How/can custom sort column in Vaadin table connected to indexed container?

Please can you advise on how to carry out the following:

vaadin v 7.4.3

Have created a table and connected to an indexed container.

For most of the columns the table can be sorted using the default mechanism (e.g. alphabetically for String types), following a user clicking on the column header.

For one of the columns (a String type), following a user clicking on the column header, the table needs to be sorted in a customised way, i.e. not alphabetically.

Thanks