This document provides an overview of Kotlin and best practices for building apps with Kotlin. Some key points: - Kotlin is a statically typed language that runs on the JVM and is fully interoperable with Java. It adds features like null safety, lambdas, and concise syntax. - Kotlin is perfect for Android development as it is compiled to Java bytecode and has full support for Android APIs. The migration process from Java to Kotlin is gradual. - Best practices for Kotlin include using features like data classes, when expressions, higher order functions, and Android extensions to write more concise and readable code compared to Java.

1. Practical tips for building
apps with kotlin
Adit Lal | @aditlal

2. Agenda
What is Kotlin?
Perfect for Android
Migration guide
Best Practices
Summary - QnA
Building with kotlin

3. 100 % interoperable
with Java
Developed by
JetBrains
Background

4. What is Kotlin?
Statically typed programming language for
the JVM, Android and the browser

6. OOP
language
with
functional
aspects
Modern and
powerful
language
Safe
concise &
readable
code
First class tool
support
What is Kotlin?

7. Kotlin
• Null safety
• Conciseness
• Lambdas
• Higher order functions
• Mutability protection
• Static typing + Smart Casts
We will cover :

8. Kotlin
Kotlin works on same byte code

9. Kotlin - Syntax
• Types follow variable/function names
• Functions start with fun keyword
• Default constructor in class signature
• Semicolons not required

10. Kotlin - Syntax
private fun sum(first: Int , second: Int): Int
{
return first + second
}
Access modifier
Keyword
Function name
Param name
Param type
Return type

11. Kotlin - Syntax
// Omit access modifier
fun sum(a: Int , b: Int): Int {
return a + b
}
// Inline return
fun sum(a: Int , b: Int): Int = a + b
// Omit return type
fun sum(a: Int , b: Int) = a + b

12. Kotlin - Syntax
val name: String ="John" // final
var name: String ="John" //Mutable
ex: name= "Johnny"
Two types of variables
val name ="John" // Types are auto-inferred

13. Kotlin - Syntax
//java
String sName = person.getSecretName()
//kotlin
person.secretName = “Batman”
var fName = person.secretName

14. Kotlin - Null Safety
NULLS ARE PART OF THE TYPE SYSTEM

15. Kotlin - Null Safety
Types default to non-nullable
Mark nullable with the addition of a ?
var a: String = "abc"
a = null
var b: String? = "abc"
b = null
var l = a.length
var l = b.length
// compilation error
// OK
// error: variable 'b' can be null

16. Kotlin - Null Safety
Helpers
Safe accessor
?
Elvis operator
?:

17. val toDisplay: String = serverResponse
Kotlin - Null
// if response is null, we'll display a default
message
val toDisplay: String = response ?: defaultMessage

18. Kotlin - Null
// cascading null safety
val maybeName: String? =
maybeNullValue?.someProperty?.name
val maybeNullValue: MyValue? = /*..*/

19. Kotlin - Null
// safe operation
maybeNullValue?.let {
//executed if not null
}
val maybeNullValue: MyValue? = /*..*/

20. Casting
if (obj instanceOf MyType) {
value =
((MyType)obj).getXValue();
} else {
// handle else
}

21. Kotlin - Smart casting
if (obj is MyType) {
value = obj.x
} else {
// handle else
}

22. Class
// Wont Compileclass Student : Person()
class Person

23. open class Person
Class
//It works nowclass Student : Person()

24. Higher order functions
Functions that can take functions as
arguments and also return function as the
output

25. fun <T> forEach(list: List<T>, funcToCall: (T) -> Unit) {
for (elm in list) {
funcToCall(elm)
}
}
val list = listOf("Alice", "Bob", "Claire")
forEach(list, ::println)
forEach(list, { str -> println(str) })
Higher order functions

26. Kotlin - Features
… and More
• Visibility Modifiers
• Companion Objects
• Nested, Sealed Classes
• Generics
• Coroutines
• Operator overloading
• Exceptions
• Annotations
• Reflection
• and more

27. Kotlin - Features
… and Even More
• Infix extension methods
• Interfaces
• Interface Delegation
• Property Delegation
• Destructuring
• Safe Singletons
• Init blocks
• Enums
• Multiline Strings
• Tail recursion

28. Perfect for
• Versatile language
• Complete interop with Java
• Compact runtime
• Enhance development speed with
less code

29. “But I’m comfortable and
experienced with Java.”
Common Concerns

30. “I don’t want to convert my entire
codebase to a new language.”
Common Concerns

31. •Compiled to byte code (like Java)
•No impact on performance
•Some Kotlin code faster
•Lambdas that can be inlined
•Built-in operations faster than DIY implementations
Kotlin

32. How to migrate
Start in small steps -
Convert a Java / Android app (side project)
Try the kotlin plugin converter
Understand how code is converted from Java to Kotlin
Experiment with kotlin features and tweak code

33. •Add Gradle dependencies (plugin, runtime, etc.)
•Start writing .kt files instead of .java ones
•No need to migrate everything at once
•Kotlin classes can co-exist with Java ones
• IntelliJ has a Java-to-Kotlin converter
•Not perfect but good start
• Works with pasted code
Try kotlin

34. Java>Kotlin

35. class HomeActivity : AppCompatActivity() {
override fun onCreate(b: Bundle?) {
super.onCreate(b)
setContentView(R.layout.activity_main)
val view = findViewById<View>(R.id.view)
view.alpha = 1f
}
}
Android & Kotlin

36. class HomeActivity : AppCompatActivity() {
override fun onCreate(b: Bundle?) {
super.onCreate(b)
setContentView(R.layout.activity_main)
val view = findViewById<View>(R.id.view)
view.alpha = 1f
}
}
Android & Kotlin

37. • View binding (like Butter Knife)
• No instance variables required
• How?
Android extensions
apply plugin: 'kotlin-android-extensions'
• Import synthetic layout
import kotlinx.android.synthetic.main..*
• Use view by ID
E.g. toolbar.title = "Home"
• Under the hood: synthetic calls replaced
by functions

38. class HomeActivity : AppCompatActivity() {
override fun onCreate(b: Bundle?) {
super.onCreate(b)
setContentView(R.layout.activity_main)
val view = findViewById<View>(R.id.view)
view.alpha = 1f
}
}
Android & Kotlin

39. Android extensions
class HomeActivity : AppCompatActivity() {
override fun onCreate(b: Bundle?) {
super.onCreate(b)
setContentView(R.layout.activity_main)
val view = findViewById<View>(R.id.view)
view.alpha = 1f
}
}

40. Android extensions
import kotlinx.android.synthetic.activity_main.*
class HomeActivity : AppCompatActivity() {
override fun onCreate(b: Bundle?) {
super.onCreate(b)
setContentView(R.layout.activity_main)
view.alpha = 1f
}
}

41. view.setOnClickListener{ toast(“Hello") }
Concise
view.setOnClickListener(object :
View.OnClickListener {
override fun onClick(v: View?) {
toast("Hello")
}
})
view.setOnClickListener{
view -> doSomethingWithView()
}

42. Concise
fun Context.inflate(
res:Int,
parent:ViewGroup? = null) : View {
return LayoutInflater.from(this)
.inflate(res, parent, false)
}
activity.inflate(R.layout.my_layout, container)
//Fragment

43. public class Person {
private String name;
private String surname;
private String id;
/*
Setters and getters - 20 lines
*/
@Override public boolean equals(Object o) {
//another few lines here
}
@Override public int hashCode() {
//some more lines
}
@Override public String toString() {
//some more
}
}
Data classes

44. Kotlin -
data class Person(
val name: String,
var surname: String = "",
val id: String
)
Data classes

45. Kotlin - Data classes
equals() & hashCode()
toString()
copy() function

46. fun evaluateObject(obj: Any): String {
return when(obj){
is String -> "Ahoy !!"
is Int -> "Lucky no is $obj"
else -> "Object is in space"
}
}
Kotlin
When expression

47. // Java
if(name.toLowerCase().contains(str))
{
...
}
Android

48. // Kotlin
if (name.contains(str, true))
{
...
}
Android

49. for (i in 1..100) { ... }
for (i in 0 until 100) { ... }
for (i in 2..10 step 2) { ... }
for (i in 10 downTo 1) { ... }
if (x in 1..10) { ... }
Android Kotlin tricks

50. fun ImageView.loadUrl(url: String) {
Glide.with(context).load(url).into(this)
}
ImageLoadingExtensions.kt
Android
//Then
ivAvatar.loadUrl("some_fancy_url")

51. Utils
fun View.visible() {
visibility = View.VISIBLE
}
fun View.gone() {
visibility = View.GONE
}
//call
view.visible()

52. Utils
fun View.setHeight(height: Int) {
val params = layoutParams
params.height = height
layoutParams = params
}
//call
tvTotalPrice.setHeight(50)

53. fun Activity.showOnUiThread(init: Activity.() -> Unit) : Activity {
if (!isFinishing) {
runOnUiThread {
init()
}
}
return this
}
Utils
//call
showOnUiThread(updateUIWithNewData())

54. fun Activity.hideKeyboard(): Boolean {
val view = currentFocus
view?.let {
val imm = getSystemService(INPUT_METHOD_SERVICE)
as InputMethodManager
return imm.hideSoftInputFromWindow(view.windowToken,
HIDE_NOT_ALWAYS)
}
return false
}
Utils

55. Adopting kotlin into
android code

56. class MyActivity : AppCompactActivity(){
// wont be init here
lateinit var obj: CustomObject
override fun onCreate(...){
obj = intent.getExtras().get("key")
// this will init the variable
}
}
lateinit Modifier

57. When to use lateinit
If you’re variable is mutable
If you’re sending a object from another component/
screen — ex Intent extras
lateinit Modifier

58. val myUtil by lazy {
SomeUtil(parameter1, parameter2)
}
Lazy Initialisation

59. val hashMap = HashMap<String, String>()
hashMap.put("keyA", "valueA")
hashMap.put("keyB", "valueB")
hashMap.put("keyC", "valueC")
//change to
val map = mapOf(
"keyA" to "valueA",
"keyB" to "valueB",
"keyC" to "valueC"
)
Kotlin

60. fun listItemsToDisplay(): List<ListItem> {
return listOf(users) + friends() + recentSearches()
}
Kotlin

61. fun parseResponse(response: Response?) =
when (response?.code()) {
null -> throw HTTPException(“Oh no”)
200, 201 -> parse(response.body())
in 400..499 -> throw HTTPException("Invalid request")
in 500..599 -> throw HTTPException("Server error")
else -> throw HTTPException("Error! Code
{response.code()}”)
}
Kotlin

62. Convert .java
Automatic migration might not always result
the best code
//Java
private static final String MY_EXTRA=
"extra_sauce";
//Kotlin
class SomeClass : AppCompatActivity() {
companion object {
}
//Other stuff here
}
private val MY_EXTRA = “extra_sauce”

63. Kotlin
Do this instead
//Kotlin
class SomeClass : AppCompatActivity() {
//Other stuff here
}
private val MY_EXTRA = “extra_sauce”

64. Kotlin
Do this instead
//Kotlin
class SomeClass : AppCompatActivity() {
//Other stuff here
}
private const val MY_EXTRA = “extra_sauce”

65. Kotlin
class SomeClass : AppCompatActivity() {
var someString = “”
var someValue : Payment = /*..*/
override fun onCreate(b: Bundle?) {
super.onCreate(b)
if (someValue != null)
someString = someValue!!.getCost()
}
}

66. Kotlin
class SomeClass : AppCompatActivity() {
var someString = “”
var someValue : Payment
override fun onCreate(b: Bundle?) {
super.onCreate(b)
if (someValue != null)
someString = someValue!!.getCost()
}
}

67. Kotlin
class SomeClass : AppCompatActivity() {
var someString = “”
var someValue : Payment
override fun onCreate(b: Bundle?) {
super.onCreate(b)
someValue?.let{
someValue!!.getCost()
}
}
}

68. run
val password: Password = PasswordGenerator().run {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
generate()
}

69. val generator = PasswordGenerator().apply {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
}
val pasword = generator.generate()
apply

70. class MyFragment : Fragment(){
companion object {
@JvmStatic
fun newInstance(b:Boolean): MyFragment {
val fragment = MyFragment()
val args = Bundle()
args.putBoolean("isValid",b)
fragment.arguments = args
return fragment
}
Fragments

71. Kotlin
fun handleView() {
for (item in list)
if (item is TextView)
item.text = getString(R.string.play)
else
doRandomOperationOn(item)
}

72. Kotlin
fun handleView() {
list.forEach { item ->
when (item) {
is TextView ->
item.text = getString(R.string.play)
else -> doRandomOperationOn(item)
}
}
}

73. class MyRecyclerAdapter(
val items: List,
val listener: (Item) -> Unit
){
….
}
Kotlin
recycler.adapter = MyRecyclerAdapter(items)
{
navigateToDetail(it)
}

74. fun View.fadeOut(duration: Long = 500):
ViewPropertyAnimator {
return animate()
.alpha(0.0f)
.setDuration(duration)
}
Optional params
icon.fadeOut() // fade out with default time (500)
icon.fadeOut(1000) // fade out with custom time

75. Kotlin
inline fun SharedPreferences.edit(
func: SharedPreferences.Editor.()-> Unit) {
val editor = edit()
editor.func()
editor.apply()
}
//Then
preferences.edit {
putString("foo", "bar")
putString("fizz", "buzz")
remove("username")
}

76. CODE
❤ Kotlin
Java to Kotlin
http://bit.ly/2glBYh8
http://bit.ly/KotlinIsAwesome
Koans -
http://bit.ly/KotlinKoans

77. Thank you
https://medium.com/@aditlal
@aditlal
http://bit.ly/devfestKotlin