Create or Edit an operation for a Service.
Go to the inputs/outputs tab.
For the inputs, if you select a collection, it is possible by clicking the <?> to specifcy the collection type. For the outputs, it is always has to be a Java set, so for example if I am passing in an ArrayList and I wanted to perform some operation on this, it will have to be returned as a Set.
It can be edited in code but any changes through the IDE change it back again.