StringBuffer sql = new StringBuffer("disp files");
System.out.println("in gettable-2");
    java.sql.Statement stmt = _conn.createStatement();       
     //Statement stmt = new com.appres.sgi.Statement(_conn, _database);
    r = stmt.executeQuery(sql.toString());

      while (r.next())
      {
            byte[][] tuple = new byte[4][0];
            tuple[0] = r.getString(1).getBytes();
            tuple[1] = r.getString(2).getBytes();
            tuple[2] = r.getString(3).getBytes();
                tuple[3] = r.getString(4).getBytes();
            // remove after success
            System.out.print(r.getString(1));
            v.addElement(tuple);
      }
      r.close();
      System.out.println("in gettable-3");
      return new com.appres.sgi.ResultSet(field1_, v);
  }
--------------------------------------------

and this is a part of Resultset.java part from where it throws the exception.
-----------------------------------------------------
public boolean next() throws java.sql.SQLException
      {

          // debug
          //System.out.println("in Resultset Next" + rows_.size());
          if (rows_.size() == 0) return false;
          if (current_row_num_ + 1 >= rows_.size()) return false;

          current_row_num_++;
          try {
          current_row_ = (Vector) rows_.elementAt(current_row_num_);
          } catch(RuntimeException ex2){
              System.out.println(ex2.getMessage());
              throw ex2;
          }
          catch(Exception ex1){
              System.out.println(ex1.getMessage()); throw ex1;

          }
          return true;
      }
-------------------------------------------------
exception is thrown at rows_.elementAt(current_row_num_);

thanks in advance for any help.