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; for some purposes, too detailed. A class diagram 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.”
- One class might depend on another in the sense that changes in one class would require changes in the other
A class diagram is a graphical representation of these relationships. For example, Figure 18.2 shows the relationships 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.
A more detailed diagram might show that a Deck actually contains a list of Cards, but built-in types like list and dict are usually not included in class diagrams.
Exercise 18.4.ReadTurtleWorld.py, World.pyandGui.pyand draw a class diagram that shows the relationships among the classes defined there.