Class Foo {

    private int a;
    private int b;
    private int c;
    private DBSaver dbSaver;

    public Foo(DBSaver dbSaver) {

        this.dbSaver = dbSaver;
    }

    public void processData() {

        processData();
        saveToDB();
    }

    private void workNumbers() {

        a = 1;
        b = 2;
        c = a + b;
    }

    private void saveToDB() {

        List<Integer> list = new ArrayList<>();
        list.add(a); list.add(b); list.add(c);
        dbSaver.save(list);
    }
}

Class FooTest {

    @Mock
    DBSaver dbSaver;
    @Mock
    Foo foo;

    @Test
    private void test() {

        when(dbSaver.save(any(ArrayList<>.class))).then(returnsFirstArg());
        foo = new Foo(dbSaver);
        Mockito.doThrow(new Exception() {
                //Now I would like to get a, b and c so I can ensure that they have the proper values.
            }
            ).when(foo).processData();
    }
}