Kotlin stdlib "let", "run", "apply", "also", "use", "with"

README.md

[TOC]

Introduction

There are very good five stdlib operates we can use.

1. let
2. run
3. apply
4. also
5. use
6. with

All of them are very interesting and useful!!

Comparison

	Connect Type	Parameter	Return	Enviroment	Comment
let	extended function	it	last line	1. Check null pointer 2. Unwrap nullable variable
run	extended function ordinary function	this	last line	1. Block context 2. Real useful operator	You can image that "run" is "let" without parameter basically.
also	extended function	it	called object	1. Builder setting 2. Context switch
apply	extended function	this	called object	1. Builder setting 2. Context switch
use	extended function	it	last line	1. try-with-resource
with	extended function	this	last line		with(x) and x.apply are almost same. "with" can't use chain pattern and return difference date type.

From Kotlin 1.1

also is the same as apply, just, also's parameter was changed to it.

Example

let

// 1. Check null pointer.
var prefs: Preferenens? = getSharedPreferences()
prefs?.let {
    // This whole block will not be executed if "prefs" is null.
    // Additionally, "it" has now been cast to a "Preferenens".
    it.putString("name", "Jieyi")
    it.commit()
}
// Oppsite is when prefs is null
prefs?:let {
    // What thing you should handle.
}

// 2. Unwrap null-safe
val filePath: String? = null
val drawable: Drawable = filePath?.let {
    // This block is null-safe.
    BitmapDrawable.createFromPath(it)
} ?: ColorDrawable(Color.TRANSPARENT)

run

fun max(numbers: Array<Int>?): Int = numbers?.run {
    var max: Int? = null
    forEach { max = Math.max(it, max) }
    return max ?: 0
} ?: 0

fun max(numbers: Array<Int>?): Int = numbers.orEmpty().run {
    var max: Int? = null
    forEach { max = Math.max(it, max) }
    return max ?: 0
}

apply & also

Basically, apply & also are the same. But inside of the block, this and it are used by apply and also.

// 1. As like a builder to be used.
val view: View = View(this).apply {
    this.background = ColorDrawable(Color.TRANSPARENT)
    this.alpha = 1f
    this.isClickable = true
}
// also is from kotlin 1.1
val view: View = View(this).also {
    it.background = ColorDrawable(Color.TRANSPARENT)
    it.alpha = 1f
    it.isClickable = true
}

// 2. Context switch.
class ExampleUnitTest {
    @Test fun addition_isCorrect() {
        ColorDrawable(Color.RED).apply {
            // Check "this" is Drawable.
            assert(this is Drawable)
        }
    }
}

use

// 1. Write a file.
File("somefile.txt").bufferedWriter().use { out ->
    history.forEach {
        out.write("${it.key}, ${it.value}\n")
    }
}

// Other example.
val prop = Properties()
FileInputStream("config.properties").use {
    prop.load(it)
}
// FileInputStream automatically closed

with

val w = Window()
with(w) {
  setWidth(100)
  setHeight(200)
  setBackground(RED)
}