Selects the index for the first instance of given object in the underlying
data model. Since the SingleSelectionModel can
only support having a single index selected at a time, this also causes
any previously selected index to be unselected.
Gets the number of items available for the selection model. If the number
of items can change dynamically, it is the responsibility of the
concrete SingleSelectionModel implementation to ensure that items are
selected or unselected as appropriate as the items change.