How to Replace The Call to A Private Method of The Class Being Tested

Question

Well, I am right now testing legacy code. And, I am somewhere near to pass this test, but its stuck at the line having comments on it. Here is the snippet

    new NonStrictExpectations(){
        SASCustomerDataAssemblerBD assembleBd;
        CustomerTOs tos;
        CustomerSASTO to;
        Another rowTo;
        SelectionJobLogBD logBd;    

        {
                SASCustomerDataAssemblerBD.getInstanceUsingEjbRef(); result = assembleBd;
                assembleBd.getData(); result = tos;
                ..
                ..
                //This line is not being invoked. 
                //Instead the actual line of code is working. Which is,
                //Collections.max(someCollection, someComparator);
                //Hence I am stuck because getting null in "to"
                invoke(Collections.class, "max", new ArrayList(), new MaxDateComparator()); result = to;
                to.getSasDataRow(); result = rowTo;
                SelectionJobLogBD.getInstanceUsingEjbRef(); result = logBd;                                 
                ..
        }
    };

    new TaskSASCustomerReading().execute(); 

Whereas, all the values of result are mocked up.

Answer 1

Solved it, in another way :). Mocked the original method -- only the method that calls Collections.max() under the hood.

    new MockUp<TaskSASCustomerReading>()
    {
        @Mock
        // This original method is invoking Collections.max(). 
        // Therefore, I just mocked this one, other methods are all original
        String findLatestSelectionDate(Collection customerTOs) {
           return "";
        }
    };

    new Expectations(){
        SASCustomerDataAssemblerBD assembleBd;
        CustomerTOs tos;         
        SelectionJobLogBD logBd;
        {
            try {
                SASCustomerDataAssemblerBD.getInstanceUsingEjbRef(); result = assembleBd;
                assembleBd.getData();  result = tos;
                SelectionJobLogBD.getInstanceUsingEjbRef();  result = logBd;                                
            }catch(Exception e){}
        }
    };

    new TaskSASCustomerReading().execute(); 

None the less, I was totally misunderstood the thing in the first place, when I asked the question. In my original question, I am in fact trying to invoke a method, instead of replacing it. (P.S. Never work after hours. ;))