3.9 - Template Method Pattern : Behavioral Design Patterns

The Template Method Pattern is a behavioral design pattern that defines the skeleton or framework of an algorithm in a method, called the template method. While the main structure of the algorithm is defined in the superclass, specific steps of the algorithm are left to be implemented by subclasses. This allows subclasses to override or extend certain parts of the algorithm without changing its overall structure.

The main idea behind this pattern is to break down complex algorithms into steps, some of which are fixed in the base class, while others are flexible and open for customization by subclasses.

Key Components of the Template Method Pattern

1. Abstract Class (Base Class): The abstract class contains the template method, which defines the order of execution for the steps of an algorithm. Some of these steps may be abstract, requiring subclasses to provide their specific implementations.

2. Concrete Class (Derived Class): The concrete class inherits from the abstract class and provides specific implementations for the abstract methods that the base class defines. It determines how certain steps are executed.

3. Template Method: This method contains the fixed steps of the algorithm, but it allows subclasses to define the implementation of some steps by overriding specific methods.

Example of Template Method Pattern

Let's look at an example to understand how the Template Method Pattern works in practice.

1. Abstract Class Defining the Template

In this example, we have an abstract class BaseClass that defines the structure of the algorithm. The method executeSteps() is the template method that defines the sequence of operations (operationA(), operationB(), and operationC()).

// Abstract class defining the template
abstract class BaseClass {

    // Template method defining the sequence of operations
    public void executeSteps() {
        operationA();
        operationB();
        operationC();
    }

    // Abstract methods to be implemented by subclasses
    protected abstract void operationA();
    protected abstract void operationB();
    protected abstract void operationC();
}

Here, the executeSteps() method defines the order in which operations will be executed. However, it doesn't specify the details of the operations themselves. These details are left to be defined by subclasses.

2. Concrete Class Implementing the Template

Next, we have a SpecificClass that extends BaseClass and provides concrete implementations for operationA(), operationB(), and operationC().

// Concrete class implementing the template
class SpecificClass extends BaseClass {

    @Override
    protected void operationA() {
        System.out.println("Executing Operation A");
    }

    @Override
    protected void operationB() {
        System.out.println("Executing Operation B");
    }

    @Override
    protected void operationC() {
        System.out.println("Executing Operation C");
    }
}

In this class, operationA(), operationB(), and operationC() are defined with specific behaviors. Whenever the template method executeSteps() is called, it will execute the operations in the defined sequence.

3. Main Class to Run the Template Method

Finally, we have the Main class that demonstrates how the template method is used.

// Main class to run the template method
public class Main {
    public static void main(String[] args) {
        BaseClass process = new SpecificClass();
        process.executeSteps();  // Executes the steps in the predefined order
    }
}

When process.executeSteps() is called, it will trigger the execution of operationA(), operationB(), and operationC() in the order defined by the template method in BaseClass.

Output of the Program:

Executing Operation A
Executing Operation B
Executing Operation C

Key Concepts of the Template Method Pattern

1. Predefined Algorithm Flow: The base class defines the structure and flow of the algorithm. This ensures that the algorithm remains consistent across different implementations.

2. Flexibility in Subclasses: The details of specific steps in the algorithm are left to be implemented by subclasses. This allows for flexibility and customization without altering the overall flow.

3. Code Reusability: By placing the common structure in the base class, the template method pattern promotes code reuse. Subclasses only need to implement specific variations of certain steps.

Benefits of the Template Method Pattern

1. Ensures Algorithm Consistency: The template method guarantees that the steps of the algorithm are executed in a specific order. Subclasses can change specific behaviors without altering the sequence of execution.

2. Promotes Code Reuse: Common parts of the algorithm are defined in the base class, while variations can be handled in subclasses. This prevents code duplication.

3. Easier Maintenance: The core logic is centralized in the base class, making it easier to maintain. Any changes to the overall structure of the algorithm are made in the base class, while changes to specific steps can be handled in subclasses.

4. Flexibility and Extensibility: Subclasses can extend the base algorithm by overriding specific methods, allowing for tailored behavior without modifying the entire process.

Drawbacks of the Template Method Pattern

1. Inheritance-Based: The pattern heavily relies on inheritance, which can limit flexibility when dealing with complex object hierarchies. Using inheritance might lead to tight coupling between base and derived classes.

2. Overridden Methods: If not properly documented, the flow of the algorithm can be harder to follow, as the method behavior can vary depending on which subclass is instantiated.

3. Rigid Structure: While the pattern ensures consistency in the algorithm's flow, it might become too rigid for scenarios where greater customization or more flexible control over the algorithm is needed.

Real-World Use Cases of the Template Method Pattern

1. Framework Design: In software frameworks, the template method pattern is widely used to provide hooks for customizing behavior without altering the overall workflow of the system. For example, GUI frameworks often define template methods for rendering windows, allowing subclasses to define specific behaviors like drawing custom components.

2. Game Development: In game development, game engines might use the template method pattern for defining the game loop. The engine might have predefined steps such as updating physics, rendering, and handling input, but specific game behaviors (like enemy AI or player control) can be customized by overriding certain methods.

3. Document Generation: In scenarios like generating documents or reports, the template method pattern can define a general process (such as opening, writing, and closing the document). Specific document types (like PDFs or Excel files) can be customized by subclassing and overriding methods for formatting and content.

Conclusion

The Template Method Pattern is a powerful tool for defining the structure of an algorithm while allowing subclasses to implement specific behaviors. By enforcing a consistent order of execution for the algorithm’s steps, the pattern promotes code reuse and flexibility. At the same time, it keeps the codebase more maintainable by separating the structure of an algorithm from its concrete implementation details. Whether it's used in frameworks, game development, or document generation, this pattern provides a structured yet flexible way to manage complex algorithms in software design.