Effective Kotlin LIGHTNING TALK 

loves 

Let’s talk about… Singleton 

// Kotlin object Singleton // Java public class Singleton { private Singleton INSTANCE; private Singleton() { } public Singleton getInstance() { if (INSTANCE != null) { INSTANCE = new Singleton(); } return INSTANCE; } } 

Let’s talk about… Primitive Obsession 

public class Api { public List doSearch( String deviceToken // other parameters ) { String customerId = getCustomerIdFromSessionToken(deviceToken); // Do something and returns the search return ArrayList(); } private String getCustomerIdFromSessionToken(String authToken) { // Retrieve the customer through the user session return "CustomerId"; } } 

public class Api { public List doSearch( String deviceToken // other parameters ) { String customerId = getCustomerIdFromSessionToken(deviceToken); // Do something and returns the search return ArrayList(); } private String getCustomerIdFromSessionToken(String authToken) { // Retrieve the customer through the user session return "CustomerId"; } } 

public class Api { public List doSearch( String deviceToken // other parameters ) { String customerId = getCustomerIdFromSessionToken(deviceToken); // Do something and returns the search return ArrayList(); } private String getCustomerIdFromSessionToken(String authToken) { // Retrieve the customer through the user session return "CustomerId"; } } 

public class Api { public List doSearch( String deviceToken // other parameters ) { String customerId = getCustomerIdFromSessionToken(deviceToken); // Do something and returns the search return ArrayList(); } private String getCustomerIdFromSessionToken(String authToken) { // Retrieve the customer through the user session return "CustomerId"; } } 

public class Api { public List doSearch( String deviceToken // other parameters ) { String customerId = getCustomerIdFromSessionToken(deviceToken); // Do something and returns the search return ArrayList(); } private String getCustomerIdFromSessionToken(String authToken) { // Retrieve the customer through the user session return "CustomerId"; } } 

IllegalArgumentException: Invalid UUID 

class Api { fun doSearch( deviceToken: String // other parameters ): List { val customerId = getCustomerIdFromSessionToken(deviceToken) // Do something and returns the search return ArrayList() } private fun getCustomerIdFromSessionToken(authToken: String): String { // Retrieve the customer through the user session return "CustomerId" } } 

class Api { fun doSearch( deviceToken: String // other parameters ): List { val customerId = getCustomerIdFromSessionToken(deviceToken) // Do something and returns the search return ArrayList() } private fun getCustomerIdFromSessionToken(authToken: String): String { // Retrieve the customer through the user session return "CustomerId" } } 

class Api { fun doSearch( sessionToken: String // other parameters ): List { val customerId = getCustomerIdFromSessionToken(sessionToken) // Do something and returns the search return ArrayList() } private fun getCustomerIdFromSessionToken(sessionToken: String): String { // Retrieve the customer through the user session return "CustomerId" } } 

class Api { fun doSearch( sessionToken: SessionToken // other parameters ): List { val customerId = getCustomerIdFromSessionToken(sessionToken) // Do something and returns the search return ArrayList() } private fun getCustomerIdFromSessionToken(sessionToken: SessionToken): String { // Retrieve the customer through the user session return "CustomerId" } } 

class SessionToken(val token: UUID) { constructor(token: String): this(UUID.fromString(token)) } 

class SessionToken(val token: UUID) { constructor(token: String): this(UUID.fromString(token)) } 

class SessionToken(val token: UUID) { constructor(token: String): this(UUID.fromString(token)) } 

inline class SessionToken(val token: UUID) { constructor(token: String): this(UUID.fromString(token)) } 

Let’s talk about… Composition over Inheritance 

// Kotlin class Foo open class Foo // Java public final class Foo {} public class Foo {} 

data class CustomerList( val customers: List ) { {…} } 

data class CustomerList( val customers: List ) : Collection { {…} } 

Class 'CustomerList' is not abstract and does not implement abstract member 

public interface Collection : Iterable { public val size: Int public fun isEmpty(): Boolean public fun contains(element: E): Boolean public fun containsAll(elements: Collection): Boolean } 

data class CustomerList( val customers: List ) : Collection { {…} } 

data class CustomerList( val customers: List ) : Collection by customers { {…} } 

Let’s talk about… State Handling 

sealed class ViewState { object Started: ViewState() object EditMode: ViewState() object ViewMode: ViewState() } fun verifyState(state: ViewState) { when (state) { is ViewState.Started -> println("Started") is ViewState.EditMode -> println("EditMode") is ViewState.ViewMode -> println("ViewMode") } } 

sealed class ViewState { object Started: ViewState() object EditMode: ViewState() object ViewMode: ViewState() class Error(val message: String = "Unknown error"): ViewState() } fun verifyState(state: ViewState) { when (state) { is ViewState.Started -> println("Started") is ViewState.EditMode -> println("EditMode") is ViewState.ViewMode -> println("ViewMode") } } 

sealed class ViewState { object Started: ViewState() object EditMode: ViewState() object ViewMode: ViewState() class Error(val message: String = "Unknown error"): ViewState() } fun verifyState(state: ViewState) { when (state) { is ViewState.Started -> println("Started") is ViewState.EditMode -> println("EditMode") is ViewState.ViewMode -> println("ViewMode") } } when expressions guarantee that their branches exhaustively check the possible subclasses of a sealed class. 

'when' expression on sealed classes is recommended to be exhaustive, add 'Error' branch or 'else' branch instead 

sealed class ViewState { object Started: ViewState() object EditMode: ViewState() object ViewMode: ViewState() class Error(val message: String = "Unknown error"): ViewState() } fun verifyState(state: ViewState) { when (state) { is ViewState.Started -> println("Started") is ViewState.EditMode -> println("EditMode") is ViewState.ViewMode -> println(“ViewMode") is ViewState.Error -> println(state.message) } } 

Let’s talk about… Contracts 

fun requireNotNull(any: Any?) { if (any == null) throw IllegalArgumentException() } 

fun requireNotNull(any: Any?) { if (any == null) throw IllegalArgumentException() } fun foo(s: String?) { requireNotNull(s) s.length } 

Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String? 

fun requireNotNull(any: Any?) { contract { returns() implies (any != null) } if (any == null) throw IllegalArgumentException() } fun foo(s: String?) { requireNotNull(s) s.length } 

Questions?