Template Method Pattern

Template Method Pattern

The Template Method Pattern is one of the most fundamental behavioral design patterns that defines the skeleton of an algorithm in a base class, letting subclasses override specific steps without changing the algorithm’s structure. Understanding the Template Method Pattern is essential for building flexible, maintainable software systems.

Why Template Method Pattern?

Imagine you’re making different types of beverages - tea and coffee. The process is similar: boil water, brew, pour, add condiments. But “brew” means steeping tea leaves for tea, and filtering coffee for coffee. The Template Method Pattern lets you define this common process once, with specific steps filled in by subclasses!

The Template Method Pattern defines the skeleton of an algorithm in a base class method, deferring some steps to subclasses. It lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.

Simple Analogy

Think of a recipe template:

• Base recipe: Preheat oven → Prepare ingredients → Mix → Bake → Serve
• Cake recipe: Fills in specific steps (mix flour, sugar, eggs)
• Bread recipe: Fills in different steps (mix flour, yeast, water)

Same skeleton, different implementations!

What’s the Use of Template Method Pattern?

The Template Method Pattern is useful when:

1. You have a common algorithm structure - Same steps, different details
2. You want to avoid code duplication - Common code in base class
3. You need to control extension points - Subclasses can only customize certain steps
4. You want to enforce algorithm structure - Subclasses can’t change the order
5. You have invariant steps - Some steps must always happen

What Happens If We Don’t Use Template Method Pattern?

Without the Template Method Pattern, you might:

• Duplicate code - Same algorithm structure in multiple places
• Inconsistent implementations - Different classes implement differently
• Hard to maintain - Changes needed in multiple places
• No control over extensions - Subclasses can change everything
• Violate DRY - Don’t Repeat Yourself principle violated

Common Problem

Without Template Method Pattern, if you have 5 classes with similar algorithms, fixing a bug in the common part means changing 5 places. With Template Method, you fix it once in the base class!

---

Simple Example: The Beverage Maker

Let’s start with a super simple example that anyone can understand!

Visual Representation

Algorithm Flow

Here’s how the template method works - same structure, different implementations:

sequenceDiagram
    participant Client
    participant Tea
    participant Coffee
    participant BaseClass as Beverage (Base)
    
    Client->>Tea: prepare_recipe()
    activate Tea
    
    Tea->>BaseClass: boil_water()
    BaseClass-->>Tea: Water boiled
    
    Tea->>Tea: brew() [steep tea]
    Note right of Tea: Subclass implementation
    
    Tea->>BaseClass: pour_in_cup()
    BaseClass-->>Tea: Poured
    
    Tea->>Tea: add_condiments() [add lemon]
    Note right of Tea: Subclass implementation
    
    Tea-->>Client: Tea ready!
    deactivate Tea
    
    Client->>Coffee: prepare_recipe()
    activate Coffee
    
    Coffee->>BaseClass: boil_water()
    BaseClass-->>Coffee: Water boiled
    
    Coffee->>Coffee: brew() [drip coffee]
    Note right of Coffee: Different implementation
    
    Coffee->>BaseClass: pour_in_cup()
    BaseClass-->>Coffee: Poured
    
    Coffee->>Coffee: add_condiments() [add milk]
    Note right of Coffee: Different implementation
    
    Coffee-->>Client: Coffee ready!
    deactivate Coffee

The Problem

You’re building a beverage maker that can make tea and coffee. Without Template Method Pattern:

Python

bad_beverage.py

# ❌ Without Template Method Pattern - Code duplication!

class Tea:
    def prepare(self):
        # Step 1: Boil water
        print("Boiling water...")

        # Step 2: Brew (specific to tea)
        print("Steeping the tea...")

        # Step 3: Pour
        print("Pouring into cup...")

        # Step 4: Add condiments (specific to tea)
        print("Adding lemon...")

class Coffee:
    def prepare(self):
        # Step 1: Boil water - DUPLICATED!
        print("Boiling water...")

        # Step 2: Brew (specific to coffee)
        print("Dripping coffee through filter...")

        # Step 3: Pour - DUPLICATED!
        print("Pouring into cup...")

        # Step 4: Add condiments (specific to coffee)
        print("Adding milk and sugar...")

class HotChocolate:
    def prepare(self):
        # Step 1: Boil water - DUPLICATED AGAIN!
        print("Boiling water...")

        # Step 2: Mix (specific to hot chocolate)
        print("Mixing chocolate powder...")

        # Step 3: Pour - DUPLICATED AGAIN!
        print("Pouring into cup...")

        # Step 4: Add condiments
        print("Adding marshmallows...")

# Problems:
# - boil_water() and pour() duplicated in EVERY class
# - If we change how we boil water, change in EVERY class
# - No enforcement of algorithm structure
# - Easy to forget a step

Java

BadBeverage.java

// ❌ Without Template Method Pattern - Code duplication!

class Tea {
    public void prepare() {
        // Step 1: Boil water
        System.out.println("Boiling water...");

        // Step 2: Brew (specific to tea)
        System.out.println("Steeping the tea...");

        // Step 3: Pour
        System.out.println("Pouring into cup...");

        // Step 4: Add condiments (specific to tea)
        System.out.println("Adding lemon...");
    }
}

class Coffee {
    public void prepare() {
        // Step 1: Boil water - DUPLICATED!
        System.out.println("Boiling water...");

        // Step 2: Brew (specific to coffee)
        System.out.println("Dripping coffee through filter...");

        // Step 3: Pour - DUPLICATED!
        System.out.println("Pouring into cup...");

        // Step 4: Add condiments (specific to coffee)
        System.out.println("Adding milk and sugar...");
    }
}

class HotChocolate {
    public void prepare() {
        // Step 1: Boil water - DUPLICATED AGAIN!
        System.out.println("Boiling water...");

        // Step 2: Mix (specific to hot chocolate)
        System.out.println("Mixing chocolate powder...");

        // Step 3: Pour - DUPLICATED AGAIN!
        System.out.println("Pouring into cup...");

        // Step 4: Add condiments
        System.out.println("Adding marshmallows...");
    }
}

// Problems:
// - boilWater() and pour() duplicated in EVERY class
// - If we change how we boil water, change in EVERY class
// - No enforcement of algorithm structure
// - Easy to forget a step

Problems:

• Code duplication - Same steps copied everywhere
• Maintenance nightmare - Change in multiple places
• No structure enforcement - Easy to forget steps
• Violates DRY principle

The Solution: Template Method Pattern

Class Structure

classDiagram
    class Beverage {
        <<abstract>>
        +prepare_recipe() void
        +boil_water() void
        #brew()* void
        +pour_in_cup() void
        #add_condiments()* void
    }
    class Tea {
        #brew() void
        #add_condiments() void
    }
    class Coffee {
        #brew() void
        #add_condiments() void
    }
    class HotChocolate {
        #brew() void
        #add_condiments() void
    }
    
    Beverage <|-- Tea : extends
    Beverage <|-- Coffee : extends
    Beverage <|-- HotChocolate : extends
    
    note for Beverage "Template method: prepare_recipe()\nConcrete: boil_water(), pour_in_cup()\nAbstract: brew(), add_condiments()"

Python

template_beverage.py

from abc import ABC, abstractmethod

# Step 1: Create the Abstract Class with Template Method
class Beverage(ABC):
    """Abstract base class with template method"""

    def prepare_recipe(self) -> None:
        """
        Template method - defines the algorithm skeleton.
        This method is final - subclasses cannot override it!
        """
        self.boil_water()
        self.brew()          # Abstract - subclasses implement
        self.pour_in_cup()
        self.add_condiments()  # Abstract - subclasses implement
        print(f"✅ {self.__class__.__name__} is ready!\n")

    def boil_water(self) -> None:
        """Concrete method - same for all beverages"""
        print("🔥 Boiling water...")

    def pour_in_cup(self) -> None:
        """Concrete method - same for all beverages"""
        print("☕ Pouring into cup...")

    @abstractmethod
    def brew(self) -> None:
        """Abstract method - subclasses must implement"""
        pass

    @abstractmethod
    def add_condiments(self) -> None:
        """Abstract method - subclasses must implement"""
        pass

# Step 2: Create Concrete Subclasses
class Tea(Beverage):
    """Concrete class - implements abstract methods for tea"""

    def brew(self) -> None:
        print("🍵 Steeping the tea for 3 minutes...")

    def add_condiments(self) -> None:
        print("🍋 Adding lemon slice...")

class Coffee(Beverage):
    """Concrete class - implements abstract methods for coffee"""

    def brew(self) -> None:
        print("☕ Dripping coffee through filter...")

    def add_condiments(self) -> None:
        print("🥛 Adding milk and sugar...")

class HotChocolate(Beverage):
    """Concrete class - implements abstract methods for hot chocolate"""

    def brew(self) -> None:
        print("🍫 Mixing chocolate powder with water...")

    def add_condiments(self) -> None:
        print("🍡 Adding marshmallows and whipped cream...")

# Step 3: Use the pattern
def main():
    print("=" * 50)
    print("Beverage Maker (Template Method Pattern)")
    print("=" * 50)

    # Make tea
    print("\n📋 Making Tea:")
    print("-" * 30)
    tea = Tea()
    tea.prepare_recipe()

    # Make coffee
    print("📋 Making Coffee:")
    print("-" * 30)
    coffee = Coffee()
    coffee.prepare_recipe()

    # Make hot chocolate
    print("📋 Making Hot Chocolate:")
    print("-" * 30)
    hot_chocolate = HotChocolate()
    hot_chocolate.prepare_recipe()

    print("=" * 50)
    print("✅ Template Method Pattern: Same algorithm, different details!")
    print("✅ Common code in base class - no duplication!")

if __name__ == "__main__":
    main()

Java

TemplateBeverage.java

// Step 1: Create the Abstract Class with Template Method
abstract class Beverage {
    /**
     * Abstract base class with template method
     */

    // Template method - defines the algorithm skeleton
    // This method is final - subclasses cannot override it!
    public final void prepareRecipe() {
        boilWater();
        brew();           // Abstract - subclasses implement
        pourInCup();
        addCondiments();  // Abstract - subclasses implement
        System.out.println("✅ " + getClass().getSimpleName() + " is ready!\n");
    }

    // Concrete method - same for all beverages
    private void boilWater() {
        System.out.println("🔥 Boiling water...");
    }

    // Concrete method - same for all beverages
    private void pourInCup() {
        System.out.println("☕ Pouring into cup...");
    }

    // Abstract method - subclasses must implement
    protected abstract void brew();

    // Abstract method - subclasses must implement
    protected abstract void addCondiments();
}

// Step 2: Create Concrete Subclasses
class Tea extends Beverage {
    /**
     * Concrete class - implements abstract methods for tea
     */
    @Override
    protected void brew() {
        System.out.println("🍵 Steeping the tea for 3 minutes...");
    }

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

class Coffee extends Beverage {
    /**
     * Concrete class - implements abstract methods for coffee
     */
    @Override
    protected void brew() {
        System.out.println("☕ Dripping coffee through filter...");
    }

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

class HotChocolate extends Beverage {
    /**
     * Concrete class - implements abstract methods for hot chocolate
     */
    @Override
    protected void brew() {
        System.out.println("🍫 Mixing chocolate powder with water...");
    }

    @Override
    protected void addCondiments() {
        System.out.println("🍡 Adding marshmallows and whipped cream...");
    }
}

// Step 3: Use the pattern
public class Main {
    public static void main(String[] args) {
        System.out.println("=".repeat(50));
        System.out.println("Beverage Maker (Template Method Pattern)");
        System.out.println("=".repeat(50));

        // Make tea
        System.out.println("\n📋 Making Tea:");
        System.out.println("-".repeat(30));
        Beverage tea = new Tea();
        tea.prepareRecipe();

        // Make coffee
        System.out.println("📋 Making Coffee:");
        System.out.println("-".repeat(30));
        Beverage coffee = new Coffee();
        coffee.prepareRecipe();

        // Make hot chocolate
        System.out.println("📋 Making Hot Chocolate:");
        System.out.println("-".repeat(30));
        Beverage hotChocolate = new HotChocolate();
        hotChocolate.prepareRecipe();

        System.out.println("=".repeat(50));
        System.out.println("✅ Template Method Pattern: Same algorithm, different details!");
        System.out.println("✅ Common code in base class - no duplication!");
    }
}

Key Benefits

✅ No code duplication - Common code in base class
✅ Algorithm structure enforced - Can’t skip or reorder steps
✅ Easy to extend - Just override abstract methods
✅ DRY principle - Don’t Repeat Yourself
✅ Single point of change - Fix bugs in one place

---

Real-World Software Example: Data Mining Pipeline

Now let’s see a realistic software example - a data mining pipeline that processes data from different sources.

The Problem

You’re building a data processing system that needs to handle CSV files, JSON APIs, and databases. The process is similar: open connection, extract data, transform, analyze, generate report, close connection. Without Template Method:

Python

bad_data_mining.py

# ❌ Without Template Method Pattern - Duplicated process!

class CSVDataMiner:
    def mine(self, path: str):
        # Open - specific to CSV
        print(f"Opening CSV file: {path}")

        # Extract - specific to CSV
        print("Parsing CSV rows...")
        data = [{"name": "Alice"}, {"name": "Bob"}]

        # Transform - COMMON!
        print("Transforming data...")
        transformed = [d["name"].upper() for d in data]

        # Analyze - COMMON!
        print("Analyzing data...")
        result = {"count": len(transformed)}

        # Generate report - COMMON!
        print(f"Report: {result}")

        # Close - specific to CSV
        print("Closing CSV file")

class JSONAPIMiner:
    def mine(self, url: str):
        # Open - specific to API
        print(f"Connecting to API: {url}")

        # Extract - specific to API
        print("Fetching JSON from API...")
        data = [{"name": "Charlie"}, {"name": "Diana"}]

        # Transform - DUPLICATED!
        print("Transforming data...")
        transformed = [d["name"].upper() for d in data]

        # Analyze - DUPLICATED!
        print("Analyzing data...")
        result = {"count": len(transformed)}

        # Generate report - DUPLICATED!
        print(f"Report: {result}")

        # Close - specific to API
        print("Closing API connection")

# Problems:
# - transform(), analyze(), generate_report() duplicated
# - If analysis logic changes, update ALL miners
# - No guarantee all miners follow same process

Java

BadDataMining.java

// ❌ Without Template Method Pattern - Duplicated process!

class CSVDataMiner {
    public void mine(String path) {
        // Open - specific to CSV
        System.out.println("Opening CSV file: " + path);

        // Extract - specific to CSV
        System.out.println("Parsing CSV rows...");
        List<Map<String, String>> data = List.of(
            Map.of("name", "Alice"),
            Map.of("name", "Bob")
        );

        // Transform - COMMON!
        System.out.println("Transforming data...");

        // Analyze - COMMON!
        System.out.println("Analyzing data...");

        // Generate report - COMMON!
        System.out.println("Report generated");

        // Close - specific to CSV
        System.out.println("Closing CSV file");
    }
}

class JSONAPIMiner {
    public void mine(String url) {
        // Open - specific to API
        System.out.println("Connecting to API: " + url);

        // Extract - specific to API
        System.out.println("Fetching JSON from API...");

        // Transform - DUPLICATED!
        System.out.println("Transforming data...");

        // Analyze - DUPLICATED!
        System.out.println("Analyzing data...");

        // Generate report - DUPLICATED!
        System.out.println("Report generated");

        // Close - specific to API
        System.out.println("Closing API connection");
    }
}

// Problems:
// - transform(), analyze(), generateReport() duplicated
// - If analysis logic changes, update ALL miners
// - No guarantee all miners follow same process

Problems:

• Common steps duplicated across all miners
• Changes need to be made in multiple places
• No enforcement of the mining process

The Solution: Template Method Pattern

Class Structure

classDiagram
    class DataMiner {
        <<abstract>>
        +mine(source) void
        #open_source(source)* void
        #extract_data()* List
        +transform_data(data) List
        +analyze_data(data) Report
        +generate_report(analysis) void
        #close_source()* void
        +hook_before_analysis() void
    }
    class CSVDataMiner {
        #open_source(source) void
        #extract_data() List
        #close_source() void
    }
    class JSONAPIMiner {
        #open_source(source) void
        #extract_data() List
        #close_source() void
    }
    class DatabaseMiner {
        #open_source(source) void
        #extract_data() List
        #close_source() void
        +hook_before_analysis() void
    }
    
    DataMiner <|-- CSVDataMiner : extends
    DataMiner <|-- JSONAPIMiner : extends
    DataMiner <|-- DatabaseMiner : extends
    
    note for DataMiner "Template method: mine()\nAbstract: open, extract, close\nConcrete: transform, analyze, report\nHook: hook_before_analysis()"

Python

template_data_mining.py

from abc import ABC, abstractmethod
from typing import List, Dict, Any
from dataclasses import dataclass

# Step 1: Define data structures
@dataclass
class AnalysisReport:
    """Report generated from data analysis"""
    source_type: str
    record_count: int
    summary: Dict[str, Any]

# Step 2: Create the Abstract Class with Template Method
class DataMiner(ABC):
    """Abstract data miner with template method"""

    def mine(self, source: str) -> AnalysisReport:
        """
        Template method - defines the data mining algorithm.
        Subclasses cannot change this structure!
        """
        print(f"\n{'='*50}")
        print(f"📊 Starting data mining: {self.__class__.__name__}")
        print(f"{'='*50}")

        # Step 1: Open source (abstract - source-specific)
        self.open_source(source)

        # Step 2: Extract data (abstract - source-specific)
        raw_data = self.extract_data()
        print(f"   📥 Extracted {len(raw_data)} records")

        # Step 3: Transform data (concrete - common for all)
        transformed_data = self.transform_data(raw_data)

        # Hook: Optional step before analysis
        self.hook_before_analysis(transformed_data)

        # Step 4: Analyze data (concrete - common for all)
        report = self.analyze_data(transformed_data)

        # Step 5: Generate report (concrete - common for all)
        self.generate_report(report)

        # Step 6: Close source (abstract - source-specific)
        self.close_source()

        return report

    # Abstract methods - MUST be implemented by subclasses
    @abstractmethod
    def open_source(self, source: str) -> None:
        """Open the data source"""
        pass

    @abstractmethod
    def extract_data(self) -> List[Dict[str, Any]]:
        """Extract raw data from source"""
        pass

    @abstractmethod
    def close_source(self) -> None:
        """Close the data source"""
        pass

    # Concrete methods - Common implementation for all
    def transform_data(self, data: List[Dict[str, Any]]) -> List[Dict[str, Any]]:
        """Transform raw data - common for all miners"""
        print("   🔄 Transforming data...")
        transformed = []
        for record in data:
            # Normalize keys to lowercase
            transformed.append({k.lower(): v for k, v in record.items()})
        return transformed

    def analyze_data(self, data: List[Dict[str, Any]]) -> AnalysisReport:
        """Analyze data - common for all miners"""
        print("   📈 Analyzing data...")

        # Calculate statistics
        summary = {
            "fields": list(data[0].keys()) if data else [],
            "sample": data[:2] if len(data) >= 2 else data
        }

        return AnalysisReport(
            source_type=self.__class__.__name__,
            record_count=len(data),
            summary=summary
        )

    def generate_report(self, report: AnalysisReport) -> None:
        """Generate report - common for all miners"""
        print("   📝 Generating report...")
        print(f"      Source: {report.source_type}")
        print(f"      Records: {report.record_count}")
        print(f"      Fields: {report.summary['fields']}")

    # Hook method - CAN be overridden, but not required
    def hook_before_analysis(self, data: List[Dict[str, Any]]) -> None:
        """Hook - optional step before analysis. Override if needed."""
        pass

# Step 3: Create Concrete Subclasses
class CSVDataMiner(DataMiner):
    """Mines data from CSV files"""

    def __init__(self):
        self._file = None

    def open_source(self, source: str) -> None:
        print(f"   📂 Opening CSV file: {source}")
        self._file = source  # Simulated file handle

    def extract_data(self) -> List[Dict[str, Any]]:
        print("   📋 Parsing CSV rows...")
        # Simulated CSV data
        return [
            {"Name": "Alice", "Age": "30", "City": "NYC"},
            {"Name": "Bob", "Age": "25", "City": "LA"},
            {"Name": "Charlie", "Age": "35", "City": "Chicago"},
        ]

    def close_source(self) -> None:
        print(f"   📁 Closing CSV file")
        self._file = None

class JSONAPIMiner(DataMiner):
    """Mines data from JSON APIs"""

    def __init__(self):
        self._connection = None

    def open_source(self, source: str) -> None:
        print(f"   🌐 Connecting to API: {source}")
        self._connection = source  # Simulated connection

    def extract_data(self) -> List[Dict[str, Any]]:
        print("   📡 Fetching JSON from API...")
        # Simulated API response
        return [
            {"id": 1, "username": "alice_dev", "active": True},
            {"id": 2, "username": "bob_admin", "active": True},
            {"id": 3, "username": "charlie_user", "active": False},
        ]

    def close_source(self) -> None:
        print(f"   🔌 Closing API connection")
        self._connection = None

class DatabaseMiner(DataMiner):
    """Mines data from databases"""

    def __init__(self):
        self._connection = None

    def open_source(self, source: str) -> None:
        print(f"   🗄️ Connecting to database: {source}")
        self._connection = source  # Simulated DB connection

    def extract_data(self) -> List[Dict[str, Any]]:
        print("   🔍 Executing SQL query...")
        # Simulated database result
        return [
            {"order_id": 1001, "amount": 99.99, "status": "completed"},
            {"order_id": 1002, "amount": 149.99, "status": "pending"},
            {"order_id": 1003, "amount": 29.99, "status": "completed"},
            {"order_id": 1004, "amount": 199.99, "status": "cancelled"},
        ]

    def close_source(self) -> None:
        print(f"   🔒 Closing database connection")
        self._connection = None

    # Override hook to add custom behavior
    def hook_before_analysis(self, data: List[Dict[str, Any]]) -> None:
        """Custom hook - validate data before analysis"""
        print("   ✅ Validating database records...")
        invalid = [d for d in data if d.get("status") == "cancelled"]
        if invalid:
            print(f"      ⚠️ Found {len(invalid)} cancelled orders")

# Step 4: Use the pattern
def main():
    print("=" * 60)
    print("Data Mining Pipeline (Template Method Pattern)")
    print("=" * 60)

    # Mine CSV data
    csv_miner = CSVDataMiner()
    csv_miner.mine("users.csv")

    # Mine JSON API data
    api_miner = JSONAPIMiner()
    api_miner.mine("https://api.example.com/users")

    # Mine Database data (with custom hook)
    db_miner = DatabaseMiner()
    db_miner.mine("postgresql://localhost/orders")

    print("\n" + "=" * 60)
    print("✅ Template Method Pattern: Same process, different sources!")
    print("✅ Common logic in base class - no duplication!")
    print("✅ Hooks allow optional customization!")

if __name__ == "__main__":
    main()

Java

TemplateDataMining.java

import java.util.*;

// Step 1: Define data structures
class AnalysisReport {
    /**
     * Report generated from data analysis
     */
    private String sourceType;
    private int recordCount;
    private Map<String, Object> summary;

    public AnalysisReport(String sourceType, int recordCount, Map<String, Object> summary) {
        this.sourceType = sourceType;
        this.recordCount = recordCount;
        this.summary = summary;
    }

    public String getSourceType() { return sourceType; }
    public int getRecordCount() { return recordCount; }
    public Map<String, Object> getSummary() { return summary; }
}

// Step 2: Create the Abstract Class with Template Method
abstract class DataMiner {
    /**
     * Abstract data miner with template method
     */

    // Template method - defines the data mining algorithm
    // Subclasses cannot change this structure!
    public final AnalysisReport mine(String source) {
        System.out.println("\n" + "=".repeat(50));
        System.out.println("📊 Starting data mining: " + getClass().getSimpleName());
        System.out.println("=".repeat(50));

        // Step 1: Open source (abstract - source-specific)
        openSource(source);

        // Step 2: Extract data (abstract - source-specific)
        List<Map<String, Object>> rawData = extractData();
        System.out.println("   📥 Extracted " + rawData.size() + " records");

        // Step 3: Transform data (concrete - common for all)
        List<Map<String, Object>> transformedData = transformData(rawData);

        // Hook: Optional step before analysis
        hookBeforeAnalysis(transformedData);

        // Step 4: Analyze data (concrete - common for all)
        AnalysisReport report = analyzeData(transformedData);

        // Step 5: Generate report (concrete - common for all)
        generateReport(report);

        // Step 6: Close source (abstract - source-specific)
        closeSource();

        return report;
    }

    // Abstract methods - MUST be implemented by subclasses
    protected abstract void openSource(String source);
    protected abstract List<Map<String, Object>> extractData();
    protected abstract void closeSource();

    // Concrete methods - Common implementation for all
    protected List<Map<String, Object>> transformData(List<Map<String, Object>> data) {
        System.out.println("   🔄 Transforming data...");
        List<Map<String, Object>> transformed = new ArrayList<>();
        for (Map<String, Object> record : data) {
            Map<String, Object> newRecord = new HashMap<>();
            for (Map.Entry<String, Object> entry : record.entrySet()) {
                newRecord.put(entry.getKey().toLowerCase(), entry.getValue());
            }
            transformed.add(newRecord);
        }
        return transformed;
    }

    protected AnalysisReport analyzeData(List<Map<String, Object>> data) {
        System.out.println("   📈 Analyzing data...");

        Map<String, Object> summary = new HashMap<>();
        summary.put("fields", data.isEmpty() ? List.of() : new ArrayList<>(data.get(0).keySet()));
        summary.put("sample", data.size() >= 2 ? data.subList(0, 2) : data);

        return new AnalysisReport(
            getClass().getSimpleName(),
            data.size(),
            summary
        );
    }

    protected void generateReport(AnalysisReport report) {
        System.out.println("   📝 Generating report...");
        System.out.println("      Source: " + report.getSourceType());
        System.out.println("      Records: " + report.getRecordCount());
        System.out.println("      Fields: " + report.getSummary().get("fields"));
    }

    // Hook method - CAN be overridden, but not required
    protected void hookBeforeAnalysis(List<Map<String, Object>> data) {
        // Default: do nothing. Override if needed.
    }
}

// Step 3: Create Concrete Subclasses
class CSVDataMiner extends DataMiner {
    /**
     * Mines data from CSV files
     */
    private String file;

    @Override
    protected void openSource(String source) {
        System.out.println("   📂 Opening CSV file: " + source);
        this.file = source;
    }

    @Override
    protected List<Map<String, Object>> extractData() {
        System.out.println("   📋 Parsing CSV rows...");
        return Arrays.asList(
            Map.of("Name", "Alice", "Age", "30", "City", "NYC"),
            Map.of("Name", "Bob", "Age", "25", "City", "LA"),
            Map.of("Name", "Charlie", "Age", "35", "City", "Chicago")
        );
    }

    @Override
    protected void closeSource() {
        System.out.println("   📁 Closing CSV file");
        this.file = null;
    }
}

class JSONAPIMiner extends DataMiner {
    /**
     * Mines data from JSON APIs
     */
    private String connection;

    @Override
    protected void openSource(String source) {
        System.out.println("   🌐 Connecting to API: " + source);
        this.connection = source;
    }

    @Override
    protected List<Map<String, Object>> extractData() {
        System.out.println("   📡 Fetching JSON from API...");
        return Arrays.asList(
            Map.of("id", 1, "username", "alice_dev", "active", true),
            Map.of("id", 2, "username", "bob_admin", "active", true),
            Map.of("id", 3, "username", "charlie_user", "active", false)
        );
    }

    @Override
    protected void closeSource() {
        System.out.println("   🔌 Closing API connection");
        this.connection = null;
    }
}

class DatabaseMiner extends DataMiner {
    /**
     * Mines data from databases
     */
    private String connection;

    @Override
    protected void openSource(String source) {
        System.out.println("   🗄️ Connecting to database: " + source);
        this.connection = source;
    }

    @Override
    protected List<Map<String, Object>> extractData() {
        System.out.println("   🔍 Executing SQL query...");
        return Arrays.asList(
            Map.of("order_id", 1001, "amount", 99.99, "status", "completed"),
            Map.of("order_id", 1002, "amount", 149.99, "status", "pending"),
            Map.of("order_id", 1003, "amount", 29.99, "status", "completed"),
            Map.of("order_id", 1004, "amount", 199.99, "status", "cancelled")
        );
    }

    @Override
    protected void closeSource() {
        System.out.println("   🔒 Closing database connection");
        this.connection = null;
    }

    // Override hook to add custom behavior
    @Override
    protected void hookBeforeAnalysis(List<Map<String, Object>> data) {
        System.out.println("   ✅ Validating database records...");
        long invalidCount = data.stream()
            .filter(d -> "cancelled".equals(d.get("status")))
            .count();
        if (invalidCount > 0) {
            System.out.println("      ⚠️ Found " + invalidCount + " cancelled orders");
        }
    }
}

// Step 4: Use the pattern
public class Main {
    public static void main(String[] args) {
        System.out.println("=".repeat(60));
        System.out.println("Data Mining Pipeline (Template Method Pattern)");
        System.out.println("=".repeat(60));

        // Mine CSV data
        DataMiner csvMiner = new CSVDataMiner();
        csvMiner.mine("users.csv");

        // Mine JSON API data
        DataMiner apiMiner = new JSONAPIMiner();
        apiMiner.mine("https://api.example.com/users");

        // Mine Database data (with custom hook)
        DataMiner dbMiner = new DatabaseMiner();
        dbMiner.mine("postgresql://localhost/orders");

        System.out.println("\n" + "=".repeat(60));
        System.out.println("✅ Template Method Pattern: Same process, different sources!");
        System.out.println("✅ Common logic in base class - no duplication!");
        System.out.println("✅ Hooks allow optional customization!");
    }
}

Key Benefits

✅ Common process - Mining algorithm defined once
✅ Easy to extend - Add new data sources easily
✅ Hooks - Optional customization points
✅ Enforced structure - Can’t skip steps
✅ DRY - No duplicated transformation/analysis code

---

Template Method Pattern Variants

1. With Hooks

Hooks are optional methods that can be overridden:

Python

hooks.py

# Template Method with Hooks
class Beverage(ABC):
    def prepare_recipe(self):
        self.boil_water()
        self.brew()
        self.pour_in_cup()

        # Hook - optional step
        if self.customer_wants_condiments():
            self.add_condiments()

    # Hook with default behavior
    def customer_wants_condiments(self) -> bool:
        """Hook - subclasses can override to change behavior"""
        return True  # Default: yes

class TeaWithHook(Beverage):
    def customer_wants_condiments(self) -> bool:
        # Ask the customer
        answer = input("Would you like lemon? (y/n): ")
        return answer.lower() == 'y'

Java

Hooks.java

// Template Method with Hooks
abstract class Beverage {
    public final void prepareRecipe() {
        boilWater();
        brew();
        pourInCup();

        // Hook - optional step
        if (customerWantsCondiments()) {
            addCondiments();
        }
    }

    // Hook with default behavior
    protected boolean customerWantsCondiments() {
        // Default: yes
        return true;
    }

    protected abstract void brew();
    protected abstract void addCondiments();
}

class TeaWithHook extends Beverage {
    @Override
    protected boolean customerWantsCondiments() {
        // Could ask the customer
        return true;
    }
}

Hooks allow:

• Optional customization without forcing implementation
• Conditional execution of steps
• Default behavior that can be changed

2. Hollywood Principle

“Don’t call us, we’ll call you” - Base class calls subclass methods:

sequenceDiagram
    participant Client
    participant BaseClass
    participant SubClass
    
    Client->>BaseClass: templateMethod()
    BaseClass->>BaseClass: step1()
    BaseClass->>SubClass: abstractStep2()
    Note right of SubClass: Subclass method called by base
    SubClass-->>BaseClass: return
    BaseClass->>BaseClass: step3()
    BaseClass-->>Client: done

Which Variant to Use?

• Basic Template Method - Fixed algorithm structure
• With Hooks - Optional customization points
• With Default Implementations - Reduce required overrides

Recommendation: Use hooks for optional customization, abstract methods for required customization!

---

When to Use Template Method Pattern?

Use Template Method Pattern when:

✅ Common algorithm structure - Same steps, different implementations
✅ Code duplication - Same structure repeated in multiple classes
✅ Control extension points - Only certain steps can be customized
✅ Enforce invariant steps - Some steps must always happen
✅ Hollywood Principle - Framework calls user code

Decision Checklist

Ask yourself:

• Do I have similar algorithms with different details? → Use Template Method
• Am I duplicating algorithm structure? → Use Template Method
• Do I need to control which steps can be customized? → Use Template Method
• Should some steps always happen in order? → Use Template Method

When NOT to Use Template Method Pattern?

Don’t use Template Method Pattern when:

❌ Algorithms are completely different - No common structure
❌ All steps need customization - Use Strategy instead
❌ Inheritance isn’t appropriate - Use composition
❌ Simple cases - Just a few steps, no duplication

Avoid Over-Use

Don’t use Template Method for simple cases! If you have only 2-3 classes with minimal common code, the pattern might add unnecessary complexity. Use it when you have clear algorithm structure with shared steps!

---

Common Mistakes to Avoid

Mistake 1: Making Template Method Overridable

Python

overridable_template.py

# ❌ Bad: Template method can be overridden
class BadBase:
    def template_method(self):  # Not protected!
        self.step1()
        self.step2()
        self.step3()
    # Subclasses can completely change the algorithm!

# ✅ Good: Template method is final (in concept)
class GoodBase(ABC):
    def template_method(self):
        """
        Template method - DO NOT OVERRIDE!
        Note: Python doesn't have 'final', but document it clearly.
        """
        self.step1()
        self.step2()
        self.step3()

Java

OverridableTemplate.java

// ❌ Bad: Template method can be overridden
class BadBase {
    public void templateMethod() {  // Not final!
        step1();
        step2();
        step3();
    }
    // Subclasses can completely change the algorithm!
}

// ✅ Good: Template method is final
abstract class GoodBase {
    public final void templateMethod() {  // final!
        step1();
        step2();
        step3();
    }
    // Subclasses cannot override the algorithm structure
}

Mistake 2: Too Many Abstract Methods

Python

too_many_abstract.py

# ❌ Bad: Everything is abstract - defeats the purpose!
class BadBase(ABC):
    def template_method(self):
        self.step1()
        self.step2()
        self.step3()
        self.step4()
        self.step5()

    @abstractmethod
    def step1(self): pass
    @abstractmethod
    def step2(self): pass
    @abstractmethod
    def step3(self): pass
    @abstractmethod
    def step4(self): pass
    @abstractmethod
    def step5(self): pass
    # All steps abstract = no code reuse!

# ✅ Good: Mix of concrete and abstract methods
class GoodBase(ABC):
    def template_method(self):
        self.setup()           # Concrete
        self.do_work()         # Abstract
        self.cleanup()         # Concrete

    def setup(self):           # Common implementation
        print("Setting up...")

    @abstractmethod
    def do_work(self): pass    # Customization point

    def cleanup(self):         # Common implementation
        print("Cleaning up...")

Java

TooManyAbstract.java

// ❌ Bad: Everything is abstract - defeats the purpose!
abstract class BadBase {
    public final void templateMethod() {
        step1();
        step2();
        step3();
        step4();
        step5();
    }

    protected abstract void step1();
    protected abstract void step2();
    protected abstract void step3();
    protected abstract void step4();
    protected abstract void step5();
    // All steps abstract = no code reuse!
}

// ✅ Good: Mix of concrete and abstract methods
abstract class GoodBase {
    public final void templateMethod() {
        setup();        // Concrete
        doWork();       // Abstract
        cleanup();      // Concrete
    }

    protected void setup() {           // Common implementation
        System.out.println("Setting up...");
    }

    protected abstract void doWork();  // Customization point

    protected void cleanup() {         // Common implementation
        System.out.println("Cleaning up...");
    }
}

Mistake 3: Not Using Hooks for Optional Steps

Python

missing_hooks.py

# ❌ Bad: Forcing subclasses to implement optional steps
class BadBase(ABC):
    def template_method(self):
        self.required_step()
        self.optional_step()  # Abstract - but not all need it!

    @abstractmethod
    def optional_step(self): pass  # Forces empty implementations

# ✅ Good: Use hooks for optional steps
class GoodBase(ABC):
    def template_method(self):
        self.required_step()
        self.optional_hook()  # Hook with default

    @abstractmethod
    def required_step(self): pass

    def optional_hook(self):
        """Hook - override only if needed"""
        pass  # Default: do nothing

Java

MissingHooks.java

// ❌ Bad: Forcing subclasses to implement optional steps
abstract class BadBase {
    public final void templateMethod() {
        requiredStep();
        optionalStep();  // Abstract - but not all need it!
    }

    protected abstract void optionalStep();  // Forces empty implementations
}

// ✅ Good: Use hooks for optional steps
abstract class GoodBase {
    public final void templateMethod() {
        requiredStep();
        optionalHook();  // Hook with default
    }

    protected abstract void requiredStep();

    protected void optionalHook() {
        // Default: do nothing. Override only if needed.
    }
}

Template Method Best Practices

Remember: Keep template methods final! Use hooks for optional steps. Don’t make everything abstract. The power is in the common code!

---

Benefits of Template Method Pattern

1. Code Reuse - Common code in base class
2. Algorithm Structure - Can’t be changed by subclasses
3. Single Point of Change - Fix bugs once
4. Controlled Extension - Only specific steps customizable
5. Inversion of Control - Hollywood Principle
6. DRY Principle - Don’t Repeat Yourself

Why It Matters

Template Method Pattern makes your code more maintainable and consistent by defining algorithm structure once. It’s essential for frameworks where users provide specific implementations!

---

Revision: Quick Catch-Up

Quick Reference

Use this section to quickly review the Template Method Pattern!

What is Template Method Pattern?

Template Method Pattern is a behavioral design pattern that defines the skeleton of an algorithm in a base class method, letting subclasses override specific steps without changing the algorithm’s structure.

Why Use It?

• ✅ Code reuse - Common steps in base class
• ✅ Algorithm structure - Fixed, can’t be changed
• ✅ Controlled extension - Only specific steps customizable
• ✅ DRY principle - No duplication
• ✅ Hollywood Principle - “Don’t call us, we’ll call you”

How It Works?

1. Create abstract base class - With template method
2. Define template method - The algorithm skeleton
3. Add concrete methods - Common implementations
4. Add abstract methods - Customization points
5. Add hooks - Optional customization
6. Create subclasses - Implement abstract methods

Key Components

Abstract Class (Template Method)
├── template_method()    → defines algorithm
├── concrete_step()      → common implementation
├── abstract_step()      → subclass implements
└── hook()              → optional override

Simple Example

class Beverage(ABC):
    def prepare_recipe(self):  # Template method
        self.boil_water()      # Concrete
        self.brew()            # Abstract
        self.pour_in_cup()     # Concrete
        self.add_condiments()  # Abstract

    def boil_water(self):
        print("Boiling water")

    @abstractmethod
    def brew(self): pass

    @abstractmethod
    def add_condiments(self): pass

When to Use?

✅ Common algorithm structure
✅ Code duplication in algorithm
✅ Need to control extension points
✅ Invariant steps must happen
✅ Framework development

When NOT to Use?

❌ Completely different algorithms
❌ All steps need customization
❌ Simple cases with no duplication

Key Takeaways

• Template Method = Algorithm skeleton in base class
• Abstract methods = Customization points
• Concrete methods = Common implementation
• Hooks = Optional customization
• Hollywood Principle = Base calls subclass methods

---

Interview Focus: Template Method Pattern

Interview Preparation

This section covers key points interviewers look for when discussing Template Method Pattern. Master these to ace your LLD interviews!

Key Points to Remember

1. Core Concept

What to say:

“Template Method Pattern is a behavioral design pattern that defines the skeleton of an algorithm in a base class method, letting subclasses override specific steps without changing the algorithm’s structure. It promotes code reuse and enforces a consistent algorithm structure.”

Why it matters:

• Shows you understand the fundamental purpose
• Demonstrates knowledge of inheritance-based patterns
• Indicates you can explain concepts clearly

2. Template Method vs Strategy Pattern

Must discuss:

• Template Method - Uses inheritance, algorithm structure fixed
• Strategy - Uses composition, entire algorithm swapped
• Key difference - Template customizes steps, Strategy replaces algorithm

Example to give:

“Template Method is like a form with blanks to fill in - the structure is fixed, you just provide specific content. Strategy is like choosing a completely different form. Template Method changes parts of an algorithm; Strategy changes the entire algorithm.”

Interview Tip

This comparison is frequently asked! Know when to use each.

3. Hollywood Principle

Must explain:

• “Don’t call us, we’ll call you”
• Base class calls subclass methods (inversion of control)
• Framework pattern - framework controls flow

Example to give:

“In Template Method, the base class controls when subclass methods are called - this is the Hollywood Principle. The subclass doesn’t call the base class to get things done; instead, the base class calls down to the subclass when it needs specific implementation.”

4. Benefits and Trade-offs

Benefits to mention:

• Code reuse - Common code in base class
• Consistent structure - Algorithm can’t be altered
• Single point of change - Fix bugs once
• Controlled extension - Limited customization points
• DRY principle - No duplication

Trade-offs to acknowledge:

• Inheritance limitation - Java single inheritance
• Harder to understand - Need to trace through hierarchy
• Tight coupling - Subclasses tied to base class structure
• Fragile base class - Changes to base affect all

Interview Tip

Always mention trade-offs! Show you understand the limitations of inheritance-based patterns.

5. Common Interview Questions

Q: “When would you use Template Method over Strategy?”

A:

“I’d use Template Method when I have a fixed algorithm structure with specific steps that vary. For example, a data processing pipeline where opening, transforming, and closing are always the same, but extracting data varies by source. I’d use Strategy when I want to swap the entire algorithm, like different sorting algorithms that have completely different approaches.”

Q: “What are hooks in Template Method?”

A:

“Hooks are methods with default (usually empty) implementations that subclasses CAN override but don’t have to. Unlike abstract methods which MUST be overridden, hooks are optional. They provide additional customization points without forcing implementation. For example, a hook before saving that’s empty by default but can add validation.”

Q: “How does Template Method relate to SOLID principles?”

A:

“Template Method supports Open/Closed Principle - the algorithm structure is closed for modification but open for extension via abstract methods. It can violate Single Responsibility if the base class does too much. It demonstrates Dependency Inversion at the method level - the high-level algorithm depends on abstractions (abstract methods), not concrete implementations.”

Interview Checklist

Before your interview, make sure you can:

• Define Template Method Pattern clearly
• Explain the difference from Strategy Pattern
• Describe the Hollywood Principle
• Implement Template Method from scratch
• List benefits and trade-offs
• Explain hooks vs abstract methods
• Give 2-3 real-world examples
• Discuss when NOT to use it
• Connect to SOLID principles

Final Interview Tip

Practice implementing a data processing pipeline! Show how different data sources (CSV, JSON, DB) can share common processing logic while customizing extraction. This is a great interview example!

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Remember: Template Method Pattern is about defining the skeleton of an algorithm - let subclasses fill in the details without changing the structure! 📋