So far we have seen stack diagrams, which show the state of a program, and object diagrams, which show the attributes of an object and their values. These diagrams represent a snapshot in the execution of a program, so they change as the program runs. They are also highly detailed, and for some applications, too detailed. A class diagrams is a more abstract representation of the structure of a program. Instead of showing individual objects, it shows classes and the relationships between them.

There are several kinds of relationship between classes:

• Objects in one class might contain references to objects in another class. For example, each Rectangle contains a reference to a Point, and each Deck contains references to many Cards. This kind of relationship is called HAS-A, as in, ‘a Rectangle has a Point.’
• One class might inherit from another. This relationship is called IS-A, as in, ‘a Hand is a kind of a Deck.’
• Once class might depend on another in the sense that changes in one class would require changes in the other.

A class diagram is graphical representation of these relationships between classes. For example, this diagram shows the relationship between Card, Deck and Hand.

The arrow with a hollow triangle head represents an IS-A relationship; in this case it indicates that Hand inherits from Deck. The standard arrow head represents a HAS-A relationship; in this case a Deck has references to Card objects. The star(*) near the arrow head is a multiplicity; it indicates how many Cards a Deck has. A multiplicity can be a simple number, like 52, a range, like 5..7 or a star, which indicates that a Deck can have any number of Cards.

from Thinking in Python

Class diagrams