Design Patterns Explained

286 Part V • Handling Variations with Design Patterns
Template Method
using Factory
Method
Figure 19-1 The Template Method (doQuery) using the Factory
Method pattern (makeDB).
In Figure 19-1, the doQuery method in the Template Method is
using makeDB to instantiate the appropriate database object.
Query-Template does not know which database object to instantiate;
it only knows that one must be instantiated and provides an interface
for its instantiation. The derived classes from Query-Template will
be responsible for knowing which ones to instantiate. Therefore, at
this level, I can defer the decisions on how to instantiate the database
to a method in the derived class.
Since there is a method involved in making an object, this approach is
called a Factory Method.
Public or protected methods?
Note that the makeDB methods are protected (as indicated by the #
signs). In this case, only the QueryTemplate class and its derivations
can access these methods. If I want objects other than
QueryTemplate to be able to access these methods, then they
should be public. This is another, quite common, way to use the
Factory Method. In this case I still have a derived class making
the decision as to which object to instantiate.