Template Method Pattern in Kotlin

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 while 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, yet 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 Kotlin . 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 a 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. Kotlin abstract class DataParserTemplate {// Template method defining the steps of the algorithm fun parseDataAndGenerateOutput() {openFile() readData() processData() writeData()} // Default implementation for opening a file (common to all subclasses) protected fun openFile() {println("Opening file for data parsing.")} // Abstract methods to be implemented by subclasses abstract fun readData() abstract fun processData() abstract fun writeData()} abstract class DataParserTemplate {// Template method defining the steps of the algorithm fun parseDataAndGenerateOutput() {openFile() readData() processData() writeData()} // Default implementation for opening a file (common to all subclasses) protected fun openFile() {println("Opening file for data parsing.")} // Abstract methods to be implemented by subclasses abstract fun readData() abstract fun processData() abstract fun 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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Kotlin

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

    // Default implementation for opening a file (common to all subclasses)
    protected fun openFile() {
        println("Opening file for data parsing.")
    }

    // Abstract methods to be implemented by subclasses
    abstract fun readData()
    abstract fun processData()
    abstract fun writeData()
}

Kotlin

class CSVDataParser : DataParserTemplate() {
    override fun readData() {
        println("Reading data from a CSV file.")
    }

    override fun processData() {
        println("Processing CSV data.")
    }

    override fun writeData() {
        println("Writing data to a CSV file.")
    }
}

class CSVDataParser : DataParserTemplate() {
    override fun readData() {
        println("Reading data from a CSV file.")
    }

    override fun processData() {
        println("Processing CSV data.")
    }

    override fun writeData() {
        println("Writing data to a CSV file.")
    }
}

Kotlin

class XMLDataParser : DataParserTemplate() {
    override fun readData() {
        println("Reading data from an XML file.")
    }

    override fun processData() {
        println("Processing XML data.")
    }

    override fun writeData() {
        println("Writing data to an XML file.")
    }
}

class XMLDataParser : DataParserTemplate() {
    override fun readData() {
        println("Reading data from an XML file.")
    }

    override fun processData() {
        println("Processing XML data.")
    }

    override fun writeData() {
        println("Writing data to an XML file.")
    }
}

Kotlin

fun main() {
    val csvParser: DataParserTemplate = CSVDataParser()
    csvParser.parseDataAndGenerateOutput()

    val xmlParser: DataParserTemplate = XMLDataParser()
    xmlParser.parseDataAndGenerateOutput()
}

/*
Expected Output:
Opening file for data parsing.
Reading data from a CSV file.
Processing CSV data.
Writing data to a CSV file.
Opening file for data parsing.
Reading data from an XML file.
Processing XML data.
Writing data to an XML file.
*/

fun main() {
    val csvParser: DataParserTemplate = CSVDataParser()
    csvParser.parseDataAndGenerateOutput()

    val xmlParser: DataParserTemplate = XMLDataParser()
    xmlParser.parseDataAndGenerateOutput()
}

/*
Expected Output:
Opening file for data parsing.
Reading data from a CSV file.
Processing CSV data.
Writing data to a CSV file.
Opening file for data parsing.
Reading data from an XML file.
Processing XML data.
Writing data to an XML file.
*/

Kotlin

class CSVDataParser : DataParserTemplate() {
    override fun readData() {
        println("Reading data from a CSV file.")
    }

    override fun processData() {
        println("Processing CSV data.")
    }

    override fun writeData() {
        println("Writing data to a CSV file.")
    }
}

Kotlin

class XMLDataParser : DataParserTemplate() {
    override fun readData() {
        println("Reading data from an XML file.")
    }

    override fun processData() {
        println("Processing XML data.")
    }

    override fun writeData() {
        println("Writing data to an XML file.")
    }
}

Kotlin

fun main() {
    val csvParser: DataParserTemplate = CSVDataParser()
    csvParser.parseDataAndGenerateOutput()

    val xmlParser: DataParserTemplate = XMLDataParser()
    xmlParser.parseDataAndGenerateOutput()
}

/*
Expected Output:
Opening file for data parsing.
Reading data from a CSV file.
Processing CSV data.
Writing data to a CSV file.
Opening file for data parsing.
Reading data from an XML file.
Processing XML data.
Writing data to an XML file.
*/

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