- Discuss Java pros and cons - Kool features in Kotlin. - How to use Kotlin in Android development. - How to migrate old Android java code to new Kotlin code.

1. Kotlin Language for
Android Development
Huỳnh Quang Thảo
Silicon Straits Saigon
Google Developer Expert on Android

2. Java1995

3. Java
Android
1995
2008

4. Java
Android
Android Studio
1995
2008
2013

5. Java
Android
Android Studio
Kotlin
1995
2008
2013
2/2016

6. Java
Android
Android Studio
Kotlin
Officially Support Kotlin
1995
2008
2013
2/2016
5/2017

7. Java
- Popular.

8. Java
- Popular.
- Strongly type language.

9. Java
- Popular.
- Strongly type language.
- Easy to learn.

10. Java
- Popular.
- Strongly type language.
- Easy to learn.
- Document.

11. Why Java sucks

12. Java culture

13. Java culture
C# has:
- 2007: lambda expressions, extension methods, first-class function
- 2010: generic covariance, contra variance …
- 2015: Null-Conditional operator.
- 2017: Pattern matching

14. Java culture
C# has:
- 2007: lambda expressions, extension methods, first-class function
- 2010: generic covariance, contra variance …
- 2015: Null-Conditional operator.
- 2017: Pattern matching
2014: Java is trying to adopt functional programming style from Java 8

15. Java culture
C# has:
- 2007: lambda expressions, extension methods, first-class function
- 2010: generic covariance, contra variance …
- 2015: Null-Conditional operator.
- 2017: Pattern matching
2014: Java is trying to adopt functional programming style from Java 8
Avoiding break compatible older API
Example: Generic API, Concurrency Collection API,….
(historical: why exist javax package)

16. Null Safety
public void printClassRoom(Person person) {
System.out.println(person.school.classRoom);
}

17. Null Safety
public void printClassRoom(Person person) {
if (person == null) return;
System.out.println(person.school.classRoom);
}

18. Null Safety
public void printClassRoom(Person person) {
if (person == null) return;
if (person.school == null) return;
System.out.println(person.school.classRoom);
}

19. Null Safety
public void printClassRoom(Person person) {
if (person == null) return;
if (person.school == null) return;
if (person.school.classRoom == null) return;
System.out.println(person.school.classRoom);
}

20. Null Safety
public void printClassRoom(Person person) {
if (person == null) return;
if (person.school == null) return;
// if (person.school.classRoom == null) return;
System.out.println(person.school.classRoom);
}

21. Null Safety
public void printClassRoom(Person person) {
if (person == null) return;
if (person.school == null) return;
// if (person.school.classRoom == null) return;
System.out.println(person.school.classRoom);
}

22. Null Safety
public void printClassRoom(Person person) {
if (person == null) return;
if (person.school.classRoom == null) return;
if (person.school == null) return;
System.out.println(person.school.classRoom);
}

23. Null Safety
public void printClassRoom(Person person) {
if (person == null) return;
if (person.school.classRoom == null) return;
if (person.school == null) return;
System.out.println(person.school.classRoom);
}

24. Null Safety
Speaking at a conference in 2009, Tony Hoare apologised for inventing
the null reference:
I call it my billion-dollar mistake. This has led to innumerable errors,
vulnerabilities, and system crashes, which have probably caused a
billion dollars of pain and damage in the last forty years.

25. Java generic type is
erasure
List<String> list = new ArrayList<String>();
list.add("Hello Google I/O");
String x = list.get(0);

26. Java generic type is
erasure
List<String> list = new ArrayList<String>();
list.add("Hello Google I/O");
String x = list.get(0);
List list = new ArrayList();
list.add("Hi");
String x = (String) list.get(0);

27. public <T>List<T> castCollection(List srcList, Class<T> clas){
List<T> list =new ArrayList<T>();
for (Object obj : srcList) {
if(obj instanceof T){
// processing code here
}
}
return list;
}

28. public <T>List<T> castCollection(List srcList, Class<T> clas){
List<T> list =new ArrayList<T>();
for (Object obj : srcList) {
if(obj instanceof T){
// processing code here
}
}
return list;
}
public <T>List<T> castCollection(List srcList, Class<T> clas){
List<Object> list =new ArrayList<Object>();
for (Object obj : srcList) {
if(obj instanceof Object){
// processing code here
}
}
}

29. Java array is covariance
A <: B => T[A] <: T[B]

30. Java array is covariance
A <: B => T[A] <: T[B]
Employee[] employees = new Employee[10];

31. Java array is covariance
A <: B => T[A] <: T[B]
Employee[] employees = new Employee[10];
employees[0] = new Employee();

32. Java array is covariance
A <: B => T[A] <: T[B]
Employee[] employees = new Employee[10];
employees[0] = new Employee();
// because Java array is covariance
Person[] people = employees;

33. Java array is covariance
A <: B => T[A] <: T[B]
Employee[] employees = new Employee[10];
employees[0] = new Employee();
// because Java array is covariance
Person[] people = employees;
people[0] = new Person();

34. Java array is covariance
A <: B => T[A] <: T[B]
Employee[] employees = new Employee[10];
employees[0] = new Employee();
// because Java array is covariance
Person[] people = employees;
people[0] = new Person();

35. … And More

37. JetBrains lead Dmitry Jemerov said that:
Most languages did not have the features they were looking
for, with the exception of Scala. However, he cited the slow
compile time of Scala as an obvious deficiency. One of the
stated goals of Kotlin is to compile as quickly as Java.
(Wikipedia)

40. Kotlin is built on JVM

41. Kotlin is built on JVM
- Can call Kotlin code from Java.

42. Kotlin is built on JVM
- Can call Kotlin code from Java.
- Can call Java code from Kotlin.

43. Kotlin Features

44. Type Inference
val a: Int = 100

45. Type Inference
val a: Int = 100
// b is implicit deduced to integer type
val b = 100

46. Type Inference
val a: Int = 100
// b is implicit deduced to integer type
val b = 100
b = "Kotlin"

47. Type Inference
val a: Int = 100
// b is implicit deduced to integer type
val b = 100
b = "Kotlin"

48. Null Safety

49. Null Safety
var a: String = "abc"
// compilation error
a = null

50. Null Safety
var a: String = "abc"
// compilation error
a = null
var b: String? = "abc"
// ok
b = null

51. Null Safety
var a: String = "abc"
// compilation error
a = null
var b: String? = "abc"
// ok
b = null
val l = a.length

52. Null Safety
var a: String = "abc"
// compilation error
a = null
var b: String? = "abc"
// ok
b = null
val l = a.length
val l = b.length

53. Null Safety
var a: String = "abc"
// compilation error
a = null
var b: String? = "abc"
// ok
b = null
val l = a.length
val l = b.length

54. val b:String? = null
if (b != null) {
// processing here
} else {
// b is null
}

55. val b:String? = null
if (b != null) {
// processing here
} else {
// b is null
}
b?.length

56. val b:String? = null
if (b != null) {
// processing here
} else {
// b is null
}
b?.length
b?.department?.head?.name

57. Kotlin array is invariant
Covariance: A < B => T[A] < T[B]

58. Kotlin array is invariant
Covariance: A < B => T[A] < T[B]
Contravariance: A < B => T[A] > T[B]

59. Kotlin array is invariant
Covariance: A < B => T[A] < T[B]
Contravariance: A < B => T[A] > T[B]
Invariance: nor Covariance or Contravariance

60. Kotlin array is invariant

61. Kotlin array is invariant

62. Higher Order Function

63. Higher Order Function
fun request(url: String, work: (url: String) -> Int) {
println("prepare request")
val result = work(url)
println("finish with result code: $result")
}

64. Higher Order Function
fun request(url: String, work: (url: String) -> Int) {
println("prepare request")
val result = work(url)
println("finish with result code: $result")
}

65. Higher Order Function
fun request(url: String, work: (url: String) -> Int) {
println("prepare request")
val result = work(url)
println("finish with result code: $result")
}
request("www.google.com", { url ->
println("sending request to $url")
200
})

66. Higher Order Function
fun request(url: String, work: (url: String) -> Int) {
println("prepare request")
val result = work(url)
println("finish with result code: $result")
}
request("www.google.com", { url ->
println("sending request to $url")
200
})
prepare request
sending request to www.google.com
finish with result code: 200

67. Higher Order Function
val strings = mutableListOf("Google", "I/O", "Extended")
// Find all strings with length > 3 —> [Google, Extended]
strings.filter { it.length > 3 }.sortedBy { it }

68. Higher Order Function
val strings = mutableListOf("Google", "I/O", "Extended")
// sort ascending —> [Extended, Google]
strings.filter { it.length > 3 }.sortedBy { it }

69. Higher Order Function
val strings = mutableListOf("Google", "I/O", "Extended")
// convert to upper case —> [EXTENDED, GOOGLE]
strings.filter { it.length > 3 }.sortedBy { it }.map { it.toUpperCase() }

70. Higher Order Function
val strings = mutableListOf("Google", "I/O", "Extended")
strings.filter { it.length > 3 }.sortedBy { it }.map { it.toUpperCase() }
[EXTENDED, GOOGLE]

71. Higher Order Function
val strings = mutableListOf("Google", "I/O", "Extended")
strings.filter { it.length > 3 }.sortedBy { it }.map { it.toUpperCase() }
[EXTENDED, GOOGLE]
public inline fun <T> Iterable<T>.filter(predicate: (T) -> Boolean): List<T> {
return filterTo(ArrayList<T>(), predicate)
}

72. Higher Order Function
val strings = mutableListOf("Google", "I/O", "Extended")
strings.filter { it.length > 3 }.sortedBy { it }.map { it.toUpperCase() }
[EXTENDED, GOOGLE]
public inline fun <T> Iterable<T>. filter (predicate: (T) -> Boolean): List<T> {
return filterTo(ArrayList<T>(), predicate)
}

73. Extension Function
val list = mutableListOf(1, 2, 3, 4, 5)
swap(list, 0, 1)

74. Extension Function
fun swap(list: MutableList<Int>, firstIndex: Int, secondIndex: Int) {
val tmp = list[firstIndex]
list[firstIndex] = list[secondIndex]
list[secondIndex] = tmp
}
val list = mutableListOf(1, 2, 3, 4, 5)
swap(list, 0, 1)

75. Extension Function
val list = mutableListOf(1, 2, 3, 4, 5)
list.swap(0, 1)
fun swap(list: MutableList<Int>, firstIndex: Int, secondIndex: Int) {
val tmp = list[firstIndex]
list[firstIndex] = list[secondIndex]
list[secondIndex] = tmp
}
val list = mutableListOf(1, 2, 3, 4, 5)
swap(list, 0, 1)

76. Extension Function
val list = mutableListOf(1, 2, 3, 4, 5)
list.swap(0, 1)

77. Extension Function
fun MutableList<Int>.swap(index1: Int, index2: Int) {
val tmp = this[index1]
this[index1] = this[index2]
this[index2] = tmp
}
val list = mutableListOf(1, 2, 3, 4, 5)
list.swap(0, 1)

78. Operator Overloading

79. Operator Overloading
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
operator fun plus(that: Point): Point = Point(x + that.x, y + that.y)
operator fun div(div: Int): Point = Point(x / div, y / div)
fun log() = println("($x,$y)")
}

80. Operator Overloading
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
operator fun plus(that: Point): Point = Point(x + that.x, y + that.y)
operator fun div(div: Int): Point = Point(x / div, y / div)
fun log() = println("($x,$y)")
}
val firstPoint = Point(6f, 5f)
val secondPoint = Point(8f, 3f)
// (7.0, 4.0)
val midPoint = (firstPoint + secondPoint) / 2
midPoint.log()

81. Operator Overloading
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
fun log() = println("($x,$y)")
}

82. Operator Overloading
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
fun log() = println("($x,$y)")
}
operator fun Point.plus(that: Point): Point = Point(x + that.x, y + that.y)
operator fun Point.div(div: Int): Point = Point(x / div, y / div)

83. Operator Overloading
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
fun log() = println("($x,$y)")
}
operator fun Point.plus(that: Point): Point = Point(x + that.x, y + that.y)
operator fun Point.div(div: Int): Point = Point(x / div, y / div)
val firstPoint = Point(6f, 5f)
val secondPoint = Point(8f, 3f)
// (7.0, 4.0)
val midPoint = (firstPoint + secondPoint) / 2
midPoint.log()

84. Reified type parameter

85. Reified type parameter
val items = listOf("one", 2, "three")
// print: [one, three]
println(items.filterIsInstance<String>())
public inline fun <reified R> Iterable<*>.filterIsInstance(): List<R> {
return filterIsInstanceTo(ArrayList<R>())
}

86. Reified type parameter
Intent i = new Intent(FirstActivity.this, SecondActivity.class);
startActivity(i);

87. Reified type parameter
Intent i = new Intent(FirstActivity.this, SecondActivity.class);
startActivity(i);
inline fun <reified T : Activity> Activity.startActivity() {
startActivity(Intent(this, T::class.java))
}

88. Reified type parameter
Intent i = new Intent(FirstActivity.this, SecondActivity.class);
startActivity(i);
inline fun <reified T : Activity> Activity.startActivity() {
startActivity(Intent(this, T::class.java))
}
startActivity<DetailActivity>()

89. Infix Call Function
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
fun multiply(k: Int) = Point(x * k, y * k)
}
val firstPoint = Point(6f, 5f)
firstPoint.multiply(3)

90. Infix Call Function
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
infix fun multiply(k: Int) = Point(x * k, y * k)
}
val firstPoint = Point(6f, 5f)
firstPoint.multiply(3)

91. Infix Call Function
class Point(xCoor: Float, yCoor: Float) {
val x = xCoor
val y = yCoor
infix fun multiply(k: Int) = Point(x * k, y * k)
}
val firstPoint = Point(6f, 5f)
firstPoint.multiply(3)
firstPoint multiply 3

92. DSL - Domain Specific
Language
language that's written to deal with a specific domain or set of concerns.
- SQL language
- Make file
- Gradle file

93. SELECT Country.name, COUNT(Customer.id) FROM Country
JOIN Customer
ON Country.id = Customer.country_id
GROUP BY Country.name
ORDER BY COUNT(Customer.id) DESC
LIMIT 1
Expose Library

94. SELECT Country.name, COUNT(Customer.id) FROM Country
JOIN Customer
ON Country.id = Customer.country_id
GROUP BY Country.name
ORDER BY COUNT(Customer.id) DESC
LIMIT 1
(Country join Customer)
.slice(Country.name, Count(Customer.id)).selectAll()
.groupBy(Country.name).orderBy(Count(Customer.id), isAsc = false).limit(1)
Expose Library

95. (Country join Customer)
.slice(Country.name, Count(Customer.id)).selectAll()
.groupBy(Country.name).orderBy(Count(Customer.id), isAsc = false).limit(1)
Customer.join(Country)

96. (Country join Customer)
.slice(Country.name, Count(Customer.id)).selectAll()
.groupBy(Country.name).orderBy(Count(Customer.id), isAsc = false).limit(1)
Customer join Country
(infix function)

97. (Country join Customer)
.slice(Country.name, Count(Customer.id)).selectAll()
.groupBy(Country.name).orderBy(Count(Customer.id), isAsc = false).limit(1)
Count: higher order function

98. (Country join Customer)
.slice(Country.name, Count(Customer.id)).selectAll()
.groupBy(Country.name).orderBy(Count(Customer.id), isAsc = false).limit(1)
isAsc = false: function with default argument

99. Integration Kotlin to
Android

100. Prepare Android Studio
- Download Android Studio 3.0 beta
- Install Kotlin plugin for Android Studio 2.x

101. Migrating from old Project

102. Migrating from old Project

103. Migrating from old Project

104. Migrating from old Project

105. Migrating from old Project

106. Starting from scratch

107. Kotlin & Java
Interoperability

108. Callling Java from Kotlin
package com.hqt.model;
public class Customer {
private String firstName;
private String lastName;
private int age;
public Customer(String firstName, String lastName, int age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
}

109. Callling Java from Kotlin
package com.hqt.model;
public class Customer {
private String firstName;
private String lastName;
private int age;
public Customer(String firstName, String lastName, int age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
} val customer = Customer("huynh", "thao", 3)
customer.firstName = "Huynh"

110. Callling Java from Kotlin

111. Callling Kotlin from Java
package com.hqt.kotlin
// Helper.kx
fun MutableList<Int>.swap(index1: Int, index2: Int) {
val tmp = this[index1]
this[index1] = this[index2]
this[index2] = tmp
}
val list = mutableListOf(1, 2, 3, 4, 5)
list.swap(0, 1)

112. Callling Kotlin from Java
package com.hqt.kotlin
// Helper.kx
fun MutableList<Int>.swap(index1: Int, index2: Int) {
val tmp = this[index1]
this[index1] = this[index2]
this[index2] = tmp
}
List<Integer> list = Arrays.asList(1, 2, 3);
com.hqt.kotlin.HelperKt.swap(list, 0, 1);
val list = mutableListOf(1, 2, 3, 4, 5)
list.swap(0, 1)

113. Callling Kotlin from Java
package com.hqt.kotlin
// Helper.kx
fun MutableList<Int>.swap(index1: Int, index2: Int) {
val tmp = this[index1]
this[index1] = this[index2]
this[index2] = tmp
}
List<Integer> list = Arrays.asList(1, 2, 3);
com.hqt.kotlin.HelperKt.swap(list, 0, 1);
val list = mutableListOf(1, 2, 3, 4, 5)
list.swap(0, 1)

114. Callling Kotlin from Java
package com.hqt.kotlin
// Helper.kx
fun MutableList<Int>.swap(index1: Int, index2: Int) {
val tmp = this[index1]
this[index1] = this[index2]
this[index2] = tmp
}
List<Integer> list = Arrays.asList(1, 2, 3);
com.hqt.kotlin.HelperKt.swap(list, 0, 1);
val list = mutableListOf(1, 2, 3, 4, 5)
list.swap(0, 1)

115. Reference
Kotlin in Action
https://kotlinlang.org/docs/tutorials/
https://github.com/irontec/android-kotlin-samples

116. Q&A