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Related Content: CS605 - VU Lectures, Handouts, PPT Slides, Assignments, Quizzes, Papers & Books of Software Engineering II
As I look at amountOf, I can see that it uses information from the rental, but does not use information from the customer. This method is thus on the wrong object, it should be moved to the rental. To move a method you first copy the code over to rental, adjust it to fit in its new home and compile.
Read more: CS605 - Software Engineering II - Lecture Handout 43