Software Engineering for Students A Programming Approach

11.5 Class–responsibility–collaborator cards 145
We shall see how the game software can be considerably simplified by making use of
inheritance.
11.4 ● Using the library
OOP is often called programming by extending the library because the libraries provided
along with OO languages are so rich and so reusable. An organization will often
also create its own library of classes that have been created in earlier projects. There are
two distinct ways of using classes in a library:
1. creating objects from classes in the library
2. defining new classes by extending (inheriting from) classes in the library.
For example, in designing the game program, we expect the library to provide
classes that implement buttons and a panel – along with classes that support the event
handling associated with these widgets. For example in the Java library we can use the
Button class directly, by creating button objects as instances of the Button class:
Button button = new Button("start");
The Java library also provides classes and methods that display the graphical images that
the program uses.
Another way in which the library is commonly used is to form new classes by
inheritance.
It is worthwhile looking in the library at the outset of design and then again at
every stage of design, in case something useful can be incorporated into the design.
This takes some self-discipline because it is tempting to write a method again, rather
than develop an understanding of someone else’s code.
11.5 ● Class–responsibility–collaborator cards
Class–responsibility–collaborator (CRC) cards are a way of helping to carry out OOD.
The technique uses ordinary index cards, made out of cardboard and widely available.
Each card describes an individual class as shown by the example of the Alien class in
Figure 11.3. The designer begins by writing the class name at the head of the card.
Then one area lists the methods provided by the class (its responsibilities). For the class
Alien, these are move, display, etc. A second area lists the classes that use this class
and the classes that are used by this class (the collaborators). For the class Alien, there
is only one, the library class Graphics that supports displaying graphical images.
The cards representing the constituent classes are placed on a table. This way the
cards can be moved around easily; their interrelationships can be visualized and adjusted
as necessary.
CRC cards offer advantages as compared with a software tool. They are cheap, readily
available and portable. Several people can easily collaborate, standing round the