just thinking out loud here.
if the comboBox only displays 'Operations' (public OperationsCellEditor(
String[] items) --> could be a problem),
maybe this might work:
1) add a field to Operation()
boolean selected = false;
2) add (or add to) a listener - when an operation is selected - change that operation's 'selected' to true
3) add the indicated line to Operation's toString()