Android & Kotlin - The code awakens #03

Android & Kotlin
The code awakens #03

Omar Miatello
Member of GDG Milano (Italy)
Android Developer @ Satispay
Personal profile
google.com/+OmarMiatello
Google+ Community: Kotlin for Android
goo.gl/mUKF1w
Google Presentation
#01 goo.gl/0jHLmE
#02 goo.gl/h3uG8M
#03 goo.gl/hnwvqu
Google Photo
#01 goo.gl/photos/cKP9L6zqZcxDRGzQ8
#02 goo.gl/photos/sXdpkbihCi5xAAnx7
#03 goo.gl/photos/P6kGhLE8yrWYnhAW6

Kotlin 1.0 is out - Read more: goo.gl/5DBQM2

dummy/HeroAdapter.java
public class HeroAdapter extends RecyclerView.Adapter<HeroAdapter.ViewHolder> {
private final List<HeroItem> mValues;
private final HeroOnClickListener mListener;
public HeroAdapter(List<HeroItem> items, HeroOnClickListener listener) {
mValues = items;
mListener = listener;
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_hero, parent, false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(final ViewHolder holder, int position) {
HeroItem item = mValues.get(position);
“Convert Java File to Kotlin File”
CTRL + ALT + SHIFT + K
(or CMD + ALT + SHIFT + K)

Previously on Android & Kotlin #01: goo.gl/0jHLmE
Properties
val a: Int = 1 // val = READ ONLY (getter)
var b: Int = 1 // var = READ/WRITE (getter/setter)
String templates
"My name is $name $surname"
Lambdas
view.setOnClickListener { Log.d("TAG", "Item clicked!") }
Delegated properties (example: lazy)
val item by lazy { MyItem() }

// val = READ ONLY (getter)
// var = READ/WRITE (getter/setter)
val onlyRead = 1
var writable = 2
var nullable: Int? = 3
fun test() {
onlyRead = 3 // Error at compile time
writable = 3
writable = "test" // Error at compile time
writable = null // Error at compile time
nullable = null
}
// Customize getter/setter
val p1: Int = 1
get() {
// add some logic
return field
}
var p2: Int = 2
get() {
return field
}
set(value) {
// add some logic
field = value
}
Properties // More examples

var name = "Omar"
fun test() {
var str: String
str = "My name is $name"
str = "My name is $name (${name.length} chars)"
str = "$name == ${name}"
str = "$name.length != ${name.length}" // IS NOT THE SAME
str = "Time is: ${System.currentTimeMillis()}ms since 1970"
}
String templates // More examples

"qwerty | 123 | abc".filter { char ->
char.isDigit() // return is implicit in lambda expressions
}
"qwerty | 123 | abc".filter {
it.isDigit() // we can use "it" with 1 parameter
}
"qwerty | 123 | abc".filterIndexed { index: Int, c: Char ->
index > 3 && c.isLetterOrDigit()
}
Lambdas // More examples

val startTime by lazy { System.currentTimeMillis() }
var test by Delegates.observable(3) {
prop, old, new ->
if (new > old) Log.w("TEST", "$new > $old")
}
fun test() {
startTime
test = 5
test = 1
test = 2
val finish = System.currentTimeMillis() - startTime
}
Delegated properties // More examples

Previously on Android & Kotlin #02: goo.gl/h3uG8M
Null safety
var a: String = null // Error at compile time
var b: String? = null
Elvis Operator
val example: String = b ?: "Default" // b may be null
Smart-cast
if (myView is TextView) { myView.setText("Ciao") }
Collections
listOf("Android", "iOS", null).filterNotNull().map { it.toUpperCase() }

var test: String? = null
fun test() {
test = "new string"
test.length // Error at compile time
test?.length // if (test != null) test.length else null
test!!.length // test MUST exist or throw NullPointerException
val len = test?.length ?: 0 // default value = 0
test?.length ?: throw IllegalStateException("Missing the main string!")
}
Null safety & Elvis Operator // More examples

var test: Any? = "My new value"
fun test() {
val _test: Any? = test
if (_test != null) {
val checkEqual =
_test.equals("My new value")
if (_test is String) {
val checkLen = _test.length
_test.capitalize()
_test.substring(1)
}
}
when (_test) {
is Int -> {
val sum = _test + 1
}
is String -> {
val len = _test.length
}
}
}
Smart-cast // More examples

val stringList = listOf("my", "items", "list")
fun test() {
stringList.first()
stringList.filterNotNull()
stringList.filter { it.length > 3 }
stringList.first { it.length > 4 }
stringList.firstOrNull() { it.length > 10 }
stringList.findLast { it.startsWith("m") }
stringList.sorted()
stringList.indexOfLast { it.startsWith("i") }
stringList.map { it.length }
stringList.maxBy { it.length }
// ...
}
Collections // More examples

Kotlin vs Java - Part 3
Extensions
Infix Notation
Operator Overloading

fun String.isBig(): Boolean {
return length() > 10
}
#9 Kotlin - Extensions
http://kotlinlang.org/docs/reference/extensions.html
vs

}
fun example() {
"why?!".isBig() // false!
"harder, better, ...".isBig() // true!
}
vs

}
fun example() {
"why?!".isBig() // false!
"harder, better, ...".isBig() // true!
}
// file MyUtils.java
class MyUtils {
static boolean isBig(String str) {
return str.length() > 10;
}
}
// file MyJavaClass.java
class MyJavaClass {
void example() {
MyUtils.isBig("why?!");
MyUtils.isBig("harder, better, ...");
}
}
vs

}
fun example() {
"why?!".isBig()
"harder, better, ...".isBig()
}
// file MyUtils.java
class MyUtils {
static boolean isBig(String str) {
return str.length() > 10;
}
}
// file MyJavaClass.java
class MyJavaClass {
void example() {
MyUtils.isBig("why?!");
MyUtils.isBig("harder, better, ...");
}
}
vs

class Hero(val power: Int) class Hero {
private final int power;
public Hero(int power) {
this.power = power;
}
}
#10 Kotlin - Infix Notation
http://kotlinlang.org/docs/reference/functions.html#infix-notation
vs

class Hero(val power: Int) {
infix fun vs(opponent: Hero): Hero {
return if (power > opponent.power) {
this
} else {
opponent
}
}
}
vs

this
} else {
opponent
}
}
}
fun example() {
val thor = Hero(7)
val ironman = Hero(8)
val spiderman = Hero(4)
}
// Skip Java version of Hero class,
// we use Kotlin class
class MyJavaClass {
void example() {
Hero thor = new Hero(7);
Hero ironman = new Hero(8);
Hero spiderman = new Hero(4);
}
}
vs

this
} else {
opponent
}
}
}
fun example() {
val thor = Hero(7)
val ironman = Hero(8)
val spiderman = Hero(4)
val theBest = thor vs ironman vs spiderman
}
// Skip Java version of Hero class,
// we use Kotlin class
class MyJavaClass {
void example() {
Hero thor = new Hero(7);
Hero ironman = new Hero(8);
Hero spiderman = new Hero(4);
Hero theBest =
thor.vs(ironman).vs(spiderman);
}
}
vs

class Male(val eyes: String)
class Female(val hair: String)
class Male {
private String eyes;
public Male(String eyes) {
this.eyes = eyes;
}
}
class Female {
private String hair;
public Female(String hair) {
this.hair = hair;
}
}
#11 Kotlin - Operator Overloading
http://kotlinlang.org/docs/reference/operator-overloading.html
vs

class Male(val eyes: String)
class Baby(val eyes: String, val hair: String)
// Skip Java version of Male, Female and Baby
// class, there is not enough space!
vs

class Male(val eyes: String) {
operator fun plus(her: Female): Baby {
return Baby(this.eyes, her.hair)
}
}
vs

}
}
fun example() {
val myBaby = Male("green") + Female("blond")
}
class MyJavaClass {
void example() {
Baby myBaby = new Male("green").plus(
new Female("blond"));
}
}
vs

}
}
fun example() {
val myBaby = Male("green") + Female("blond")
}
class MyJavaClass {
void example() {
Baby myBaby =
new Male("green").plus(new Female("blond"));
}
}
vs

Android Example
from REAL code!

Extensions // More examples (1)
fun String.encodeUrl(charsetName: String = "UTF-8") = URLEncoder.encode(this, charsetName)
fun String.isValidEmail() = Patterns.EMAIL_ADDRESS.matcher(this).matches()
// INFO: https://en.wikipedia.org/wiki/Non-breaking_space
fun String.useNonBreakingSpace() = replace(' ', 'u00A0')
// http://developer.android.com/reference/android/content/Intent.html#ACTION_VIEW
fun Uri.viewIntent() = Intent(Intent.ACTION_VIEW).setData(this)
fun <T : View> T.onClick(onClick: (T) -> Unit) = setOnClickListener(onClick as (View) -> Unit)
fun View.postDelayed(delayMillis: Long, action: () -> Unit) = postDelayed(action, delayMillis)
val EditText.string: String
get() = text.toString()

Extensions // More examples (2)
fun View.hideSoftInput() {
val imm = context.getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager
imm.hideSoftInputFromWindow(windowToken, 0)
}
/**
* Kotlin version of method Utils.isViewInBounds(container, view) by Alex Lockwood
* URL: https://github.com/alexjlockwood/activity-
transitions/blob/c4fa3a21c941691fb6bbe53e37cf3965842ee254/app/src/main/java/com/alexjlockwood/activity/transi
tions/Utils.java
*
* Returns true if {@param view} is contained within {@param container}'s bounds.
*/
fun View.isInBounds(container: View): Boolean {
val containerBounds = Rect()
container.getHitRect(containerBounds)
return getLocalVisibleRect(containerBounds)
}

Extensions + Infix Notation // More examples
infix fun String?.orNotNull(defaultValue: String): String =
if (!this.isNullOrBlank()) this!! else defaultValue
infix fun String?.or(defaultValue: String?): String? =
if (!this.isNullOrBlank()) this else defaultValue
// val a: String? = null
// val b: String? = null
// val c: String? = "test"
// val myText = a or b or c orNotNull "default"

Extensions + Operator Overloading // More examples
operator fun StringBuilder.plusAssign(s: String) { appendln(s) }
// val sb = StringBuilder()
// sb += "test"
// sb += "test2"

Questions?
Developers playground - #EX3
- In KotlinExample use: Extensions, Infix Notation, Operator Overloading
Start with: https://github.com/jacklt/KotlinExample/tree/ex2
Solution: https://github.com/jacklt/KotlinExample/tree/ex3 (at some point maybe …) :P

THANKS!
Omar Miatello, Member of GDG Milano (Italy)
Android Developer @ Satispay

Android & Kotlin - The code awakens #03

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