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#31DaysOfKotlin — Week 1 Recap

Florina Muntenescu
Apr 3, 2018 · 4 min read

Day 1: Elvis operator

Handling nulls in style? Check out the elvis operator ?:, to cut your “null-erplate”. It’s just a small bit of syntax sugar to replace nulls with a default value or even return! Docs: Elvis operator.

val name: String = ?: “unknown”
val age = person.age ?: return

Day 2: String templates

Formatting Strings? Refer to variables and expressions in string literals by putting $ in front of the variable name. Evaluate expressions using ${expression}. Docs: string templates.

val language = “Kotlin”// “Kotlin has 6 characters”
val text = “$language has ${language.length} characters”

Day 3: Destructuring declarations

Now with prisms? Android KTX uses destructuring to assign the component values of a color. You can use destructuring in your classes, or extend existing classes to add destructuring. Docs: destructuring declarations.

// now with prisms
val (red, green, blue) = color
// destructuring for squares
val (left, top, right, bottom) = rect
// or more pointedly
val (x, y) = point

Day 4: When expressions

A switch statement with superpowers? Kotlin’s when expression can match on just about anything. Literal values, enums, ranges of numbers. You can even call arbitrary functions! Docs: when

class Train(val cargo: Number?) {
override fun toString(): String {
return when (cargo) {
null, 0 -> "empty"
1 -> "tiny"
in 2..10 -> "small"
is Int -> "big inty"
else -> "$cargo"

Day 5: For loops, range expressions and destructuring

For loops get superpowers when used with two other Kotlin features: range expressions and destructuring. Docs: ranges, destructuring.

// iterating in the range 1 to 100
for(i in 1..100) {…}
// iterating backwards, in the range 100 to 1
for(i in 100 downTo 1){…}
// iterating over an array, getting every other element
val array = arrayOf(“a”, “b”, “x”)
for(i in 1 until array.size step 2 ){…}
// iterating over an array with the item index and destructuring
for((index, element) in array.withIndex()) {…}
// iterating over a map
val map = mapOf(1 to “one”, 2 to “two”)
for( (key, value) in map){…}

Day 6: Properties

In Kotlin, classes can have mutable and read-only properties, with getters and setters generated by default. You can also implement custom ones if required. Docs: properties.

class User {
// properties
val id: String = “” // immutable. just getter
var name: String = “” // default getter and setter var surname: String = “” // custom getter, default setter
get() = surname.toUpperCase() // custom getter declaration
var email: String = “” // default getter, custom setter
set(value) { // custom setter declaration
// “value” = name of the setter parameter
// “field” = property’s backing field; generated
if(isEmailValid(value)) field = value

Day 7: Data classes and equality

Creating classes with one role: to hold data? Mark them as “data” classes. The default implementation of equals() is generated (so are hashCode(), toString(), and copy()) and checks for structural equality. Docs: data classes, equality

data class User(
val name: String,
val email: String,
val address: Address,

public class UserListDiffCallback: DiffUtil.Callback() { override fun areContentsTheSame(
oldItemPosition: Int,
newItemPosition: Int
): Boolean {
// use the generated equals method
return newUserList[newItemPosition] ==

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