Welcome to our blog series on software design patterns! In this instalment, we will delve into the world of the Template Design Pattern and explore its various applications in software development.

The Template Design Pattern is classified as a behavioral design pattern, focusing on defining the skeleton of an algorithm while allowing subclasses to provide specific implementations of certain steps. It enables developers to create a blueprint for an algorithm or process and allows for customization without modifying the overall structure.

The Template Design Pattern stands out for its ability to simplify the development process and promote code reusability.

Real World Analogy of Template Pattern

Let’s consider another real-world analogy for the Template Method pattern: building a house.

When constructing a house, there are several sequential steps that need to be followed, such as laying the foundation, erecting the walls, installing the plumbing and electrical systems, and finishing the interior. However, the specific materials, techniques, and design choices may vary depending on the type of house being built, such as a modern-style home versus a traditional-style home.

In this analogy, the Template Method pattern can be applied to the construction process.

The overall process of building a house represents the template or the high-level algorithm. It consists of a series of defined steps that are common to all house construction projects, such as site preparation, framing, installation, and finishing.

However, the specific implementation of each step can differ based on the style, design, and specific requirements of the house. For example, while constructing the walls, a modern-style house may use steel and glass materials, while a traditional-style house may use wooden beams and bricks. These variations in individual steps represent the customizable parts of the algorithm.

By utilizing the Template Method pattern, we can create a base “HouseBuilder” class that defines the overall construction algorithm and provides default implementations for each step. The template method, representing the overall process, can be called “buildHouse()” in this case. The individual steps, such as “layFoundation()”, “erectWalls()”, and “finishInterior()”, are defined as separate methods within the template.

Subclasses can then inherit from the base class and override specific methods to provide their own implementations. For example, a subclass representing the construction of a modern-style house can override the “erectWalls()” method to use steel and glass materials, while a subclass representing a traditional-style house can override the same method to use wooden beams and bricks.

By applying the Template Method pattern, we can achieve code reusability, avoid duplication, and maintain a structured approach to building houses with variations in design and materials. The template method provides a consistent framework for the overall construction process, while allowing customization in specific steps to accommodate different house styles.

Applicability of Template Design Pattern

The Template Method pattern should be used

• To implement the invariant parts of an algorithm once and leave it up to subclasses to implement the behavior that can vary
• When common behavior among subclasses should be factored and localized in a common class to avoid code duplication. This is a good example of “refactoring to generalize” . You first identify the differences in the existing code and then separate the differences into new operations. Finally, you replace the differing code with a template method that calls one of these new operations
• To control subclasses extensions. You can define a template method that calls “hook” operations at specific points, thereby permitting extensions only at those points

Applications of Template Design Pattern

1. Frameworks and Libraries: Many software frameworks and libraries utilize the Template Method pattern to define the overall structure of an application or module while allowing customization in specific steps.
   For example, web development frameworks often provide template methods for handling request processing, authentication, and database interactions, enabling developers to build applications with consistent patterns.
2. Data Access and Object-Relational Mapping (ORM): In data access layers and ORM frameworks, the Template Method pattern is commonly used to provide a standardized way of interacting with databases. The base class defines the overall database access algorithm, while subclasses can override specific methods to handle different database operations, such as querying, inserting, or updating data.
3. Test Automation: The Template Method pattern is useful in test automation frameworks. A base test class can define the overall test structure, including setup, execution, and teardown steps. Subclasses can then override specific methods to provide test-specific implementations while reusing the common test flow.
4. Code Generation: Template Method pattern finds application in code generation tools or code scaffolding frameworks. The base template defines the overall structure of the generated code, while subclasses can provide specific implementations for different code components or functionalities.
5. Document Processing: Document processing frameworks often use the Template Method pattern to handle common operations such as parsing, formatting, and generating documents. The base template provides a structure for processing documents, while subclasses can specialize in handling different document formats or transformations.
6. Game Development: Game development engines and frameworks can benefit from the Template Method pattern to define the core game loop and gameplay mechanics. The base template establishes the overall game structure, while subclasses can override specific methods to implement game-specific logic, such as rendering, input handling, and collision detection.
7. Workflow and Business Process Management: Workflow engines and business process management systems often employ the Template Method pattern to define the sequence of steps and actions in a workflow or business process. The base template outlines the overall flow, while subclasses can customize specific steps based on the business rules and requirements.

Template Design Pattern Implementation

To implement the Template Design Pattern, you can follow these steps:

• Identify the algorithm or process that you want to implement with variations in certain steps.
• Create an abstract base class (or interface) that defines the overall algorithm and declares methods representing the individual steps. These methods can be implemented or left abstract, depending on whether they have a default behavior or require subclass implementation.
• Implement the common functionality of the algorithm in the base class, using template methods to define the sequence of steps. These template methods can call the individual step methods as needed.
• Identify the parts of the algorithm that need customization and mark them as abstract methods in the base class. These abstract methods will be implemented by subclasses to provide the specific behavior for each step.

// Abstract class defining the template
abstract class AbstractAlgorithm {
    
    // Template method defining the overall algorithm
    public void executeAlgorithm() {
        stepOne();
        stepTwo();
        stepThree();
    }
    
    // Abstract methods representing individual steps to be implemented by subclasses
    protected abstract void stepOne();
    protected abstract void stepTwo();
    
    // Default implementation of step three
    protected void stepThree() {
        System.out.println("Executing default Step Three");
    }
}

• Create concrete subclasses that inherit from the base class. Override the abstract methods in the subclasses to provide the customized implementations for the specific steps.

// Concrete subclass implementing the template
class ConcreteAlgorithm extends AbstractAlgorithm {
    
    // Implementation of step one
    protected void stepOne() {
        System.out.println("Executing Step One");
    }
    
    // Implementation of step two
    protected void stepTwo() {
        System.out.println("Executing Step Two");
    }
    
    // Overriding default implementation of step three
    protected void stepThree() {
        System.out.println("Executing Custom Step Three");
    }
}

• Clients of your code will interact with the concrete subclasses, using the base class (or interface) as the type. This allows for flexibility in choosing different implementations while adhering to the common structure defined by the base class.
• Clients can invoke the template method on the base class, which will internally call the individual step methods, including the overridden methods in the subclasses.

// Client code
public class Main {
    public static void main(String[] args) {
        AbstractAlgorithm algorithm = new ConcreteAlgorithm();
        algorithm.executeAlgorithm();
    }
}

Example

// Abstract class defining the template
abstract class HouseBuilder {
    
    // Template method defining the overall house construction algorithm
    public final void buildHouse() {
        constructFoundation();
        constructWalls();
        constructRoof();
        decorateHouse();
    }
    
    // Abstract methods representing individual construction steps
    protected abstract void constructFoundation();
    protected abstract void constructWalls();
    protected abstract void constructRoof();
    
    // Hook method for optional step, can be overridden in subclasses
    protected void decorateHouse() {
        System.out.println("Default decoration of the house");
    }
}

// Concrete subclass implementing the house construction template
class ConcreteHouseBuilder extends HouseBuilder {
    
    protected void constructFoundation() {
        System.out.println("Constructing foundation of the house");
    }
    
    protected void constructWalls() {
        System.out.println("Constructing walls of the house");
    }
    
    protected void constructRoof() {
        System.out.println("Constructing roof of the house");
    }
    
    protected void decorateHouse() {
        System.out.println("Adding custom decoration to the house");
    }
}

// Client code
public class Main {
    public static void main(String[] args) {
        HouseBuilder builder = new ConcreteHouseBuilder();
        builder.buildHouse();
    }
}

In this example, we have an abstract class HouseBuilder that defines the template method buildHouse(). The buildHouse() method calls the individual construction steps: constructFoundation(), constructWalls(), constructRoof(), and the optional step decorateHouse().

The constructFoundation(), constructWalls(), and constructRoof() methods are declared as abstract in the abstract class, which means they must be implemented by concrete subclasses. In this case, the ConcreteHouseBuilder class extends the HouseBuilder and provides its own implementations for these methods.

The decorateHouse() method is a hook method in the abstract class, which provides a default implementation but can be overridden in the concrete subclass. In the ConcreteHouseBuilder class, we override the decorateHouse() method to provide custom decoration for the house.

Pros & Cons of Template Design Pattern

The keyword in all of the “cons” above was “may”, “can” or “could”, if the best practices are not followed.

Let’s looks at some best practices & design consideration to be followed while implementing template Design Pattern.

Best Practices & Design Consideration

When implementing the Template Design Pattern, it’s important to consider the following best practices and design considerations:

1. Identify the Common Algorithm: Clearly identify the parts of the algorithm that remain consistent across different implementations. These common steps should be defined in the base class.
2. Define Template Methods: Declare template methods in the base class to represent the individual steps of the algorithm. These methods can be either abstract, allowing subclasses to provide their own implementations, or they can have default behavior in the base class.
3. Use Hook Methods: Incorporate hook methods in the base class to provide optional steps or customization points. Hook methods have a default implementation but can be overridden in subclasses to allow for additional flexibility.
4. Follow the Open-Closed Principle: Design the base class and subclasses in a way that allows for easy extension without modifying the existing code. New variations or steps should be added by extending the base class or creating new subclasses.
5. Avoid Code Duplication: Extract and reuse common code within the base class whenever possible. Eliminate redundancy by centralizing shared functionality.
6. Consider Extensibility: Plan for potential future modifications or additions to the algorithm. Design the base class and subclasses with extensibility in mind, making it easier to incorporate new variations or customization points.
7. Understand Performance Implications: Analyze the performance impact of using the Template Design Pattern, especially in cases where there are many variations and subclasses. Evaluate whether the pattern introduces any overhead that may impact the overall system performance.
8. Provide Clear Documentation: Clearly document the purpose and usage of the template methods and hook methods in the base class to guide developers who will implement the subclasses.
9. Follow Naming Conventions: Use meaningful and descriptive names for template methods, hook methods, and subclasses to enhance code readability and maintainability.
10. Test Thoroughly: Ensure comprehensive testing of the base class and subclasses to verify the correctness and proper interaction of the template methods and their implementations.

Relation with other Patterns

1. Strategy Pattern: The Template Method pattern and the Strategy pattern share a similar motivation but differ in their implementation. Both patterns aim to encapsulate algorithms, but while the Template Method uses inheritance and a fixed structure, the Strategy pattern uses composition and allows for interchangeable algorithms at runtime.
2. Factory Method Pattern: The Template Method pattern can be combined with the Factory Method pattern. The Factory Method can be used to create instances of subclasses that implement the varying steps in the template. This combination allows for dynamic creation of objects with customized behavior while adhering to the defined template structure.
3. Composite Pattern: The Template Method pattern is often used in conjunction with the Composite pattern. The Composite pattern allows objects to be composed into tree-like structures. The Template Method pattern can be used to define a common algorithm in a base component class, with the algorithm applied recursively to each component in the composite structure.
4. Hook Method: The Template Method pattern uses hook methods to provide optional steps or customization points in the algorithm. This concept is similar to the “hook” concept used in other patterns like the Template Method pattern itself or the Observer pattern, where subclasses or observers can choose to override or extend certain methods to customize behavior.
5. Decorator Pattern: The Template Method pattern can be used in conjunction with the Decorator pattern. The Template Method defines the overall structure of the algorithm, while the Decorator pattern allows for the dynamic addition of behavior to an object. The Template Method can serve as a base for different decorators, each providing a different customization to the algorithm’s steps.
6. Bridge Pattern: The Template Method pattern can be combined with the Bridge pattern to decouple the abstraction from its implementation. The Template Method defines the high-level algorithm structure, while the Bridge pattern allows different implementations of the steps to be selected at runtime.