Practical tips for building apps with kotlin

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

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

100 % interoperable
with Java
Developed by
JetBrains
Background

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

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

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

Kotlin
Kotlin works on same byte code

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

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

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

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

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

Kotlin - Null Safety
NULLS ARE PART OF THE TYPE SYSTEM

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

Helpers
Safe accessor
?
Elvis operator
?:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

•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

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

•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

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

• 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

Android extensions
super.onCreate(b)
val view = findViewById<View>(R.id.view)
view.alpha = 1f
}
}

Android extensions
import kotlinx.android.synthetic.activity_main.*
super.onCreate(b)
view.alpha = 1f
}
}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Adopting kotlin into
android code

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

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

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

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

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

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

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”

Kotlin
Do this instead
//Kotlin
//Other stuff here
}
private val MY_EXTRA = “extra_sauce”

Kotlin
Do this instead
//Kotlin
//Other stuff here
}
private const val MY_EXTRA = “extra_sauce”

Kotlin
var someString = “”
var someValue : Payment = /*..*/
super.onCreate(b)
if (someValue != null)
someString = someValue!!.getCost()
}
}

Kotlin
var someValue : Payment
super.onCreate(b)
if (someValue != null)
someString = someValue!!.getCost()
}
}

Kotlin
var someValue : Payment
super.onCreate(b)
someValue?.let{
someValue!!.getCost()
}
}
}

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

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

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

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

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

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

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

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")
}

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

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

Practical tips for building apps with kotlin

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