code: public Class<?> getColumnType(int c) {
  
          for (int r = 0; r < m_data.size(); r++) {
              Object val = m_data.get(r).get(c);
              if (val != null) {
                  return val.getClass();
              }
          }
          return Object.class;
      }
docs: Gets the type to use for the Vaadin table column corresponding to the c-th column in this result.
  
  @param c the column index
  @return the class to use for the c-th Vaadin table column