Template Method Pattern

Hello and welcome to the lesson on the Template Method pattern! This pattern is an integral part of Behavioral Design Patterns, focusing on defining the skeleton of an algorithm and allowing subclasses to refine certain steps without changing the algorithm's structure. It's quite common in frameworks and libraries where a series of steps must follow a specific order, but each step can differ based on the context.

What You Will Learn

By the end of this lesson, you will: Gain a solid understanding of the Template Method pattern . Learn how to implement and use this pattern in Java . Understand the advantages of separating constant and variable parts of an algorithm.

Implementing the Template Method Pattern

The Template Method pattern defines a skeleton for an algorithm within a method, outlining the sequence of steps while allowing subclasses to implement specific steps. This ensures consistent algorithm flow while enabling variations in individual steps, promoting code reuse and flexibility. To illustrate, consider data parsing from different file formats. We start with an abstract class, DataParserTemplate, defining the workflow in parseDataAndGenerateOutput . This method includes concrete steps common to all subclasses and abstract methods for steps that differ. Subclasses like CSVDataParser and XMLDataParser implement these abstract methods, preserving the overall structure and allowing specific implementations for different file formats.

Step 1: Define the Template Abstract Class

Our first step is to create an abstract class that defines the template method. Java public abstract class DataParserTemplate {// Template method defining the steps of the algorithm public final void parseDataAndGenerateOutput() {openFile(); readData(); processData(); writeData();} // Default implementation for opening a file (common to all subclasses) private void openFile() {System.out.println("Opening file for data parsing.");} // Abstract methods to be implemented by subclasses public abstract void readData(); public abstract void processData(); public abstract void writeData();} public abstract class DataParserTemplate {// Template method defining the steps of the algorithm public final void parseDataAndGenerateOutput() {openFile(); readData(); processData(); writeData();} // Default implementation for opening a file (common to all subclasses) private void openFile() {System.out.println("Opening file for data parsing.");} // Abstract methods to be implemented by subclasses public abstract void readData(); public abstract void processData(); public abstract void writeData();} The parseDataAndGenerateOutput method is the template method. It defines the sequence of steps to parse data and generate output, ensuring a consistent structure across all subclasses. Our abstract class provides a default implementation for openFile, which is common to all subclasses, and declares three abstract methods: readData, processData, and writeData . Subclasses will provide specific implementations for these methods.

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Java

public abstract class DataParserTemplate {
    // Template method defining the steps of the algorithm
    public final void parseDataAndGenerateOutput() {
        openFile();
        readData();
        processData();
        writeData();
    }

    // Default implementation for opening a file (common to all subclasses)
    private void openFile() {
        System.out.println("Opening file for data parsing.");
    }

    // Abstract methods to be implemented by subclasses
    public abstract void readData();
    public abstract void processData();
    public abstract void writeData();
}

Java

public class CSVDataParser extends DataParserTemplate {
    @Override
    public void readData() {
        System.out.println("Reading data from CSV file.");
    }

    @Override
    public void processData() {
        System.out.println("Processing CSV data.");
    }

    @Override
    public void writeData() {
        System.out.println("Writing data to CSV file.");
    }
}

public class CSVDataParser extends DataParserTemplate {
    @Override
    public void readData() {
        System.out.println("Reading data from CSV file.");
    }

    @Override
    public void processData() {
        System.out.println("Processing CSV data.");
    }

    @Override
    public void writeData() {
        System.out.println("Writing data to CSV file.");
    }
}

Java

public class XMLDataParser extends DataParserTemplate {
    @Override
    public void readData() {
        System.out.println("Reading data from XML file.");
    }

    @Override
    public void processData() {
        System.out.println("Processing XML data.");
    }

    @Override
    public void writeData() {
        System.out.println("Writing data to XML file.");
    }
}

public class XMLDataParser extends DataParserTemplate {
    @Override
    public void readData() {
        System.out.println("Reading data from XML file.");
    }

    @Override
    public void processData() {
        System.out.println("Processing XML data.");
    }

    @Override
    public void writeData() {
        System.out.println("Writing data to XML file.");
    }
}

Java

public class Main {
    public static void main(String[] args) {
        DataParserTemplate csvParser = new CSVDataParser();
        csvParser.parseDataAndGenerateOutput();

        DataParserTemplate xmlParser = new XMLDataParser();
        xmlParser.parseDataAndGenerateOutput();
    }
}

public class Main {
    public static void main(String[] args) {
        DataParserTemplate csvParser = new CSVDataParser();
        csvParser.parseDataAndGenerateOutput();

        DataParserTemplate xmlParser = new XMLDataParser();
        xmlParser.parseDataAndGenerateOutput();
    }
}

Java

public class CSVDataParser extends DataParserTemplate {
    @Override
    public void readData() {
        System.out.println("Reading data from CSV file.");
    }

    @Override
    public void processData() {
        System.out.println("Processing CSV data.");
    }

    @Override
    public void writeData() {
        System.out.println("Writing data to CSV file.");
    }
}

Java

public class XMLDataParser extends DataParserTemplate {
    @Override
    public void readData() {
        System.out.println("Reading data from XML file.");
    }

    @Override
    public void processData() {
        System.out.println("Processing XML data.");
    }

    @Override
    public void writeData() {
        System.out.println("Writing data to XML file.");
    }
}

Java

public class Main {
    public static void main(String[] args) {
        DataParserTemplate csvParser = new CSVDataParser();
        csvParser.parseDataAndGenerateOutput();

        DataParserTemplate xmlParser = new XMLDataParser();
        xmlParser.parseDataAndGenerateOutput();
    }
}

Template Method Pattern

What You Will Learn

Implementing the Template Method Pattern

Step 1: Define the Template Abstract Class

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Step 2: Implement Concrete Subclasses

Step 3: Using the Template Method Pattern

Why It Matters