title: uml class diagram - when should use usage, aggregation or association?
body: <p>I am brand new in UML desgin. Here, I have a question about it. Let us say I have two class, one is main caller class A, and it will new another class B. Should I use usage to represent the relationship between A and B (or aggregation)? Moreover, if a class C calls a method in another class D. What should I use? Association? or usage? Please help. Thanks a lot.</p>