I found the difficulties of test design most of the time comes from the design of the program being tested. For example, for the City Class and Visitor Class I wrote, there are some private methods, and some methods do not explicitly return a value, this makes me to use fake class and method to make it easier to test, and I think it will be better if I add more getters to the private method and private variables within class scope. Another difficulties I found is to test the probability requirement. I was still wondering how to test such requirement with assertion, I think it will be better to use statistic method or analyze the logic of the program.