Kotlin Extensions
Add new functionality to existing classes without inheritance
🔧 What are Kotlin Extensions?
Extension functions allow you to add new functionality to existing classes without modifying their source code. They're like adding superpowers to classes you can't change directly.
// Add a function to String class
fun String.isEmail(): Boolean {
return this.contains("@") && this.contains(".")
}
// Usage
val email = "[email protected]"
println(email.isEmail()) // true
Types of Extensions
Extension Functions
Add new methods to existing classes
fun Int.isEven(): Boolean = this % 2 == 0Extension Properties
Add computed properties to classes
val String.wordCount: Int
get() = this.split(" ").sizeNullable Extensions
Extend nullable types safely
fun String?.isNullOrEmpty(): Boolean =
this == null || this.isEmpty()Generic Extensions
Work with any type using generics
fun T.println(): T {
println(this)
return this
} 🔹 Basic Extension Functions
Create your first extension function:
// Extend String class
fun String.removeSpaces(): String {
return this.replace(" ", "")
}
// Extend Int class
fun Int.squared(): Int {
return this * this
}
// Usage examples
fun main() {
val text = "Hello World"
println(text.removeSpaces()) // "HelloWorld"
val number = 5
println(number.squared()) // 25
}Output:
HelloWorld
25
🔹 Extension Properties
Add computed properties to existing classes:
// Extension property for String
val String.lastChar: Char
get() = this[this.length - 1]
// Extension property for List
val List.secondOrNull: T?
get() = if (this.size >= 2) this[1] else null
// Usage
fun main() {
val name = "Kotlin"
println("Last character: ${name.lastChar}") // n
val numbers = listOf(1, 2, 3, 4, 5)
println("Second element: ${numbers.secondOrNull}") // 2
} Output:
Last character: n
Second element: 2
🔹 Practical Examples
Real-world extension functions:
// Date formatting extension
fun Long.toDateString(): String {
val date = java.util.Date(this)
val format = java.text.SimpleDateFormat("yyyy-MM-dd")
return format.format(date)
}
// Collection utilities
fun List.second(): T = this[1]
fun List.secondOrNull(): T? = if (size >= 2) this[1] else null
// String validation
fun String.isValidEmail(): Boolean {
return this.matches(Regex("[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}"))
}
// Usage
fun main() {
val timestamp = System.currentTimeMillis()
println("Date: ${timestamp.toDateString()}")
val fruits = listOf("apple", "banana", "cherry")
println("Second fruit: ${fruits.second()}")
val email = "[email protected]"
println("Valid email: ${email.isValidEmail()}")
}