What is Template pattern?

The Template pattern is a behavioral design pattern that defines the skeleton or outline of an algorithm in a base class while allowing subclasses to provide specific implementations for certain steps or variations of the algorithm. It enables the reusability and extensibility of an algorithm while providing a consistent structure.

The Template pattern follows the “Hollywood Principle”: “Don’t call us, we’ll call you.” The base class or template defines the overall structure of the algorithm and controls the flow by defining abstract or hook methods. Concrete subclasses then provide the specific implementation for those methods.

Key components of the Template pattern:

  1. Abstract Template: The abstract template class provides the overall structure of the algorithm. It defines the steps or operations that make up the algorithm, including abstract or hook methods that the subclasses must implement.
  2. Concrete Template: Concrete templates are the subclasses that extend the abstract template and provide the implementation for the abstract or hook methods. They customize or specialize certain steps of the algorithm while keeping the overall structure intact.

Benefits and use cases of the Template pattern:

  1. Code reuse and consistency: The Template pattern promotes code reuse by providing a reusable structure for the algorithm. The base class encapsulates the common steps, and subclasses provide specific implementations, ensuring consistency in the overall algorithm.
  2. Flexibility and extensibility: The Template pattern allows the algorithm to be easily extended and customized by providing different implementations for specific steps. Subclasses can override or customize certain parts of the algorithm while inheriting the rest of the structure.
  3. Maintenance and code organization: The Template pattern improves code maintenance and organization. The overall algorithm is centralized in the base class, making it easier to understand and modify. Changes made in the base class automatically propagate to all subclasses, ensuring consistency across implementations.
  4. Enforcing constraints: The Template pattern allows certain steps or methods to be defined as abstract or final, enforcing constraints on the algorithm’s structure. This ensures that subclasses adhere to a specific order or perform certain operations, preventing deviations from the intended design.

Example of the Template pattern in Java:

// Abstract template class defining the algorithm structure
public abstract class AbstractClass {
    public void templateMethod() {
        // Step 1: Common implementation
        commonOperation();

        // Step 2: Abstract method to be implemented by subclasses
        specificOperation();

        // Step 3: Common implementation
        commonOperation();
    }

    protected void commonOperation() {
        // Common implementation
        System.out.println("Performing common operation");
    }

    protected abstract void specificOperation();
}

// Concrete subclasses providing specific implementations
public class ConcreteClass1 extends AbstractClass {
    protected void specificOperation() {
        // Specific implementation for ConcreteClass1
        System.out.println("Performing specific operation for ConcreteClass1");
    }
}

public class ConcreteClass2 extends AbstractClass {
    protected void specificOperation() {
        // Specific implementation for ConcreteClass2
        System.out.println("Performing specific operation for ConcreteClass2");
    }
}

// Usage:
AbstractClass class1 = new ConcreteClass1();
class1.templateMethod();

AbstractClass class2 = new ConcreteClass2();
class2.templateMethod();

In the above example, the AbstractClass represents the abstract template class. It defines the templateMethod(), which is the skeleton of the algorithm. It includes common steps (commonOperation()) and an abstract method (specificOperation()) that subclasses must implement.

The ConcreteClass1 and ConcreteClass2 are concrete subclasses that extend the AbstractClass and provide specific implementations for the specificOperation() method.

By calling the templateMethod() on different concrete subclasses, you can execute the algorithm while utilizing the customized behavior provided by each subclass.

The Template pattern provides a way to define an algorithm’s structure in a base class while allowing subclasses to provide specific implementations for certain steps. It promotes code reuse, consistency, and extensibility, making it easier to maintain and modify the overall algorithm.

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