This design pattern comes under behavioral design pattern
Definition
Define the skeleton of an algorithm in an
operation, deferring some steps to client subclasses. Template Method lets
subclasses redefine certain steps of an algorithm without changing the
algorithm's structure.
Basically base class defines the Placeholders,
skeleton of the operation and derived classes implement those Placeholders as necessary.
UML Diagram
Main components of the design pattern
AbstractClass - The base class
All the abstract primitive operations are
defined here
A template method which defines
skeleton/structure of the algorithm. This method calls the primitive operations
and other necessary methods which resides in the abstract class.
ConcreteClass – Sub class
Implements the abstract operations which
defined in the base abstract class as needed
Usage
Mostly using when creating frameworks. A
framework normally has multiple steps (Placeholders) and finally they all call by
a base method. Those steps can be overridden in different ways in different
scenarios. But that base method is common
to all scenarios and it should be a final method which is no one can override
it.
All reusable code is declared in the
framework's base classes, and then clients of the framework are free to do
customizations by creating derived classes as needed.
Example
Think, that we are going to implement a common
database access class which can be used to retrieve data from multiple database
servers like “SQL Server”, “MySql”, “Oracle” and Etc.
Abstract class = DataAccess
Template Method = RunQuery() -
Abstact Methods = OpenConnection(),
OpenTransaction(), ProcessQuery(),EndTransaction(),CloseConnection()
Concreate Classes = SQLDataAccess ,
MySQLDataAccess - All the above abstract
methods must implement on these concreate classes
DataAccess Class - Base Class
SQLDataAccess Class
MySQLDataAccess Class
When to implement template
method
A template pattern should be used when there is an algorithm
with many implementations but when we can find significant similarity among
them. Simply where a number of components/scenarios shared the
same process but the implementation was slightly different from each other.
Sometimes Template method referred as “The Hollywood principle”,
when we think from the superclass point of view simply it means “Don’t call us,
we will call you”. That means instead of
calling the methods from base class in the subclasses, the methods from the subclass
are called by the template method in base class. Hence the template method
implemented by the base class should not be overridden. So the Template method
in the base class should be a “Final“and non-abstract method.
