I have a base code with interfaces and some classes, the application in general has to simulate an agenda and the only thing that had to be modified were a few classes because the interfaces and some methods were ready to be used, however when trying to implement it that marked the comments did not frame error but it does not work well.
in this link I leave the code link
And more than the code, I would like you to be able to explain to me the way in which I should have created the methods that I used