If I understand you correctly, you want to manipulate/use a list of mendix objects in a java action?
If that is the case, I'd suggest setting the input parameter of the java action to "list" and sending it as a parameter in that way.
The wacky alternative is to create a Domain Entity that has a referenceset to the list of objects that you want to pass, then add that entity to the java action as a parameter, and retrieve the list inside the java action.