The command pattern can be used to share common implementations of operations. This class provides an abstract implementation for common operations on books that other commands can share. Figure 15-3 shows the class diagram for a number of the use-cases.
This class implements a number of convenience methods that can be used to do the following:
Query generic fields by passing “field” and “field value,”
Return found Book instance as a subclass.
Ensure that only a single instance result is returned; else an error is thrown.
Copies of a book can be found by “back pointer” or join ...