Welcome to the third lesson of our course, where we explore the concept of the Parameter Object to address a common code smell: complex function signatures. Throughout this course, we work on eliminating code smells to enhance the readability, maintainability, and scalability of your codebase, with a strong emphasis on Test Driven Development (TDD).

In our previous lessons, we addressed code smells such as duplicated code and long methods using the Extract Method technique. Today, we confront the code smell associated with long parameter lists and introduce the Parameter Object as an effective solution. We will utilize Kotlin's object-oriented capabilities and use JUnit for testing. Understanding and mastering the TDD cycle — Red, Green, Refactor — will be critical as we work to eliminate code smells in this lesson.

Long parameter lists are considered a code smell because they complicate method signatures, making the code harder to read, maintain, and test. This complexity arises from several issues:

1. Readability: When a method has too many parameters, it becomes challenging to understand what each parameter represents without referring to the documentation or method implementation. It clutters the method definition and makes the code less intuitive.

2. Maintainability: Modifying a method with a long parameter list becomes cumbersome. Adding or removing parameters can lead to errors in existing method calls across the codebase, increasing the risk of bugs.

3. Error-Prone: It's easy to mix up or misorder parameters, especially when their types are similar. Even with type-checking, logical errors can occur if parameters are passed in the wrong order or are misunderstood.

4. Testing Challenges: Long parameter lists make writing and maintaining tests more difficult, as test cases need to supply many arguments. This complexity can discourage thorough testing and make tests brittle against changes.

5. Lack of Cohesion: A long list of parameters may indicate that not all parameters are related or that the method is doing too much, violating the single responsibility principle.

By recognizing long parameter lists as a code smell and refactoring them into a Parameter Object, we can improve how we work with and maintain our code, making it more robust and easier to understand.

Let's start by identifying the problematic long parameter list in the existing code. Consider the processExamScore function in ExamProcessor.kt. The function has numerous parameters, making it difficult to read and error-prone when changes are made.

Kotlin
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1fun processExamScore(
2    examScore: Double,
3    isHomeworkComplete: Boolean,
4    attendanceScore: Int,
5    bonusActivities: List<String>,
6    examWeight: Double,
7    homeworkWeight: Double,
8    attendanceWeight: Double,
9    bonusPointsPerActivity: Int,
10    passingThreshold: Double,
11    isRetake: Boolean,
12    courseCode: String
13): ExamResult {
14    // Function implementation...
15}

This complexity can lead to various issues, such as incorrectly ordered parameters during function calls and increased difficulty when refactoring or adding new features.

Additionally, these long parameter lists make the testing process cumbersome, as seen in ExamProcessorTest.kt.

Kotlin
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1class ExamProcessorTest {
2
3    @Test
4    fun calculatesMaximumScoreWithCompletedHomework() {
5        val examProcessor = ExamProcessor()
6        val result = examProcessor.processExamScore(
7            85.0,
8            true,
9            3,
10            listOf("extra_credit", "participation"),
11            0.7,
12            0.1,
13            0.1,
14            2,
15            75.0,
16            false,
17            "MATH101"
18        )
19
20        assertEquals(100, result.finalScore)
21    }
22}

Notice how difficult it is to read this test! What do the values 0.7, 0.1, 2, and 75.0 even mean?

Kotlin
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1data class ScoreWeights(
2    val examWeight: Double,
3    val homeworkWeight: Double,
4    val attendanceWeight: Double
5)
6
7data class CoursePolicy(
8    val bonusPointsPerActivity: Int,
9    val passingThreshold: Double
10)
11
12fun processExamScore(
13    courseCode: String,
14
15    // Individual Performance
16    examScore: Double,
17    isHomeworkComplete: Boolean = true,
18    attendanceScore: Int = 3,
19    bonusActivities: List<String> = listOf(),
20    isRetake: Boolean = false,
21
22    // Configuration
23    scoreWeights: ScoreWeights = getDefaultScoreWeights(),
24    coursePolicy: CoursePolicy = getDefaultCoursePolicy()
25): ExamResult {
26    // Implementation
27}

Notice the following:

• Re-ordering parameters: Parameters have been reorganized in the function signature to group related values, enhancing readability and logical flow. For instance, parameters related to the course information are grouped together, separate from those related to individual performance or scoring configurations.

• Parameter Objects: The function has been refactored to utilize two parameter objects (ScoreWeights and CoursePolicy), which encapsulate multiple related parameters into a single cohesive object. This reduces complexity in the function signature and clarifies its intent.

• Defaults: Default values have been assigned to certain parameters, simplifying function calls and reducing potential errors.

The refactored test example below demonstrates improved readability by eliminating the need to decipher multiple unordered parameters:

Kotlin
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1class ExamProcessorTest {
2
3    @Test
4    fun calculatesMaximumScoreWithCompletedHomework() {
5        val examProcessor = ExamProcessor()
6        val result = examProcessor.processExamScore("MATH101", 100.0)
7
8        assertEquals(100, result.finalScore)
9    }
10}

This clarity is achieved by utilizing a simplified method call, where essential parameters like courseCode and examScore are explicitly specified, making it immediately clear what values are being tested. This concise form minimizes cognitive load, reduces potential errors related to parameter misordering, and increases maintainability, ultimately making the test more intuitive and focused on its intent.

Parameter Objects are exceptionally useful in real-world scenarios where methods require multiple related parameters. For example, consider an e-commerce platform where you handle payment and shipping details through parameter objects, simplifying both method calls and testing.

However, it's crucial to acknowledge scenarios where alternatives like optional parameters or builder patterns might be more effective, especially if parameters don't naturally form a cohesive group.

In this lesson, we've learned to refactor long parameter lists by introducing Parameter Objects:

• Recognized issues with long parameter lists.
• Created a Parameter Object to bundle related parameters, simplifying function signatures.
• Re-organized the parameters for clarity.
• Introduced defaults.
• Utilized Kotlin data classes and constructors for object organization.
• Updated tests to accommodate the refactored design.

Prepare to apply these techniques in the upcoming exercises, where you'll enhance your skills in refactoring and testing with Parameter Objects. These exercises will enable you to solidify your understanding of the TDD workflow — Red, Green, Refactor — as you work toward maintaining a robust and scalable codebase.