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.

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.

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.

Our first step is to create an abstract class that defines the template method.

Kotlin
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1abstract class DataParserTemplate {
2    // Template method defining the steps of the algorithm
3    fun parseDataAndGenerateOutput() {
4        openFile()
5        readData()
6        processData()
7        writeData()
8    }
9
10    // Default implementation for opening a file (common to all subclasses)
11    protected fun openFile() {
12        println("Opening file for data parsing.")
13    }
14
15    // Abstract methods to be implemented by subclasses
16    abstract fun readData()
17    abstract fun processData()
18    abstract fun writeData()
19}

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.

Now, let's create concrete classes that implement the abstract methods for different file formats.

Kotlin
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1class CSVDataParser : DataParserTemplate() {
2    override fun readData() {
3        println("Reading data from a CSV file.")
4    }
5
6    override fun processData() {
7        println("Processing CSV data.")
8    }
9
10    override fun writeData() {
11        println("Writing data to a CSV file.")
12    }
13}

CSVDataParser extends DataParserTemplate and provides concrete implementations for readData, processData, and writeData, specifically for handling operations related to CSV files.

Kotlin
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1class XMLDataParser : DataParserTemplate() {
2    override fun readData() {
3        println("Reading data from an XML file.")
4    }
5
6    override fun processData() {
7        println("Processing XML data.")
8    }
9
10    override fun writeData() {
11        println("Writing data to an XML file.")
12    }
13}

Similarly, XMLDataParser implements the abstract methods for handling XML files, ensuring that each step is tailored to processing XML data correctly.

Finally, let's see how to use these classes in a real-world scenario.

Kotlin
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1fun main() {
2    val csvParser: DataParserTemplate = CSVDataParser()
3    csvParser.parseDataAndGenerateOutput()
4
5    val xmlParser: DataParserTemplate = XMLDataParser()
6    xmlParser.parseDataAndGenerateOutput()
7}
8
9/*
10Expected Output:
11Opening file for data parsing.
12Reading data from a CSV file.
13Processing CSV data.
14Writing data to a CSV file.
15Opening file for data parsing.
16Reading data from an XML file.
17Processing XML data.
18Writing data to an XML file.
19*/

In the main function, we create instances of CSVDataParser and XMLDataParser. We then call the parseDataAndGenerateOutput method on each instance to demonstrate how each subclass processes its specific file format, showcasing the flexibility and consistency provided by the Template Method pattern.

The Template Method pattern brings several advantages in software design:

1. Code Reuse: By putting the unchanged parts of the algorithm in the template method and the variable parts in abstract methods, you promote code reuse.
2. Flexibility: Subclasses can redefine certain steps without altering the overall algorithm's structure.
3. Maintainability: Any changes in the invariant steps of the algorithm only need to be made in the template method, reducing the risk of errors.

The Template Method pattern is particularly useful in designing libraries and frameworks, where consistent behavior with customizable variations is crucial.

Excited to practice? Now it's your turn to implement the Template Method pattern. Let's get started and see how you can apply this powerful design pattern to real-world scenarios.