From java to kotlin beyond alt+shift+cmd+k

From Java to Kotlin beyond
Alt+Shift+Cmd+K
Fabio Collini

@fabioCollini
linkedin.com/in/fabiocollini
github.com/fabioCollini
medium.com/@fabioCollini
codingjam.it
Android programmazione avanzata
Android Developers Italia
Ego slide

Kotlin can be conﬁgured in one click

A Java class can be converted easily

LAMB
DAS
DATACLASSES
COLLE
CTIONS
ASYNC
CODE
DELE
GATES
Agenda

/**
* Returns the first element matching the given [predicate].
* @throws [NoSuchElementException] if no such element is found.
*/
inline fun <T> Iterable<T>.first(predicate: (T) -> Boolean): T {
for (element in this) {
if (predicate(element)) {
return element
}1
}2
throw NoSuchElementException(
"Collection contains no element matching the predicate.")
}3

val list = listOf("Java", "Kotlin")
val s = list.first({_v: String ->
v.length > 4
_})

val s = list.first({_v ->
v.length > 4
_})

val s = list.first({_
it.length > 4
_})

val s = list.first {_
it.length > 4
_}

val s = list.first { it.length > 4_}

public static <T> T first(Iterable<T> list, Predicate<T> predicate) {_
for (T t : list) {__
if (predicate.test(t)) {___
return t;
}____
}_____
throw new NoSuchElementException(
"Collection contains no element matching the predicate.");
}______
inline fun <T> Iterable<T>.first(predicate: (T) -> Boolean): T {_
for (element in this) {__
if (predicate(element)) {___
return element
}____
}____
}_____

List<String> list = Arrays.asList("Java", “Kotlin");
String s = first(list, p -> p.length() > 4);

List<String> list = Arrays.asList("Java", “Kotlin");
String s = first(list, p -> p.length() > 4);
val s = list.first { it.length > 4 }

return element
}
}
}
public static <T> T first(List<T> list, Predicate<T> predicate) {
for (T t : list) {
if (predicate.test(t)) {
return t;
}
}
throw new NoSuchElementException(
“Collection contains no element matching the predicate.");
}

val s = list.first { it.length > 4 }
return element
}
}
}

var ret: String? = null
for (element in list) {
if (element.length > 4) {
ret = element;
break
}
}
if (ret == null)
val s = ret

class Person(var name:_String)_
public class Person {
private String name;
public Person(String name) {
this.name = name;
}1
public String getName() {
return name;
}2
public void setName(String name) {
this.name = name;
}3
}4

public class Person {
private long id;
private int age;
public Person(long id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
public long getId()_{_return id;_}
public String getName()_{_return name;_}
public int getAge()_{_return age;_}
}4
class Person(
val id: Long,
val name: String,
val age: Int
)_

@Parcelize
data class Person(
val id: Long,
val name: String,
val age: Int
)_: Parcelable
public class Person implements Parcelable {
private long id;
private int age;
public Person(long id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}1
protected Person(Parcel in) {
id = in.readLong();
name = in.readString();
age = in.readInt();
}2
public long getId()_{_return id;_}
public String getName()_{_return name;_}
public int getAge()_{_return age;_}
@Override public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person that = (Person) o;
if (id != that.id) return false;
if (age != that.age) return false;
return name != null ? name.equals(that.name) : that.name == null;
}3
@Override public int hashCode() {
int result = (int) (id ^ (id >>> 32));
result = 31 * result + (name != null ? name.hashCode() : 0);
result = 31 * result + age;
return result;
}4
@Override public String toString() {
return "Person{" +
"id=" + id +
", name='" + name + ''' +
", age=" + age +
'}';
}5
@Override
public void writeToParcel(Parcel dest, int flags) {
dest.writeLong(id);
dest.writeString(name);
dest.writeInt(age);
}6
@Override
public int describeContents() {
return 0;
}7
public static final Creator<Person> CREATOR = new Creator<Person>() {
@Override
public Person createFromParcel(Parcel in) {
return new Person(in);
}8
@Override
public Person[] newArray(int size) {
return new Person[size];
}9
};
}_

@AutoValue
public abstract class Person
implements Parcelable {_
public static Person create(
long id, String name, int age) {
return new AutoValue_Person(
id, name, age);
}__
public abstract long id();
public abstract String name();
public abstract int age();
public abstract Person withName(String name);
}_
@Parcelize
data class Person(
val id: Long,
val name: String,
val age: Int
)_: Parcelable

val p = Person(1, "name", 10)
val p1 = p.copy(name = "newName", age = 20)_
Person p = Person.create(1, "name", 10);
Person p1 = p.withName("newName");

val p = Person(1, "name", 10)
val (_, name, age) = p
println("$name $age")

data class Pair<out A, out B>(
val first: A,
val second: B
) : Serializable {
override fun toString(): String = "($first, $second)"
}_

data class Pair<out A, out B>(
val first: A,
val second: B
) : Serializable {
override fun toString(): String = "($first, $second)"
}_
val myPair: Pair<Int, Int> = Pair(10, 20)
val otherPair: Pair<Int, Int> = 10 to 20
infix fun <A, B> A.to(that: B): Pair<A, B> = Pair(this, that)

fun myMethod(): Pair<Int, Int> {
//...
return 10 to 20
}
val (a, b) = myMethod()

System.out.println(b.toString());
List<Person> people = Arrays.asList(...);
StringBuilder b = new StringBuilder();
for (String s : cities) {
if (b.length() > 0) {
b.append(", ");
}3
b.append(s);
}4
for (Person person : people) {
City city = person.getAddress().getCity();
if (city.getRegion().equals("Tuscany")) {
cities.add(city.getName() + " (" + city.getCode() + ")");
}1
}2
Set<String> cities = new TreeSet<>();

val people = listOf(...)
val s = people
.map { it.address.city }
.filter { it.region == "Tuscany" }
.distinct()
.sortedBy { it.name }
.joinToString { "${it.name} (${it.code})" }
println(s)

String s = Stream.of(people)
.map(it -> it.getAddress().getCity())
.filter(it -> it.getRegion().equals("Tuscany"))
.distinct()
.sortBy(City::getName)
.map(it -> it.getName() + " (" + it.getCode() + ")")
.collect(Collectors.joining(", "));
System.out.println(s);

val people = listOf(...)
val s = people
.asSequence()
.map { it.address.city }
.map { "${it.name} (${it.code})" }
.first { it.region == "Tuscany" }
println(s)

val youngest = people.minBy { it.age }
val peopleByCity: Map<City, List<Person>> =
people.groupBy { it.address.city }
val all: Boolean = people.all {
it.address.city.name == "Florence"
}1
val any: Boolean = people.any {
it.address.city.name == "Florence"
}2
val (adults: List<Person>, minors: List<Person>) =
people.partition { it.age >= 18 }

val readOnlyList: List<Int> = listOf(10, 20, 30)
val secondElement = readOnlyList[1]
val mutableList: MutableList<Int> = mutableListOf(10, 20, 30)
mutableList[1] = 21
val map = mapOf(1 to "ABC", 2 to "DEF")
val abc = map[1]
val mutableMap = mutableMapOf(1 to "ABC", 2 to "DEF")
mutableMap[3] = "XYZ"
val otherList = (mutableList - readOnlyList) + mutableMap.keys

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val myTextView = findViewById<TextView>(R.id.text)
myTextView.text = "Loading..."
Thread.sleep(2000)
myTextView.text = "Loading something else..."
Thread.sleep(2000)
myTextView.text = "Done"
}_

/**
* Delays coroutine for a given time without blocking
* a thread and resumes it after a specified time.
* ...
*/
suspend fun delay(time: Long, unit: TimeUnit = MILLISECONDS) {
//...
}

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val myTextView = findViewById<TextView>(R.id.text)
async(UI) {
myTextView.text = "Loading..."
delay(2000)
myTextView.text = "Loading something else..."
delay(2000)
myTextView.text = "Done"
}async
}end

interface StackOverflowService {
@GET("/users")
fun getTopUsers(): Deferred<List<User>>
}A
async(UI) {
try {
val topUsers = service.getTopUsers().await()
val usersUi = topUsers.map { createUserUi(it) }
showInUi(usersUi)
} catch (e: Exception) {
showError(e)
}__
}___

interface MyService {
fun login(): Deferred<String>
fun loadData(token: String): Deferred<Data>
}A
async(UI) {
try {
val token = service.login().await()
val data = service.loadData(token).await()
showInUi(data)
showError(e)
}__
}___

service.login()
.flatMap { service.loadData(it) }
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{ data -> showInUi(data) },
{ showError(it) }
)end
RxJavaCoroutines
async(UI) {
try {
showInUi(data)
showError(e)
}__
}___

RxJava Coroutines->
RxJavaCoroutines ->
rxCompletable, rxMaybe, rxSingle, rxObservable, rxFlowable
CompletableSource.await, MaybeSource.await, MaybeSource.awaitOrDefault,
MaybeSource.openSubscription, SingleSource.await,
ObservableSource.awaitFirst, ObservableSource.awaitFirstOrDefault,
ObservableSource.awaitFirstOrElse, ObservableSource.awaitFirstOrNull,
ObservableSource.awaitLast, ObservableSource.awaitSingle,
ObservableSource.openSubscription, ObservableSource.iterator
github.com/Kotlin/kotlinx.coroutines/tree/master/reactive/kotlinx-coroutines-rx2

My 2 cents
Suspending methods are easier to
use than RxJava Singles
Observables/Flowables are the best
abstraction for a stream of data

async(UI) {
try {
showInUi(data)
showError(e)
}__
}___

async(UI) {
try {
val otherData = service.loadOtherData(token).await()
showInUi(data, otherData)
showError(e)
}__
}___

async(UI) {
try {
val data = service.loadData(token)
val otherData = service.loadOtherData(token)
showInUi(data.await(), otherData.await())
showError(e)
}__
}___

async(UI) {
try {
updateProgress()
showError(e)
}__
}___

async(UI) {
try {
withTimeout(10, SECONDS) {
val token = retry(3) {
service.login().await()
}T
updateProgress()
}B
showError(e)
}__
}___

async(UI) {
try {
withTimeout(10, SECONDS) {
val token = retry(3) {
service.login().await()
}T
updateProgress()
}B
showError(e)
}__
}___
suspend fun <T> retry(times: Int, block: suspend () -> T): T {
repeat(times - 1) {
try {
return block()
} catch (ignored: Exception) {
}
}
return block()
}

class TokenHolder {0
var token = ""
}1

class TokenHolder(val prefs: SharedPreferences) {0
var token = ""
}1

class TokenHolder(val prefs: SharedPreferences) {
var token
get() = prefs.getString("token", "")
set(value) = prefs.edit().putString("token", value).apply()
}1

class TokenHolder(prefs: SharedPreferences) {
var token by prefs.string()
}1

}1
fun SharedPreferences.string(
defaultValue: String = "",
key: String? = null
): ReadWriteProperty<Any, String> {
return object : ReadWriteProperty<Any, String> {
override fun getValue(thisRef: Any, property: KProperty<*>): String {
return getString(key ?: property.name, defaultValue)
}2
override fun setValue(thisRef: Any, property: KProperty<*>,
value: String) {
edit().putString(key ?: property.name, value).apply()
}3
}4
}5

}1
tokenHolder.token += "ABC"
prefs.edit().putString(
"token",
prefs.getString("token", "") + "ABC"
).apply()

Wrapping up
inline methods to avoid extra classes
functional code is natural in Kotlin
code can be simplified using data
classes, coroutines and delegates

Links
AutoValue
https://github.com/google/auto/blob/master/value/userguide/index.md
Lightweight Stream
https://github.com/aNNiMON/Lightweight-Stream-API
Guide to kotlinx.coroutines by example
https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md
Kotlin coroutines, a deeper look
https://medium.com/@elizarov/kotlin-coroutines-a-deeper-look-180536305c3f
Kotlin delegates in Android development - part 1
https://hackernoon.com/kotlin-delegates-in-android-development-part-1-50346cf4aed7
Kotlin delegates in Android development - part 2
https://proandroiddev.com/kotlin-delegates-in-android-development-part-2-2c15c11ff438

QUESTIONS?
Android Developers Italia
androiddevs.it

From java to kotlin beyond alt+shift+cmd+k

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