Lesson 2
Refactoring in Kotlin: Extract, Rename, and Substitute Techniques
Introduction

Hello, Explorer! Today is about refactoring. Consider it as organizing your favorite toys in the toybox. We're going to learn about the Extract Function, Rename Function, and Substitute Algorithm refactorings. Refactoring helps us make our code cleaner and neater while keeping the functionality the same!

Refactoring Overview

Imagine having a complex map. Refactoring transforms it into simpler directions. Our code gets rearranged to make it more readable and efficient without altering what it does. Let's consider a small code snippet before and after refactoring:

Kotlin
1// Before refactoring
2fun calculate(total: Double, quantity: Int): Double {
3    val price = total / quantity
4    val tax = price * 0.2
5    val totalPrice = price + tax
6    return totalPrice
7}
8
9// After refactoring
10fun calculateTotalPrice(total: Double, quantity: Int): Double {
11    val price = calculatePrice(total, quantity)
12    val tax = calculateTax(price)
13    return price + tax
14}
15
16fun calculatePrice(total: Double, quantity: Int): Double {
17    return total / quantity
18}
19
20fun calculateTax(price: Double): Double {
21    return price * 0.2
22}

Both versions of the code do the same thing, but the latter is simpler and easier to understand!

Understanding the Extract Function

Imagine a large recipe for a complete breakfast. The Extract Function technique is like having separate recipes for eggs, toast, coffee, etc., instead of one large recipe. Take a look at this code:

Kotlin
1// Before refactoring
2fun greetUser(username: String): String {
3    val cleanUsername = username.trim().lowercase() // Prepare the username
4    val message = "Hello, $cleanUsername!" // Prepare the message
5    return message // Return the prepared message
6}
7
8// After refactoring
9fun cleanUsername(username: String): String {
10    return username.trim().lowercase() // Returns a cleaned version of the username
11}
12
13fun greetUser(username: String): String {
14    val cleanUsername = cleanUsername(username) // Clean the username
15    val message = "Hello, $cleanUsername!" // Prepare and return the message
16    return message
17}

Here, we moved the username preparation from greetUser into its own function cleanUsername. Nice and tidy!

Using Rename Function

Clear function names make it easy to understand our code, just as clear street names make navigating a city more accessible. Let's have a look at renaming a function:

Kotlin
1// Before refactoring
2fun fx(x: Double): Double {
3    return 3.14 * (x * x) // Calculates a value that is pi times the square of x
4}
5
6// After refactoring
7fun calculateCircleArea(radius: Double): Double {
8    return 3.14 * (radius * radius) // Calculates the area of a circle with a given radius
9}

Renaming the function fx to calculateCircleArea makes it easier to understand its purpose.

Discovering Substitute Algorithm

Substitute Algorithm involves replacing a part of a code (an algorithm) with a simpler one, analogous to discovering a faster route to school. Here's an example:

Kotlin
1// Before refactoring
2fun findSmallest(numbers: IntArray): Int {
3    var smallest = Int.MAX_VALUE
4    for (num in numbers) {
5        if (num < smallest) {
6            smallest = num
7        }
8    }
9    return smallest
10}
11
12// After refactoring
13fun findSmallest(numbers: IntArray): Int? {
14    return numbers.minOrNull() // Returns the smallest number from 'numbers'
15}

Just like the minOrNull() function in Kotlin, it performs the same job as our previous function, but with less code.

Lesson Summary

Great work! We've learned how to use the Extract Function, Rename Function, and Substitute Algorithm to keep our code clean and efficient. Now, it's time for some hands-on practice with real examples. Remember, practice makes perfect. Let's do some refactoring!

Enjoy this lesson? Now it's time to practice with Cosmo!
Practice is how you turn knowledge into actual skills.