You firstly need to create a list of IMendixObject. Then you create your Mendix entities and add them to the list as MendixObjects. Finally you return them from the Java Action.
Here is some example code that should help you.
// Create the List List<IMendixObject> returnList = new ArrayList<IMendixObject>(); // Create a new BatchList entity BatchList batchList = new BatchList(getContext()); batchList.setMachineName("Test Machine"); batchList.setPlantName("Test Plant"); // Add to the List returnList.add(batchList.getMendixObject()); // Return the List from the Java Action return returnList;