Introduction to Kotlin

Presentation slides of our presentation at Kotlin Tel Aviv User Group meetup

Transcript

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2. Hello :)
   

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3. Together.
   Learn.
   

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4. Agenda
   • Quick history / what is Kotlin / why Kotlin / For whom
   • Variables
   • Functions & extension functions
   • Classes & Properties
   • Lambdas & Collections
   

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5. What is Kotlin
   • Programming Language developed by Jetbrains (IntelliJ)
   • Open sourced in 2011, Kotlin 1.0 on Feb 15th 2016
   • 100% interoperable with Java
   • On I/O 2017 became first class citizen language on Android
   • Consice, Safe, Interoperable,
   

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6. Concise?
   public class JavaPerson {
   private String firstName;
   private String lastName;
   public JavaPerson(String firstName, String lastName) {
   this.firstName = firstName;
   this.lastName = lastName;
   }
   public String getFirstName() {
   return firstName;
   }
   public void setFirstName(String firstName) {
   this.firstName = firstName;
   }
   public String getLastName() {
   return lastName;
   }
   public void setLastName(String lastName) {
   this.lastName = lastName;
   }
   @Override
   public String toString() {
   return "Person{" +
   "firstName='" + firstName + '\'' +
   ", lastName='" + lastName + '\'' +
   '}';
   }
   @Override
   public boolean equals(Object o) {
   if (this == o) return true;
   if (o == null || getClass() != o.getClass()) return false;
   JavaPerson person = (JavaPerson) o;
   if (firstName != null ? !firstName.equals(person.firstName) : person.firstName != null) return
   false;
   return lastName != null ? lastName.equals(person.lastName) : person.lastName == null;
   }
   @Override
   public int hashCode() {
   int result = firstName != null ? firstName.hashCode() : 0;
   result = 31 * result + (lastName != null ? lastName.hashCode() : 0);
   return result;
   }
   }
   

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7. Concise!
   data class Person(var firstName: String,
   var lastName: String)
   

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8. Why Kotlin?
   • Costs nothing to adopt
   • Concise language
   • No runtime overhead
   • Solves the Nullability problem
   • Lots of modern features
   

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9. For whom?
   • Android developers (JVM)
   • Backend developers (JVM)
   • iOS developers (LLVM)
   • Windows developers (LLVM)
   • Javascript developers
   

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10. Introduction to Kotlin
    

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11. Packages and Files
    • Can be same as Java
    • Functions do not need to be in a class to work
    • All code shown may be under the same file
    

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12. Packages and Files
    

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13. Variables
    

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14. Val (read-only)
    val firstName: String = "Rhaquel"
    val firstName = "Rhaquel"
    firstName = “Alex” // cannot be reassigned
    

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15. Var (mutable)
    var lastName: String = "Valencia"
    var lastName = “Valencia"
    lastName = "Gherschon"
    lastName = 5 // Int not type of String
    

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16. Object Instantiation
    sb.append(" is some").append(" text!”)
    println(sb)
    val sb: StringBuilder = StringBuilder(“Here") // no ‘new’ keyword
    

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17. Functions / Extension Functions
    

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18. Functions
    fun maximum(first: Int, second: Int): Int {
    if (first > second) {
    return first
    } else {
    return second
    }
    }
    

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19. Functions - ‘if’ is an expression
    fun maximum(first: Int, second: Int): Int {
    return if (first > second) {
    first
    } else {
    second
    }
    }
    

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20. Functions - Default values
    fun maximum(first: Int = 3, second: Int = 4): Int {
    return if (first > second) {
    first
    } else {
    second
    }
    }
    

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21. Calling Functions
    var max = maximum(1, 2) // Regular call
    max = maximum(first = 1, second = 2)// Named Parameters
    max = maximum(second = 2, first = 1)
    max = maximum(second = 8)
    

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22. Functions - concise?
    fun maximum(first: Int, second: Int): Int {
    return if (first > second) {
    first
    } else {
    second
    }
    }
    

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23. Functions - concise!
    fun maximum(first: Int, second: Int): Int =
    if (first > second) {
    first
    } else {
    second
    }
    

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24. Functions - concise!!
    fun maximum(first: Int, second: Int) =
    if (first > second) {
    first
    } else {
    second
    }
    

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25. Functions - concise!!!
    fun maximum(first: Int, second: Int) =
    if (first > second) first else second
    fun maximum(first: Int, second: Int) = if (first > second) first else second
    

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26. Functions - When
    fun testResult(value: Any?) = when {
    value == 3 -> "value is exactly 3"
    value is Int -> "double the value = ${value * 2}"
    value == "What the fudge?" -> "Swich case + if statement!"
    else -> "No value"
    }
    val value: Any? = 3
    println(testResult(value))
    

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27. Functions - When - Concise
    fun testResult(value: Any?) = when (value) {
    3 -> "value is exactly 3"
    is Int -> "double the value = ${value * 2}"
    "What the fudge?" -> "Swich case + if statement!"
    else -> "No value"
    }
    val value: Any? = 3
    println(testResult(value))
    

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28. Extension functions
    fun Int.maximum(other: Int) =
    if (this > other) this else other
    max = 3.maximum(4)
    

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29. Extension functions - infix
    infix fun Int.maximum(other: Int) =
    if (this > other) this else other
    max = 3 maximum 4
    

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30. Null Safety
    

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31. A look at the nullability problem
    String aString = "Not Null";
    // Some time passes
    aString = null;
    // Some more time passes
    aString.equals(“some value”);
    

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32. Enter the Nullable Type
    val a: String = “cannot be null"
    val b: String? = null
    

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33. Unwrapping Nullables
    b?.equals(“Other string”) // Safe
    b!!.equals(“Other String”) // Unsafe
    

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34. Nullable Chaining
    b?.toDoubleOrNull()?.compareTo(1)
    if (b != null) {
    if (Double.valueOf(b) != null) {
    return Double.valueOf(b).compareTo(1.0);
    }
    }
    return null;
    

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35. Elvis Operator
    val result = b?.toDoubleOrNull()?.equals(1) ?: false
    

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36. Classes
    

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37. Classes
    class Person(val firstName: String,
    var lastName: String)
    

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38. Classes
    class Person constructor(val firstName: String,
    var lastName: String)
    

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39. Classes
    class Person constructor(fName: String, lName: String) {
    val firstName: String = fName
    var lastName = lName
    }
    

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40. Classes public final class Person {
    @NotNull
    private final String firstName;
    @NotNull
    private String lastName;
    @NotNull
    public final String getFirstName() {
    return this.firstName;
    }
    @NotNull
    public final String getLastName() {
    return this.lastName;
    }
    public final void setLastName(@NotNull String var1) {
    Intrinsics.checkParameterIsNotNull(var1, "");
    this.lastName = var1;
    }
    public Person(@NotNull String fName, @NotNull String lName) {
    Intrinsics.checkParameterIsNotNull(fName, "fName");
    Intrinsics.checkParameterIsNotNull(lName, "lName");
    super();
    this.firstName = fName;
    this.lastName = lName;
    }
    }
    

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41. Properties
    class Person constructor(private val firstName: String,
    private var lastName: String) {
    fun getFullName() = "$firstName $lastName”
    }
    

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42. Properties
    public final class Person {
    private final String firstName;
    private String lastName;
    @NotNull
    public final String getFullName() {
    return "" + this.firstName + ' ' + this.lastName;
    }
    public Person(@NotNull String firstName, @NotNull String lastName) {
    Intrinsics.checkParameterIsNotNull(firstName, "firstName");
    Intrinsics.checkParameterIsNotNull(lastName, "lastName");
    super();
    this.firstName = firstName;
    this.lastName = lastName;
    }
    }
    

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43. Properties
    class Person(private var firstName: String,
    private var lastName: String) {
    var fullName
    get() = "$firstName $lastName"
    set(value) {
    val split = value.split(" ")
    firstName = split[0]
    lastName = split[1]
    }
    }
    

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44. Properties
    public final class Person {
    private String firstName;
    private String lastName;
    @NotNull public final String getFullName() {
    return "" + this.firstName + ' ' + this.lastName;
    }
    public final void setFullName(@NotNull String value) {
    List split = StringsKt.split$default(…);
    this.firstName = (String)split.get(0);
    this.lastName = (String)split.get(1);
    }
    public Person(@NotNull String firstName, @NotNull String lastName) {
    (…)
    }
    }
    

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45. Lambdas
    

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46. Lambdas
    val sum3: { x: Int, y: Int -> x + y }
    val sum: (x: Int, y: Int) -> Int = { x, y -> x + y }
    val sum2: (Int, Int) -> Int = { x, y -> x + y }
    

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47. Lambdas
    val sum: (x: Int, y: Int) -> Int = { x, y -> x + y }
    /** A function that takes 2 arguments. */
    public interface Function2 : Function {
    /** Invokes the function with the specified arguments. */
    public operator fun invoke(p1: P1, p2: P2): R
    }
    

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48. Collections
    val result = list.map { it + 2 }
    .filter { value -> value % 2 == 0 }
    .firstOrNull{ it > 5 }
    val list = arrayOf(1, 2, 3, 4, 5)
    

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49. Sequences
    val result = list.asSequence()
    .map { it + 2 }
    .filter { value -> value % 2 == 0 }
    val list = arrayOf(1, 2, 3, 4, 5)
    .firstOrNull{ it > 5 }
    

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50. Questions?
    Almost done :)
    

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51. Resources
    • https://www.meetup.com/KotlinTLV/
    • https://www.facebook.com/groups/KotlinTLV
    • https://github.com/KotlinTLV
    • http://slack.kotlinlang.org/
    • https://try.kotlinlang.org/
    • http://kotlinlang.org/docs/reference/
    • https://kotlin.link/
    

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52. Thank you!
    

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