This document discusses how Kotlin can help improve Android app quality by eliminating common issues like crashes and null pointer exceptions. It highlights Kotlin features like non-nullable types, the safe call operator, and extension functions. Kotlin offers improvements over Java like optional and collection APIs without nullability and verbosity issues. It also maintains full interoperability with Java while providing better tools for static analysis and debugging.

1. Better Apps with Kotlin!
Pavel Shackih, SBT
t.me/PavelShackih

2. Motivation
Android core app quality: the app does not crash, force
close, freeze, or otherwise function abnormally on any
targeted device.

3. Motivation
Android core app quality: the app does not crash, force
close, freeze, or otherwise function abnormally on any
targeted device.

4. JSR-305, Android Support Annotations
@Nonnull
@Override
public User save(@Nonnull User user) {
// ...
userDao.save(user);
// ...
return user;
}

5. JSR-305, Android Support Annotations
1. Hard to maintain.
2. Verbose, but could be simplified using @ParametersAreNonnullByDefault or
@ParametersAreNullableByDefault.
3. Just warning.
4. “Leak” NPE.
5. ~6500 in SBOL.

6. Optional<T>
@Override
public Optional<User> findById(long id) {
return Optional.ofNullable(userDao.findById(id));
}
public void printUser(long id) {
Optional<User> optional = findById(id);
optional.ifPresent(user -> System.out.println(user));
}

7. Optional<T>
1. Verbose.
2. GC overhead.
3. Problem with third-party libraries.

9. Here comes Kotlin
fun main(args: Array<String>) {
val hello = "Hello, Innopolis!"
println(hello)
}

10. Here comes Kotlin
● statically-typed, object-oriented, functional;
● target: JVM, Android, JS, Native (in progress);
● fully interoperable with Java;
● compact runtime lib, good tooling;
● runs as fast as an equivalent on Java;
● easy to learn for Java developers;
● official Android development language.

11. Not nullable types by default
var foo: String = "Foo" // var foo = "Foo"
foo = null // Compile error: Null can not be a value of a non-null type String
var bar: String? = "Bar"
bar = null // Ok
bar = foo // Ok
foo = bar // Error

12. Not nullable types by default
String
String?
String? = String or null

13. Deal with nullable types
val user: User? = null
println(user.address.street)
// Error: only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable
receiver of type kotlin.String?

14. Smartcast
val user: User? = null
if (user != null && user.address != null) {
println(user.address.street) // Now: user: User, address: Address
}
User?
User
Address?
Address

15. Safe call operator ?.
val user: User? = null
println(user?.address)
if (user != null) {
return user.getAddress();
} else {
return null;
}
user?.address
user.address
null
*Kotlin in Action
user != null
user == null
Safe call operator always returns
nullable type of <T>, so <T> result type
transform to type <T?>
println(user?.address?.street)
Chaining safe call operators:

16. Elvis operator ?:
val user: User? = null
println(user ?: ADMIN)
if (user != null) {
return user;
} else {
return ADMIN;
}
user
user
ADMIN
*Kotlin in Action
user != null
user == null
println(user?.address?.street ?: UNKNOWN_STREET)

17. Let function
val user: User? = null
user?.let { println(it) }
if (user != null) {
System.out.println(user);
}
user
println(user)
user != null
*Kotlin in Action

18. Not null assertions: !! operator
val user: User? = null
println(user!!.address)
System.out.println(user.getAddress());
user
user.address
NullPointerException
*Kotlin in Action
user != null
user == null

19. Platform types and Java interoperability
Java Kotlin
@Nullable + String String?
@NotNull + String String
String String!
String! = String or String?
val stringValue = Java.getString()
println(stringValue.length) // Ok
println(stringValue?.length) // Ok

20. Kotlin nullability: Examples
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main2)
val id = savedInstanceState.getString(ID) // compile error
}
}
class MyIntentService : IntentService("MyIntentService") {
override fun onHandleIntent(intent: Intent?) {
val param1 = intent.getStringExtra(EXTRA_PARAM1) // compile error
}
}

21. Intrinsics
● Apply Intrinsics.checkParameterIsNotNull() for every public function for each
non-null parameter.
● Disable using -Xno-param-assertions or Proguard.
JDK 1.8 Kotlin
@Override
public void forEachRemaining(DoubleConsumer
action) {
Objects.requireNonNull(action);
if (count == -2) {
action.accept(first);
count = -1;
}
}
@NotNull
public User save(@NotNull User user) {
Intrinsics.checkParameterIsNotNull(user, "user");
this.userDao.save(user);
return user;
}

22. Kotlin nullability
1. Hard to maintain built-in type system.
2. Verbose special syntax for nullable values.
3. Not error, just a warning, no guarantees compile time error.
4. “Leak” NPE kotlin intrinsics.
5. GC overhead no overhead at all.
6. Problem with third-party libraries kotlin compiler could read default java
annotations from external byte code.

23. Extension functions
Java Kotlin
public static boolean isEmpty(@Nullable String string) {
return string == null || string.isEmpty();
}
fun String?.isEmpty(): Boolean {
return this == null || this.length == 0
}
String text = ...;
if (isEmpty(text)) {
System.out.println("Empty text!");
}
val text = ...
if (text.isEmpty()) {
println("Empty text!")
}

24. Extension functions: Lambdas
fun String.printWithCondition(predicate: (s: String) -> Boolean) {
if (predicate(this)) {
println(this)
}
}
val text = "Hello"
text.printWithCondition { it == "Hello" }

25. Extension functions: Lambdas with inline
inline fun String.printWithCondition(predicate: (s: String) -> Boolean) {
if (predicate(this)) {
println(this)
}
}
val text = "Hello"
text.printWithCondition { it == "Hello" }
String text = "Hello";
if(Intrinsics.areEqual(text, "Hello")) {
System.out.println(text);
}

26. Extension functions: AutoCloseable
val fos = FileOutputStream("data.txt")
val result = fos.use {
// do some stuff with stream
"result"
}
println(result)
val resultSet: ResultSet? = ...
resultSet?.use {
println(it.getString(USER_ID))
}

27. Extension functions: Collections
val list = listOf("Foo", "Bar", "Buzz")
list.filter { it.startsWith("B") } // [Bar, Buzz]
list.map { it.toUpperCase() } // [FOO, BAR, BUZZ]
list.flatMap { it.asIterable() } // [F, o, o, B, a, r, B, u, z, z]
list.groupBy { it.first() } // {F=[Foo], B=[Bar, Buzz]}
list.fold(0, { acc, s -> acc + s.length }) // 10
list.reduce { acc, s -> s + acc } // BuzzBarFoo

28. Extension functions: Collections
1. Immutable by default.
2. Mutable collections should be created explicitly, e.g. by mutableListOf(),
mutableSetOf(), mutableMapOf() or using constructors.
3. Not lazy by default: each transformation → new collection.
4. Sequences are lazy like a Java 8 Streams. No parallelism.
5. Default implementations: ArrayList, HashSet and HashMap.
6. Array supports same functional operators.

29. Extension functions: Lambda Extensions
inline fun String.printWithCondition(predicate: String.() -> Boolean) {
if (predicate()) { // predicate(this)
println(this)
}
}
val text = "Hello"
text.printWithCondition { length > 3 }

30. Extension functions: Standard functions
public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }
val user = User().apply {
id = 1L
name = "John Smith"
}
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
user?.let {
println(user.name)
}

31. Conclusion: tools
1. Dagger 2, Butterknife, RxJava 2, Retrofit 2, Realm, Room...
2. Built-in plugin in AS 3.0.
3. Static analysis - https://github.com/arturbosch/detekt.
4. https://kotlinlang.org/docs/reference/coding-conventions.html.
5. Proguard - ok.
6. Debugging - as java.
7. Compile speed - as java - https://habrahabr.ru/company/badoo/blog/329026/.
8. Java projects: Spring 5, Reactor, Corda, Gradle, Spark.
9. Android projects: Hotellook, Avito.

32. Conclusion: next
1. Intro to Kotlin at Google IO 17: https://youtu.be/X1RVYt2QKQE.
2. Jake Wharton and Christina Lee at Google IO 17:
https://youtu.be/fPzxfeDJDzY.
3. https://developer.android.com/kotlin/index.html.
4. https://kotlinlang.org/docs/reference/.
5. 10 Kotlin Tricks in 10 ish minutes by Jake Wharton:
https://youtu.be/0sPzDwS55wM.
6. Kotlin in Action: https://www.manning.com/books/kotlin-in-action.
7. Kotlin type system: http://natpryce.com/articles/000818.html.
8. Perfomance: https://goo.gl/J2FVSm and https://goo.gl/CDKxw9.

33. Conclusion: community
1. https://t.me/kotlin_lang @kotlin_lang.
2. http://slack.kotlinlang.org/.
3. http://www.kotlinweekly.net/.