The difference between association and dependency
Relationship: class A needs an object reference or pointer of class B as a parameter when it is instantiated
Dependency: a method of class A uses class B, either because the parameter of the method is class B or because an instance of class B is obtained in the method
A class appears in another class in the form of member variables, the two are associated; a class appears in the form of local variables in another class, the two are dependent.
Aggregation relationship: weak correlation, whole and part can exist independently
Combinatorial relationship: strong relevance, the whole and the part are inseparable and cannot exist independently.
