TwinColSelect and setting values in right column

I've read through a couple of posts on the same topic. The problem is always the same: there are no visible values in the right hand column of a TwinColSelect although getValue() does indeed return a Set with some members of the type expected.

In my case the TwinColSelect is bound to a bean which ultimately comes from JPA.
My class is called UserGroup and it implements toString(), equals() and hash().

Both the left-hand side and the right-hand side contain an underlying value which in both cases is Set<UserGroup>, as expected. The left-hand side is able to display visible values just fine, supposedly using the toString() method on UserGroup.

Couple of questions:

1. Disregarding the problem with the display of values in the right-hand side why is the left-hand side able to display those exact same object types just fine?  (I would expect some symmetry: either none of the sides work or both of them work).
2. The real question: What am I missing that makes the values in the right-hand side invisible?

Thanks.

Peter

So I found out what the question was and I cannot find this documented anywhere ...although there's some logic to it.

Here's the correct method of populating a TwinColSelect:

1. Before the beans binding you must set potential values in the left-hand column using one of TwinColSelect.addItem..  methods. You must set all potential values, disregarding if they are currently a selected value in your bean or not. Yes, the full list of potential values.
2. Now do the bean binding, e.g.  BeanFieldGroup.bindFieldsBuffered(userGroup, this) or whatever. This sets the right-hand column. Vaadin will automatically remove those values from the left-hand column that you set in step 1 and which are now in right-hand column.

So left-hand column first, then right-hand column.
If you do not comply with this order of events you'll get the result that the right-hand column seemingly displays nothing.

Peter