The Template Design Pattern is a behavioral pattern that defines the skeleton of an algorithm in a base class but allows subclasses to override specific steps of the algorithm without changing its structure. This pattern is useful when you have a common sequence of steps that are shared among multiple subclasses but may vary in certain details.

Components of the Template Design Pattern:

  1. Abstract Class (Template): Defines the template method that outlines the algorithm’s structure and includes some default behavior. It may also include abstract methods that must be implemented by subclasses.
  2. Concrete Class: Implements the abstract methods defined in the abstract class and may override some steps of the algorithm.

Simple Example:

Let’s say we want to create a system for preparing beverages. The process of preparing a beverage involves several common steps, but the details may vary for different types of beverages.

1. Abstract Class (Template):

// Abstract class defining the template method
abstract class Beverage {
    // Template method
    final void prepareRecipe() {
        boilWater();
        brew();
        pourInCup();
        addCondiments();
    }

    abstract void brew();
    abstract void addCondiments();

    void boilWater() {
        System.out.println("Boiling water");
    }

    void pourInCup() {
        System.out.println("Pouring in cup");
    }
}

2. Concrete Classes:

// Concrete class for Tea
class Tea extends Beverage {
    @Override
    void brew() {
        System.out.println("Steeping the tea");
    }

    @Override
    void addCondiments() {
        System.out.println("Adding lemon");
    }
}

// Concrete class for Coffee
class Coffee extends Beverage {
    @Override
    void brew() {
        System.out.println("Dripping coffee through filter");
    }

    @Override
    void addCondiments() {
        System.out.println("Adding sugar and milk");
    }
}

3. Client Code:

public class TemplatePatternDemo {
    public static void main(String[] args) {
        Beverage tea = new Tea();
        Beverage coffee = new Coffee();

        System.out.println("Preparing tea...");
        tea.prepareRecipe();

        System.out.println("\nPreparing coffee...");
        coffee.prepareRecipe();
    }
}

Explanation:

  1. Abstract Class (Template): Beverage defines the prepareRecipe method, which is a template method outlining the steps to prepare a beverage. It includes some default behavior (e.g., boilWater, pourInCup) and abstract methods (brew, addCondiments) that subclasses need to implement.
  2. Concrete Classes: Tea and Coffee extend the Beverage class and provide specific implementations for the brew and addCondiments methods.
  3. Client Code: Demonstrates how to use the template method to prepare different types of beverages using the common algorithm defined in the abstract class.

Benefits of the Template Design Pattern:

  • Code Reusability: Common code is defined in the base class, reducing duplication and ensuring consistency.
  • Flexibility: Subclasses can provide specific implementations for certain steps of the algorithm, allowing customization while preserving the overall structure.
  • Control Over Algorithm: The base class controls the algorithm’s structure, ensuring that all necessary steps are executed in the correct order.

Use Cases of the Template Design Pattern:

  • Frameworks: When creating a framework, you can use the template pattern to define a set of steps that the framework will follow, allowing users to customize specific steps.
  • Workflow Systems: Systems that follow a common workflow but allow for varying details in different scenarios (e.g., data processing pipelines, report generation).
  • Algorithmic Processes: When implementing algorithms with a fixed structure but varying implementations (e.g., game AI behaviors, data parsing).