Your System Under Test (SUT) calls a method on a collaborator, sending in a List as a parameter.
The collaborator adds an element to the list before returning.
The SUT uses the list with the added element.
You need to mock the collaborator and test the SUT.
There is no argument matcher in EasyMock supporting a scenario like this, so I created one. Would it be interesting to include such a matcher in the list of predefined argument matchers for EasyMock?