Effective Kotlin

A lightning talk about how Kotlin helps you to write effective code based on Effective Java by Joshua Block.

Transcript

1. Effective Kotlin
   LIGHTNING TALK
   

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2. loves
   

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3. Let’s talk about…
   Singleton
   

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

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5. Let’s talk about…
   Primitive Obsession
   

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6. 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";
   }
   }
   

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7. 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";
   }
   }
   

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8. 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";
   }
   }
   

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9. 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";
   }
   }
   

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10. 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";
    }
    }
    

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11. IllegalArgumentException: Invalid UUID
    

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12. 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"
    }
    }
    

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13. 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"
    }
    }
    

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14. 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"
    }
    }
    

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15. 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"
    }
    }
    

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

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

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

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

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20. Let’s talk about…
    Composition over Inheritance
    

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

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

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

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

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

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

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

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28. Let’s talk about…
    State Handling
    

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29. 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")
    }
    }
    

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30. 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")
    }
    }
    

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31. 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.
    

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

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33. 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)
    }
    }
    

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34. Let’s talk about…
    Contracts
    

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

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

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

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

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39. Questions?
    

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