docs: protected List ensureColumns(List columns, List existingColumns, StringBuffer buf)
      {
          if (columns == null || columns.isEmpty())
          {
              return existingColumns;
          }
  
          Iterator iter = columns.iterator();
          int ojb_col = existingColumns.size() + 1;
  
          while (iter.hasNext())
          {
              FieldHelper cf = (FieldHelper) iter.next();
              if (!existingColumns.contains(cf.name))
              {
                  existingColumns.add(cf.name);
                  
                  buf.append(",");
                  appendColName(cf.name, "ojb_col_" + ojb_col, false, null, buf);
                  ojb_col++;
              }
          }
          
          return existingColumns;
      }