Template Pattern

Template pattern is all about encapsulating algorithm or process. It defines a skeleton of an algorithm with some abstract methods, which allows subclasses to redefine certain steps with out changing the algorithm structure. These abstract methods allow subclasses to be a part of the algorithm so that the pre defined algorithm can be reused.

Why Template:

1. Encapsulates algorithm or process, So that it can be reused.
2. Allows sub classes to be a part of the process, so this pattern is often seen in different frameworks.

How to use Template:

It is a common scenario that we need to authenticate and authorize an user before letting him to execute some secured facility. The user only gets to call the method if he is valid. Here the process sequence is the same. First check if he is valid and then let him execute the method depending on validation. We can encapsulate the process and create "hook up" or "plug in" abstract methods. The client sub class can implement the abstract methods and become part of the process. Here what varies is the logic to verify the user and eventually the logic to execute. So we let the client sub classes to define that part by implementing the abstract methods.

Class Diagram :

Java Code :

AbstractPage.cs

public abstract class AbstractPage {

   private String userName;

   public void setUserName(String userName) {
       this.userName = userName;
   }

   public void run() {
       //a simple process for validation
       //which is encapsulated in the template class
       if (isValid(userName)) {
           execute();
       }
   }
   //abstract hook up methods which is defined in sub classes

   public abstract boolean isValid(String name);

   public abstract void execute();
}

Page.cs 

public class Page extends AbstractPage {

   public boolean isValid(String name) {       //my code to validate user is in here      
       return true;//just a dummy return  
   }

   public void execute() {
       //my code to execute is here  
   }
}

TestTemplate.cs 

class TestTemplate {

   private AbstractPage abstractPage;

   public TestTemplate(AbstractPage abstractPage) {
       this.abstractPage = abstractPage;
   }

   public void runProcess() {
       abstractPage.execute();
   }
}

Object Oriented Principals used in Template : 

1. The Hollywood principle(don't call us, we'll call you)
2. Code to interface or abstraction not to concrete class.
3. High level modules should not depend upon low level modules. Both should depend upon abstractions. Abstractions should not depend upon details. Details should depend upon abstractions.[Dependency Inversion Principal]

We see the Hollywood principle in this pattern because the client sub class is being used by the template class. The sub class is being called and becoming part of the process. And eventually the control belongs to the template class.

The template class is dependent on abstract methods which is later implemented by client sub classes. Template do not have any idea on concrete implementation, it is coded to abstraction. 

Dependency Inversion Principal is often used in deferent design patterns. In Template Pattern the high level class is the template class and the implementation is lower level class. Template class is not directly dependent on sub classes rather it is dependent on abstract methods or abstraction. And the client sub class is also dependent on the abstract template class. 

Observation : 

Object oriented pattern is all about encapsulation. Template is a trick to encapsulate process or algorithm, so that the process can be re used, which is achieved by inheritance. But one object oriented principal states that, "prefer composition over inheritance". In template we are using inheritance, which can be replaced by Strategy pattern. A template needs some feed or it needs to provide some hook up, which are achieved by abstract methods. These methods can be interfaced out of the abstract class of Template and convert inheritance in to composition. But it is always a design choice and context dependent decision.

Template is widely used in frameworks for it's ability to allow others to become a part of the system. Similar implementation is seen in Spring controllers, where the frameworks has the control and client controllers are plugged in the system by instantiating the abstract controllers. 

References : 

http://sourcemaking.com/design_patterns/template_method