Skip to main content

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 a 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 documents. 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 the 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 the 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 hooking 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 detailed 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 the Template Method allows a generic algorithm to manipulate many possible detailed implementations, the Strategy by fully conforming to Dependency Injection Principle (DIP) allows each detailed implementation to be manipulated by many different generic algorithms.

References:
[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
[3]. https://en.wikipedia.org/wiki/Template_method_pattern

Labels:

Comments

Post a Comment

Popular posts from this blog

How to apply Lean - Kanban for your business

This is the topic of Scrum Breakfast meetup this time, speaker: Ms. Phuong Bui - Technical Project Manager of YOOSE Pte. Ltd. http://www.meetup.com/Scrum-Breakfast-Vietnam-Agile-and-Scrum-Meetup/events/230313727/ Lean comes from Lean manufacturing is a method that focuses on elimination of wastes. In other words, this is a set of principles for archiving the quality, speed and customer alignment. The first time I knew about the term "Lean" is  from the book Software Craftsmanship . Sandro recommends if we want to transform our pet projects into a real business, we should get familiar with Lean Startup concepts. In this talk, Ms. Phuong pointed out some major wastes includes information (ex: unclear requirements), processes (ex: waiting), physical environment and people. Knowing what the problems should be the best way to eliminate them. The difference between  Single item flow and Batch processing is the second main point; and it is the Lean's idea. Batch pr...
Post a Comment
Read more

[Snippet] CSS - Child element overlap parent

I searched from somewhere and found that a lot of people says a basic concept for implementing this feature looks like below: HTML code: <div id="parent">  <div id="child">  </div> </div> And, CSS: #parent{   position: relative;   overflow:hidden; } #child{   position: absolute;   top: -1;   right: -1px; } However, I had a lot of grand-parents in my case and the above code didn't work. Therefore, I needed an alternative. I presumed that my app uses Boostrap and AngularJs, maybe some CSS from them affects mine. I didn't know exactly the problem, but I believed when all CSS is loaded into my browser, I could completely handle it. www.tom-collinson.com I tried to create an example to investigated this problem by Fiddle . Accidentally, I just changed: position: parent; to position: static; for one of parents -> the problem is solved. Look at my code: <div class="modal-body dn-placeholder-parent-positi...
Post a Comment
Read more

Improving the execution time of CI pipelines

Executing a large number of tests, especially integration tests, takes a lot of time. For instance, the pipeline of one of our projects for each Pull Request previously took nearly 30 minutes, including over 1 thousand test cases. This article guides you through several good techniques that we have discovered and applied to improve the time-consuming process. Parallel stages Analyze the current phases in your pipeline and categorize them in parallel. For example, we can separate the build and verify code of Node.js and Maven modules simultaneously in our Jenkins pipelines. Please mind using the setting failFast whether you want to abort the pipeline immediately. Read more: Parallel stages with Declarative Pipeline 1.2 (jenkins.io) Parallel test execution If you use Maven, t he plugin maven-failsafe-plugin is used to execute integration tests during phases integration-test and verify  the build lifecycle. It allows us to execute tests in parallel. There are many settings related ...
Post a Comment
Read more

Sharing a virtualenv across several Python projects using Pipenv

There is a standard library for all projects in Python. However, several projects don’t always have the same dependencies all the time. That is where virtual environments come to play. You can follow this official document to use two separated tools  virtualenv and pip to  fulfill that need. My preferred alternative is to use pipenv . Pipenv is easy to use and convenient. The following are my steps to make a shared virtualenv for my all projects which requires the same dependencies. Step 1. Create an isolated virtualenv. python -m venv my-shared-env Step 2. Create a symbolic link to the created virtualenv. cd project_1 ln -s ~/.local/share/virtualenvs/my-shared-env .venv I have encountered the following issue at step 1. FileNotFoundError: [Errno 2] No such file or directory: '{my_project_path}/.venv/bin/pip': '{my_project_path}/.venv/bin/pip' The root cause was I tried to create virtualenv by running pipenv install and renaming the generated virtualenv to ...
Post a Comment
Read more

The HelloWorld example of JSF 2.2 with Myfaces

I just did by myself create a very simple app "HelloWorld" of JSF 2.2 with a concrete implementation Myfaces that we can use it later on for our further JSF trying out. I attached the source code link at the end part. Just follow these steps below: 1. Create a Maven project in Eclipse (Kepler) with a simple Java web application archetype "maven-archetype-webapp". Maven should be the best choice for managing the dependencies , so far. JSF is a web framework that is the reason why I chose the mentioned archetype for my example. 2. Import dependencies for JSF implementation - Myfaces (v2.2.10) into file pom.xml . The following code that is easy to find from  http://mvnrepository.com/  with key words "myfaces". <dependency> <groupId>org.apache.myfaces.core</groupId> <artifactId>myfaces-api</artifactId> <version>2.2.10</version> </dependency> <dependency> <groupId>org.apache.myfaces.core<...
Post a Comment
Read more