Extension Functions

Extensions let you add functions and properties to a type without modifying its source — even types you do not own, like String or List.

Extension functions

fun String.isPalindrome(): Boolean {
    val clean = lowercase().filter { it.isLetterOrDigit() }
    return clean == clean.reversed()
}

"Level".isPalindrome()        // true
"hello".isPalindrome()        // false

Inside the function, this refers to the receiver (the String). You can omit this, as with lowercase() above.

Extension properties

val String.firstWord: String
    get() = substringBefore(" ")

"hello world".firstWord       // "hello"

Extension properties cannot have backing fields, so they must be computed.

`infix` functions

An infix function can be called without the dot and parentheses:

infix fun Int.clampedTo(max: Int): Int = if (this > max) max else this

10 clampedTo 5                // 5
3 clampedTo 5                 // 3

(to, used to build map pairs, is itself an infix extension.)

`vararg` and local helpers, revisited

Extensions combine well with other features:

fun <T> List<T>.second(): T = this[1]

fun StringBuilder.appendLineIf(condition: Boolean, line: String) {
    if (condition) appendLine(line)
}

How extensions work

Extensions are resolved statically by the declared type of the receiver — they are syntactic sugar for a function that takes the receiver as its first argument. They do not actually modify the class or allow overriding its members.

Exercises

Write an extension Int.isEven(): Boolean.
Write String.isPalindrome(): Boolean that ignores case and non-letters (so "A man, a plan, a canal: Panama" is a palindrome).

Solution

   fun String.isPalindrome(): Boolean {
       val clean = lowercase().filter { it.isLetterOrDigit() }
       return clean == clean.reversed()
   }

</details>

This solution is in examples/core/part3/ and is tested by CI.

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Extension Functions