What is the difference between the extension functions let, run, also, and apply in Kotlin?

let: Calls a closure on an object and returns the result of the closure. Allows using the object as an argument in a lambda expression (it). Suitable for working with nullable objects.

// Example of using let
val nullableString: String? = "Hello"
val length = nullableString?.let {
    // it is a non-null String
    it.length
}

run: Executes a block of code on an object and returns the result of the block. Inside the block, the object is accessible as this. Useful for initializing an object and then calling methods.

// Example of using run
val result = buildString {
    // this is a StringBuilder
    append("Start\n")
    append("Middle\n")
}.run {
    // this is a String
    uppercase()
}

also: Executes a closure on an object and returns the object itself. Allows performing additional actions (e.g., logging or configuration) on the object without modifying it.

// Example of using also
val list = mutableListOf("a", "b", "c").also {
    // it is a MutableList
    println("List created: $it")
}

apply: Executes a closure on an object and returns the object itself. Inside the block, the object is accessible as this. Convenient for setting properties of an object.

// Example of using apply
val textView = TextView(context).apply {
    // this is a TextView
    text = "Hello, world!"
    textSize = 16f
    setTextColor(Color.BLACK)
}

Main differences:

Function	How the object is accessed	What it returns	Typical use
let	it	Result of lambda	Nullable objects, transformation
run	this	Result of lambda	Initialization and method calls
also	it	The object itself	Additional actions (side effects)
apply	this	The object itself	Setting properties