This document provides an overview of key Kotlin concepts compared to Java, including: - Null safety in Kotlin prevents null reference exceptions at compile time by distinguishing nullable and non-nullable types. - The Elvis operator (?:) provides a shorthand for checking for null values and returning a default. - Smart casts allow the compiler to infer the type of a variable after checking its type against an if or when condition without needing an explicit cast. - Kotlin collections APIs provide concise ways to filter, map, sort and iterate over collections compared to Java by leveraging functions like filterNotNull(), sortedBy(), map(), forEach(). The document demonstrates these Kotlin features through code examples

1. Android & Kotlin
The code awakens #02

2. 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

3. 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() }

4. 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)

5. Kotlin vs Java - Part 2
Null-safety
Elvis Operator
Smart-cast
Collections

6. class MyKotlinClass {
val a: String = "ciao"
val b: String = null // Error at compile time
}
#5 Kotlin - Null Safety
http://kotlinlang.org/docs/reference/null-safety.html
vs

7. class MyKotlinClass {
val a: String = "ciao"
val b: String = null // Error at compile time
val c: String? = null
val d: String? = "ok"
}
#5 Kotlin - Null Safety
http://kotlinlang.org/docs/reference/null-safety.html
vs

8. class MyKotlinClass {
val a: String = "ciao"
val b: String = null // Error at compile time
val c: String? = null
val d: String? = "ok"
fun example(e: String, f: String?) {
e.length()
f.length() // Error at compile time
}
}
static class MyUtils {
void example(String e, String f) {
e.length(); // throw NullPointerException?
f.length(); // throw NullPointerException?
}
}
#5 Kotlin - Null Safety
http://kotlinlang.org/docs/reference/null-safety.html
vs

9. class MyKotlinClass {
val a: String = "ciao"
val b: String = null // Error at compile time
val c: String? = null
val d: String? = "ok"
fun example(e: String, f: String?) {
e.length()
f.length() // Error at compile time
f?.length()
}
}
static class MyUtils {
void example(String e, String f) {
e.length(); // throw NullPointerException?
f.length(); // throw NullPointerException?
}
}
#5 Kotlin - Null Safety
http://kotlinlang.org/docs/reference/null-safety.html
vs

10. class MyKotlinClass {
val a: String = "ciao"
val b: String = null // Error at compile time
val c: String? = null
val d: String? = "ok"
fun example(e: String, f: String?) {
e.length()
f.length() // Error at compile time
f?.length()
if (f != null) {
f.length()
}
}
}
static class MyUtils {
void example(String e, String f) {
e.length(); // throw NullPointerException?
f.length(); // throw NullPointerException?
if (e != null) {
e.length();
}
if (f != null) {
e.length();
}
}
}
#5 Kotlin - Null Safety
http://kotlinlang.org/docs/reference/null-safety.html
vs

11. val map = mapOf(
"dog" to "woof",
"cat" to "meow",
"bird" to "tweet",
"mouse" to "squeek")
fun sound(animal: String): String? {
return map.get(animal)
}
class MyUtils {
static Map<String, String> map = // ...
static String sound(String animal) {
return map.get(animal);
}
}
#6 Kotlin - Elvis Operator
http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand
vs

12. val map = mapOf(
"dog" to "woof",
"cat" to "meow",
"bird" to "tweet",
"mouse" to "squeek")
fun sound(animal: String): String? {
return map.get(animal)
}
fun example() {
val foxSay = sound("fox") ?: "No one knows"
}
class MyUtils {
static Map<String, String> map = // ...
static String sound(String animal) {
return map.get(animal);
}
static void example() {
String s = sound("fox");
String foxSay =
s != null ? s : "No one knows";
}
}
#6 Kotlin - Elvis Operator
http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand
vs

13. val map = mapOf(
"dog" to "woof",
"cat" to "meow",
"bird" to "tweet",
"mouse" to "squeek")
fun sound(animal: String): String? {
return map.get(animal)
}
fun example() {
val foxSay = sound("fox") ?: "No one knows"
}
class MyUtils {
static Map<String, String> map = // ...
static String sound(String animal) {
return map.get(animal);
}
static void example() {
String s = sound("fox");
String foxSay =
s != null ? s : "No one knows";
}
}
#6 Kotlin - Elvis Operator
http://kotlinlang.org/docs/reference/idioms.html#if-not-null-and-else-shorthand
vs

14. class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
}
}
}
#7 Kotlin - Smart Cast
http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
vs

15. class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
ImageView imageView =
(ImageView) myView;
}
}
}
#7 Kotlin - Smart Cast
http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
vs

16. class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
ImageView imageView =
(ImageView) myView;
imageView.setImageAlpha(10);
}
}
}
#7 Kotlin - Smart Cast
http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
vs

17. class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
ImageView imageView =
(ImageView) myView;
imageView.setImageAlpha(10);
} else if (myView instanceof TextView) {
TextView textView = (TextView) myView;
textView.setText("Ciao");
}
}
}
#7 Kotlin - Smart Cast
http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
vs

18. fun example1(myView: View) {
if (myView is ImageView) {
myView.setImageAlpha(10)
} else if (myView is TextView) {
myView.setText("Ciao")
}
}
class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
ImageView imageView =
(ImageView) myView;
imageView.setImageAlpha(10);
} else if (myView instanceof TextView) {
TextView textView = (TextView) myView;
textView.setText("Ciao");
}
}
}
#7 Kotlin - Smart Cast
http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
vs

19. fun example1(myView: View) {
if (myView is ImageView) {
myView.setImageAlpha(10)
} else if (myView is TextView) {
myView.setText("Ciao")
}
}
fun example2(myView: View) {
when (myView) {
is ImageView -> myView.imageAlpha = 10
is TextView -> myView.text = "Ciao"
}
}
class MyUtils {
static void example(View myView) {
if (myView instanceof ImageView) {
ImageView imageView =
(ImageView) myView;
imageView.setImageAlpha(10);
} else if (myView instanceof TextView) {
TextView textView = (TextView) myView;
textView.setText("Ciao");
}
}
}
#7 Kotlin - Smart Cast
http://kotlinlang.org/docs/reference/typecasts.html#smart-casts
vs

20. fun example() {
val os = listOf("Android", "iOS", null,
"Windows Phone")
}
class MyUtils {
static void example() {
List<String> os = Arrays.asList("Android",
"iOS", null, "Windows Phone");
}
}
#8 Kotlin - Using collections
http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections
vs

21. fun example() {
val os = listOf("Android", "iOS", null,
"Windows Phone")
os.filterNotNull()
}
class MyUtils {
static void example() {
List<String> os = Arrays.asList("Android",
"iOS", null, "Windows Phone");
List<String> osNotNull = new ArrayList<>();
for (String name : os) {
if (name != null) osNotNull.add(name);
}
}
}
#8 Kotlin - Using collections
http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections
vs

22. fun example() {
val os = listOf("Android", "iOS", null,
"Windows Phone")
os.filterNotNull().sortedBy { it.length() }
}
class MyUtils {
static void example() {
List<String> os = Arrays.asList("Android",
"iOS", null, "Windows Phone");
List<String> osNotNull = new ArrayList<>();
for (String name : os) {
if (name != null) osNotNull.add(name);
}
Collections.sort(osNotNull, new
Comparator<String>() { @Override
public int compare(String l, String r) {
return r.length() - l.length();
}
});
}
}
#8 Kotlin - Using collections
http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections
vs

23. fun example() {
val os = listOf("Android", "iOS", null,
"Windows Phone")
os.filterNotNull().sortedBy { it.length() }
.map { it.toUpperCase() }
}
class MyUtils {
static void example() {
List<String> os = Arrays.asList("Android",
"iOS", null, "Windows Phone");
List<String> osNotNull = new ArrayList<>();
for (String name : os) {
if (name != null) osNotNull.add(name);
}
Collections.sort(osNotNull, new
Comparator<String>() { @Override
public int compare(String l, String r) {
return l.length() - r.length();
}
});
for (String name : osNotNull) {
String value = name.toUpperCase();
}
}
}
#8 Kotlin - Using collections
http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections
vs

24. fun example() {
val os = listOf("Android", "iOS", null,
"Windows Phone")
os.filterNotNull().sortedBy { it.length() }
.map { it.toUpperCase() }
.forEach { print(it) }
}
class MyUtils {
static void example() {
List<String> os = Arrays.asList("Android",
"iOS", null, "Windows Phone");
List<String> osNotNull = new ArrayList<>();
for (String name : os) {
if (name != null) osNotNull.add(name);
}
Collections.sort(osNotNull, new
Comparator<String>() { @Override
public int compare(String l, String r) {
return l.length() - r.length();
}
});
for (String name : osNotNull) {
String value = name.toUpperCase();
print(value);
}
} }
#8 Kotlin - Using collections
http://kotlinlang.org/docs/reference/basic-syntax.html#using-collections
vs

25. Android Example
From Java to Kotlin!
https://github.com/jacklt/KotlinExample/tree/ex1

26. dummy/Items.kt
data class HeroItem(val name: String, val gender: String, val power: Int)

27. dummy/HeroDummyContent.kt
object HeroDummyContent {
private val MALE = "Male"
private val FEMALE = "Female"
val ITEMS = listOf(
HeroItem("Hulk", MALE, 9),
HeroItem("Iron Man", MALE, 8),
HeroItem("Thor", MALE, 8),
HeroItem("Jessica Jones", FEMALE, 2),
HeroItem("Spiderman", MALE, 4)
)
}

28. dummy/HeroDummyContent.kt > object vs class
object HeroDummyContent {
private val MALE = "Male"
private val FEMALE = "Female"
val ITEMS = listOf(
HeroItem("Hulk", MALE, 9),
HeroItem("Iron Man", MALE, 8),
HeroItem("Thor", MALE, 8),
HeroItem("Jessica Jones", FEMALE, 2),
HeroItem("Spiderman", MALE, 4)
)
}

29. dummy/HeroDummyContent.kt > listOf(...)
object HeroDummyContent {
private val MALE = "Male"
private val FEMALE = "Female"
val ITEMS = listOf(
HeroItem("Hulk", MALE, 9),
HeroItem("Iron Man", MALE, 8),
HeroItem("Thor", MALE, 8),
HeroItem("Jessica Jones", FEMALE, 2),
HeroItem("Spiderman", MALE, 4)
)
}

30. dummy/HeroAdapter.kt
class HeroAdapter(val mValues: List<HeroItem>, val mListener: Function1<HeroItem, Unit>?) :
RecyclerView.Adapter<HeroAdapter.ViewHolder>() {
// ...
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
val hero = mValues[position]
holder.item = hero
holder.nameView.text = "${hero.name} (Power: ${hero.power})"
holder.genderView.text = hero.gender
holder.itemView.setOnClickListener {
mListener?.invoke(hero)
}
}
// ...
}

31. dummy/HeroAdapter.kt > String template
class HeroAdapter(val mValues: List<HeroItem>, val mListener: Function1<HeroItem, Unit>?) :
RecyclerView.Adapter<HeroAdapter.ViewHolder>() {
// ...
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
val hero = mValues[position]
holder.item = hero
holder.nameView.text = "${hero.name} (Power: ${hero.power})"
holder.genderView.text = hero.gender
holder.itemView.setOnClickListener {
mListener?.invoke(hero)
}
}
// ...
}

32. MainActivity.kt
class MainActivity : AppCompatActivity() {
private var currentHero: HeroItem? = null
private val adapter by lazy {
HeroAdapter(HeroDummyContent.ITEMS) {
// onItemClick ...
}
}
// ...
}

33. MainActivity.kt > adapter (lazy)
HeroAdapter(HeroDummyContent.ITEMS) { // onClick
val previousHero = currentHero
if (previousHero != null) { // fight
val conclusion = if (previousHero.power == it.power) {
"It's a draw"
} else if (previousHero.power > it.power) {
"${previousHero.name} wins!"
} else {
"${it.name} wins!"
}
Snackbar.make(fab, "${previousHero.name} vs ${it.name}!n$conclusion", Snackbar.LENGTH_LONG).show()
currentHero = null
} else { // assign current hero
Snackbar.make(fab, "Choose: ${it.name}", Snackbar.LENGTH_SHORT).show()
currentHero = it
}
}

34. MainActivity.kt > adapter (lazy) > Smart cast
HeroAdapter(HeroDummyContent.ITEMS) { // onClick
val previousHero = currentHero
if (previousHero != null) { // fight
val conclusion = if (previousHero.power == it.power) {
"It's a draw"
} else if (previousHero.power > it.power) {
"${previousHero.name} wins!"
} else {
"${it.name} wins!"
}
Snackbar.make(fab, "${previousHero.name} vs ${it.name}!n$conclusion", Snackbar.LENGTH_LONG).show()
currentHero = null
} else { // assign current hero
Snackbar.make(fab, "Choose: ${it.name}", Snackbar.LENGTH_SHORT).show()
currentHero = it
}
}

35. MainActivity.kt > adapter (lazy) > return if expression
HeroAdapter(HeroDummyContent.ITEMS) { // onClick
val previousHero = currentHero
if (previousHero != null) { // fight
val conclusion = if (previousHero.power == it.power) {
"It's a draw"
} else if (previousHero.power > it.power) {
"${previousHero.name} wins!"
} else {
"${it.name} wins!"
}
Snackbar.make(fab, "${previousHero.name} vs ${it.name}!n$conclusion", Snackbar.LENGTH_LONG).show()
currentHero = null
} else { // assign current hero
Snackbar.make(fab, "Choose: ${it.name}", Snackbar.LENGTH_SHORT).show()
currentHero = it
}
}

36. Questions?
Developers playground - #EX2
- Aggiungere le proprietà a HeroItem: hair (String?), eyes (String?)
- Mostrarle (se presenti) nella lista
- Se vengono scelti 2 eroi di sesso opposto, viene creato un nuovo eroe
- Si possono usare le proprietà dei genitori, oppure dei valori di default se
mancano
Start with: https://github.com/jacklt/KotlinExample/tree/ex1
Solution: https://github.com/jacklt/KotlinExample/tree/ex2

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