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 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


Popular posts from this blog

AngularJS - Build a custom validation directive for using multiple emails in textarea

AngularJS already supports the built-in validation with text input with type email. Something simple likes the following:
<input name="input" ng-model="email.text" required="" type="email" /> <span class="error" ng-show="myForm.input.$"> Not valid email!</span>
However, I used a text area and I wanted to enter some email addresses that's saparated by a comma (,). I had a short research and it looked like AngualarJS has not supported this functionality so far. Therefore, I needed to build a custom directive that I could add my own validation functions. My validation was done only on client side, so I used the $validators object.

Note that, there is the $asyncValidators object which handles asynchronous validation, such as making an $http request to the backend.

This is just my implementation on my project. In order to understand that, I supposed you already had experiences with Angular…

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 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</groupId&g…

Only allow input number value with autoNumeric.js

autoNumeric is a jQuery plugin that automatically formats currency and numbers as you type on form inputs. I used autoNumeric 1.9.21 for demo code.

1. Dowload autoNumeric.js file from
2. Import to project
<script src=""></script> <script type="text/javascript" src="js/autoNumeric.js"></script> 3. Define a function to use it
<script type="text/javascript"> /* only number is accepted */ function txtNumberOnly_Mask() { var inputOrgNumber = $("#numberTxt"); inputOrgNumber.each(function() { $(this).autoNumeric({ aSep : '', aDec: '.', vMin : '0.00' }); }); } </script>
4. Call the function by event
<form> <input type="text" value="" id="numberTxt"/>(only number) </form> <script type="te…

Styling Sort Icons Using Font Awesome for Primefaces' Data Table

So far, Primefaces has used image sprites for displaying the sort icons. This leads to a problem if we want to make a different style for these icons; for example, I would make the icon "arrow up" more blurry at the first time the table loading because I want to highlight the icon "arrow down". I found a way that I can replace these icons with Font Awesome icons.

We will use "CSS Pseudo-classes" to achieve it. The hardest thing here is that we should handle displaying icons in different cases. There is a case both "arrow up" and "arrow down" showing and other case is only one of these icons is shown.

.ui-sortable-column-icon.ui-icon.ui-icon-carat-2-n-s { background-image: none; margin-left: 5px; font-size: 1.1666em; position: relative; } .ui-sortable-column-icon.ui-icon.ui-icon-carat-2-n-s:not(.ui-icon-triangle-1-s)::before { content: "\f106"; font-family: "FontAwesome"; position: absolut…

Attribute 'for' of label component with id xxxx is not defined

I got the warning in the log file when I have used the tag <h:outputLabel> without attribute "for" in xhtml file. It was really polluting my server log files.

The logged information actually makes sense anyway! We could find an answer as the following:

"Having h:outputLabel without a "for" attribute is meaningless. If you are not attaching the label, you should be using h:outputText instead of h:outputLabel."

However, these solutions are not possible just for my situation. Instead of using h:outputText for only displaying text, my team has used h:outputLabel too many places. We were nearly in our release time (next day) so it is quite risky and takes much efforts if we try to correct it. Because the style (with CSS) is already done with h:ouputLabel. The alternative by adding attribute "for" the existing h:outputLabel is not reasonable either. I really need to find another solution.
Fortunately, I came across a way if I change to use p:out…