Extensions on built-in types in Kotlin - Mini Project: Build & Apply

Extensions on built-in types in Kotlin

📖 Scenario: You are working on a Kotlin project where you want to add new useful functions to existing types without changing their original code. This is common when you want to make your code cleaner and easier to read.

🎯 Goal: You will create an extension function on the String type to count vowels, then use it in a program to show how many vowels a word has.

📋 What You'll Learn

Create an extension function on the String type called countVowels

The countVowels function returns the number of vowels in the string

Use the extension function on a sample string

Print the result

💡 Why This Matters

🌍 Real World

Extension functions let you add new abilities to existing types like strings or numbers. This makes your code cleaner and easier to read, like adding new tools to your toolbox without changing the tools themselves.

💼 Career

Many Kotlin projects use extension functions to improve code quality and maintainability. Knowing how to write and use them is important for Kotlin developers in app development and backend services.

1

Create a sample string

Create a variable called word and set it to the string "Kotlin".

Kotlin

// Create a variable called word and set it to "Kotlin"
// Your code here

Need a hint?

Use val word = "Kotlin" to create the string variable.

2

Create an extension function to count vowels

Create an extension function on String called countVowels that returns an Int. It should count the vowels a, e, i, o, u in the string ignoring case.

Kotlin

val word = "Kotlin"

// Create an extension function countVowels on String
// Your code here

Need a hint?

Use fun String.countVowels(): Int and inside use this.count { it.lowercaseChar() in "aeiou" }.

3

Use the extension function on the string

Create a variable called vowelCount and set it to the result of calling countVowels() on the variable word.

Kotlin

val word = "Kotlin"

fun String.countVowels(): Int {
    return this.count { it.lowercaseChar() in "aeiou" }
}

// Call countVowels on word and store in vowelCount
// Your code here

Need a hint?

Use val vowelCount = word.countVowels() to call the extension function.

4

Print the vowel count

Print the text "Number of vowels in Kotlin: " followed by the value of vowelCount using println.

Kotlin

val word = "Kotlin"

fun String.countVowels(): Int {
    return this.count { it.lowercaseChar() in "aeiou" }
}

val vowelCount = word.countVowels()

// Print the number of vowels in word
// Your code here

Need a hint?

Use println("Number of vowels in Kotlin: $vowelCount") to show the result.