Design Patterns Explained

28O Part V • Handling Variations with Design Patterns
... but the details
differ
Standardizing on the
steps
The details are
varying
4. Send the database the SELECT command.
5. Return the selected dataset.
The specific implementations of the databases differ, however,
requiring slightly different formatting procedures.
The Template Method Pattern
The Template Method is a pattern intended to help one abstract out a
common process from different procedures. According to the
Gang of Four, the intent of the Template method is to
Define the skeleton of an algorithm in an operation,
deferring some steps to subclasses. Redefine the steps in
an algorithm without changing the algorithm's structure. 1
In other words, although there are different methods for connecting
and querying Oracle databases and SQL Server databases, they share
the same conceptual process. The Template Method gives us a way to
capture this common ground in an abstract class while
encapsulating the differences in derived classes. The Template
Method pattern is about controlling a sequence common to different
processes.
Applying the Template Method
to the Case Study
In this case study, the variations in database access occur in the particular
implementations of the steps involved. Figure 18-1 illustrates
this.
1. Gamma, E., Helm, R., Johnson, R., Vlissides, J., Design Patterns: Elements of
Reusable Object-Oriented Software, Reading, Mass.: Addison-Wesley, 1995, p. 325.