Software Engineering for Students A Programming Approach

154 Chapter 12 ■ Design patterns
But the converse is not true.
The moral is:
■ anything that can be accomplished by inheritance can be achieved through delegation
■ everything that can be accomplished by delegation cannot be achieved through
inheritance. Delegation is more general mechanism than inheritance.
■ inheritance can be useful for modeling static “is-a” situations
■ inheritance is not appropriate for modeling “is-a-role-played-by” situations
■ delegation is more widely used than inheritance.
SELF-TEST QUESTION
12.4 ● Singleton
Black White CrownedBlack CrownedWhite
Uses
Piece
Figure 12.2 Game of draughts using delegation
12.1 A soldier in the army is a private, a sergeant or a general. Do we model
this as inheritance or delegation?
In some systems there only needs to be one instance of a class. Normally, of course,
someone writes a class so that any number of objects can be created from it. But occasionally
there should only be one. An example is an object to manage the communication
between a system and the database. It would be confusing if any number of
classes were interacting with the database. Now it would be possible to try to achieve
this affect by telling all the programmers on the team that there is one copy of the
object, with such-and-such a name, written by Mo. But in a large and complex system,
this could be forgotten. We can instead legislate (with the help of compiler checking)
using the Singleton pattern.
Another example of the Singleton pattern is evident in the cyberspace invaders game
(Appendix A), where there should only be one object representing the defender.
Uses