template-pattern

🧠 Concept

The Template Method Pattern defines the skeleton of a process in a base class but lets subclasses override certain steps without changing the overall algorithm structure.

👉 The base class provides a template method that outlines the report generation steps. Subclasses fill in the specific details (e.g., data source, format).

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🧱 Key Roles

Role	Description
AbstractClass	Defines the template method and abstract steps.
ConcreteClass	Implements specific details for each report type.

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💻 Java Example – Report Generator

// Abstract class (template)
abstract class ReportGenerator {
    // Template method (final – cannot be overridden)
    public final void generateReport() {
        fetchData();
        processData();
        formatReport();
        exportReport();
    }

    // Abstract methods for subclasses to implement
    abstract void fetchData();
    abstract void processData();
    abstract void formatReport();

    // Common method shared by all
    void exportReport() {
        System.out.println("Exporting report to PDF...");
    }
}

// Concrete implementation 1
class SalesReportGenerator extends ReportGenerator {
    void fetchData() { System.out.println("Fetching sales data from database..."); }
    void processData() { System.out.println("Calculating total sales and growth rates..."); }
    void formatReport() { System.out.println("Formatting sales data into summary tables..."); }
}

// Concrete implementation 2
class EmployeeReportGenerator extends ReportGenerator {
    void fetchData() { System.out.println("Retrieving employee records..."); }
    void processData() { System.out.println("Analyzing performance scores..."); }
    void formatReport() { System.out.println("Creating charts and ranking tables..."); }
}

✅ Usage Example:

ReportGenerator salesReport = new SalesReportGenerator();
salesReport.generateReport();

/*
Fetching sales data from database...
Calculating total sales and growth rates...
Formatting sales data into summary tables...
Exporting report to PDF...
*/

ReportGenerator employeeReport = new EmployeeReportGenerator();
employeeReport.generateReport();

/*
Retrieving employee records...
Analyzing performance scores...
Creating charts and ranking tables...
Exporting report to PDF...
*/

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🧠 Flow Summary (Step-by-Step)

Step	Action	Base Template Method	Subclass Override	Example Output
1️⃣	fetchData()	—	Custom per report	“Fetching sales data from database...”
2️⃣	processData()	—	Custom per report	“Calculating total sales…”
3️⃣	formatReport()	—	Custom per report	“Formatting data into summary tables…”
4️⃣	exportReport()	Common (base class)	Shared by all	“Exporting report to PDF…”

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🪜 Summary

• Base class defines the overall workflow (generateReport()).

• Subclasses fill in details for individual steps.

• Encourages code reuse and consistent process structure.

• Follows the Hollywood Principle — “Don’t call us, we’ll call you.” (Base class calls subclass methods.)

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📊 Real-world Analogy

🧾 Report Templates in BI Tools — Every report follows the same process: Fetch → Process → Format → Export, but each report type defines its own implementation for each step.