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Template Method Pattern: Don't Call Us, We'll Call You!

So far, the Template Method has been my most used design pattern. That is the reason why this post is quite long. J

Definition from Wiki

The Template Method defines the program skeleton of an algorithm in an method, deferring some steps to subclasses. It lets one redefine certain steps of an algorithm without changing the algorithm's structure.

Template Method UML.svg

A Real World Use Case

Imagine that you have many different kinds of document. You want to generate a pdf file from a corresponding word template. Each type has its own small modifications but the main process for document generating is the same.

We apply Template Method for this case. We define a final method including some steps (such as preparing for content, generating file) at a superclass. There are three possibilities for these steps at subclasses:
  1. Must be overridden: abstract methods.
  2. Not mandatory to be overridden: default protected methods.
  3. Can not be overridden: default private methods.

Dissecting the Pattern

Simply, we can see the following attributes:
  • Superclass has a "template" method which should be final to prevent subclasses from reworking the algorithm.
  • The template method defines the sequence of steps, each represented by a method.
  • Some methods are handled by superclass.
  • Some methods are handled by subclasses.

Benefit? Why?

  • Reuse, avoiding duplication and subclasses just need to implement a couple of methods.
  • The algorithm lives in one place and code changes only need to be made there.

Hooked on Template Method

A hook is a method that is declared in the abstract class, but only given an empty or default implementation. This gives subclasses the ability to "hook into" the algorithm at various points, if they wish; a subclass is also free to ignore the hook.

The Hollywood Principle

We allow low-level components to hook themselves into a system, but high-level components determine when they are needed, and how. In other words, the high-level components give the low-level components a "don't call us, we'll call you" treatment.

Template Method & Strategy: Inheritance vs. Delegation

Template Method and Strategy both solve the problem of separating a generic algorithm from a detail context. They can often be used interchangeably. However, Template Method uses inheritance to solve the problem, whereas Strategy uses composition/delegation.

Inheritance was a highlight feature in the early days of OO. That is, given some class that did something almost useful to us, we could create a subclass and change only the bits we didn't like. However, inheritance was very easy to overuse; that is a reason why we have heard of a lot of "Favor object composition over class inheritance". That is, inheritance is a very strong relationship. Derivatives are inextricably bound to their base classes. Anyway, that doesn't mean we should avoid using inheritance.

The Strategy provides one extra benefit over the Template Method. Whereas Template Method allows a generic algorithm to manipulate many possible detailed implementation, the Strategy by fully conforming to Dependency Injection Principle (DIP) allows each detailed implementation to be manipulated by many different generic algorithms.
[1]. Bert Bates, Kathy Sierra, Eric Freeman and Elisabeth Robson, Head First Design Patterns, A Brain-Friendly Guide
[2]. Robert C. Martin, Agile Software Development, Principles, Patterns, and Practices


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