Template design pattern

• it is a design pattern that defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure

Problem

• when creating a data mining app that analyzes corporate documents
  • user feed the app documents in various formats (pdf, doc, csv)
  • and tries to extract meaningful data from these docs in a uniform format
• 1st version of the app could only work only with DOC files
  • next version able to support CSV files
  • later it can extract PDF files
• at some point, you realized that all 3 classes have a lot of similar code
  • while the code for dealing with various data formats was entirely different in all classes
    • the code for data processing and analysis is almost identical
    • would be great to get rid of code duplication
      • and leave the algorithm structure intact
• there is another problem related to client code that used these classes
  • it had lots of conditionals that picked a proper course of action depeneding on the class of the processing object
  • if all 3 processing classes had a common interface or a base class
    • one can eliminate the conditionals in client code
    • and use polymorphism when calling methods on a processing object

Solution

• the pattern suggest that you break down an algorithm into a series of steps
  • turn these steps into methods
  • and put a series of calls to these methods inside a single template method
  • the steps may either be abstract or have some default implementation
  • to use the algorithm, the client is supposed to provide its own subclass
    • implement all abstract steps and override some of the optional ones if needed but not the template method itself
• a base class can be created for all algorithms
  • the class defines a template method consisting of a series of calls to various document processing steps
• at first, we can declare all steps abstract
  • forcing the subclasses to provide their own implementations for these methods
  • for this case, subclasses already have all necessary implementations
    • the only thing needed is to adjust signatures of the methods to match the methods of the superclass
• to get rid of duplicate code
  • the code for opening/closing files and extracting/parsing data is different for various data formats
    • no point touching these methods
  • however, implementation of other steps such as analyzing raw data & composing reports is similar
  • so it can pulled up into the base class where subclasses can share that code
• there are 2 types of steps
  • abstract steps must be implemented by every subclass
  • optional steps already have some default implementation, but still can be overridden if needed
• there is another type of step, called hooks
  • it is an optional step with an empty body
  • a template method would work even if a hook isn't overridden
  • usually hooks are placed before and after crucial steps of algorithms
    • provding subclasses with additional extension points for an algorithm

Analogy

• the template method approach can be used in mass housing construction
  • the architectural plan for building a standard house may contain several extension points that would let a potential owner adjust some details of the resulting house
• each building step such as laying the foundation, framing, building walls, installing plumbing and wiring for water and electricity, can be slightly changed to make the resulting house a little bit different from others

Structure

1. The Abstract Class declares methods that act as steps of an algorithm, as well as the actual template method which calls these methods in a specific order.

   • The steps may either be declared abstract or have some default implementation.

2. Concrete Classes can override all of the steps, but not the template method itself.

How to use

• Use the Template Method pattern when you want to let clients extend only particular steps of an algorithm, but not the whole algorithm or its structure
  • The Template Method lets you turn a monolithic algorithm into a series of individual steps which can be easily extended by subclasses while keeping intact the structure defined in a superclass
• Use the pattern when you have several classes that contain almost identical algorithms with some minor differences
  • As a result, you might need to modify all classes when the algorithm changes
  • When you turn such an algorithm into a template method, you can also pull up the steps with similar implementations into a superclass, eliminating code duplication
  • Code that varies between subclasses can remain in subclasses

How to implement

1. Analyze the target algorithm to see whether you can break it into steaps
   • consider which steps are common to all subclasses and which ones will always be unique
2. create the abstract base class and declare the template method and a set of abstract methods representing the algorithm's steps
   • outline the algorithm's structure in the template method by executing corresponding steps
   • consider making the template method final to prevent final to prevent subclasses from overriding it
3. it is okay if all steps end by being abstract
   • however, some steps might benefit from having a default implementation
   • subclasses don't have to implement those methods
4. think of adding hooks between the crucial steps of the algorithm
5. for each variation of the algorithm, create a new concrete subclass
   • it must implement all of the abstract steps, but may also override some of the optional ones

Pros and Cons

Pros

• can let clients override only certain parts of a large algorithm
  • making them less affected by changes that happen to other parts of the algorithm
• can pull the duplicate code into a superclass

Cons

• some clients may be limited by the provided skeleton of an algorithm
• you might violate the Liskov Substitution Principle by suppressing a default step implementation via a subclass
• template methods tend to be harder to maintain the more steps they have