5.5. Summary

In this chapter, you learned

• That an association describes a relationship between classes—that is, a potential relationship between objects of two particular types/classes.

• That we define the multiplicity of an association between classes X and Y in terms of how many objects of type X can be linked to a given object of type Y, and vice versa. Possible multiplicities are one-to-one (1:1), one-to-many (1:m), and many-to-many (m:m). In all these cases, the involvement of the objects at either end of the relationship may be optional or mandatory.

• That an aggregation is a special type of association that implies containment.

• How to derive new classes based on existing classes through inheritance and what the do's and don'ts are when deriving these new classes. Specifically, how we can extend a base class by adding members or specialize a base class by overriding methods.

• How class hierarchies develop over time, and what we can do to try to avoid "ripple effects" to our application as the class hierarchy changes with evolving requirements.

• How overloading can be used to create multiple methods with the same name but with different argument signatures.

• Why multiple inheritance can be so troublesome to implement in an OOPL.