Builder pattern is one pattern that belongs to the category
of Creational Pattern. It is used to create the object for the client. There are some situation where the object creation is to be done via multiple
steps. Builder pattern comes up for this
situations. With Builder pattern you
can have the fine-grained control over the some steps in the creation process.
In the below example to make a policy object it needs
several steps: intializePolicy, addProduct and calculateDiscount. After
these three steps one policy objects can be constructed completely. For different Policy classes you may be able
to differentiate the each step creation in the concrete policy maker
classes. Here NZPolicyMaker will make the NZPolicy
and AusPolicyMaker will make the AusPolicy. In each step you can decide what to do based
on the requirement for NZPolicy or AustPolicy.
So in short, Builder pattern applies to the situation where
one object creation is multiple predefined steps and you can have the control
over each steps in concrete Builder based on the requirement.
Template method pattern is another category of design
pattern. It belongs to Behavioural
pattern. This pattern is used to implement
one algorithm which is executed in multiple steps and the interface defines the
skeleton of the algorithm while the concrete class can implement the steps
which vary.
The below is one example of Template method pattern. The interface FleetCardHandler defines the
methods to be used to process the request message from the fleet card. But different fleet cards need the different
processing logic. These varying logic
are implemented in the concrete classes: SUCFleetCardHanlder and
TekFleetCardHandler.
From here we can see the similarity
between Builder pattern and Template method pattern. Both these patterns have the predefined steps
to perform one process(creation or algorithm)
and these steps can be implemented based on the varying business logic. Builder and Template Method pattern give you
the fine-grained control over the either creation or algorithm.
