The introduction of Kotlin

1. To Kotlin or not to Kotlin,
That is the question

2. 自我介紹
汪 永興
LINE 京都開發室
Freddie Wang

3. 1.5M 50+ 250+
Developers Team size Contributors

4. Kotlin is designed for modernized
Java

5. • Readability (functional)
• Reuse (extensible, DSL-friendly)
• Interoperability with JVM
• Safety tooling (except the doc generation tool...)

6. Standing on the shoulder of giants

7. But definitely not

8. The improvement(?) from Java:
• Nullability
• Lambda + inline
• Properties
• Extension
• String template
• Operator overloading
• Data class
• Inner function
• No checked exception

9. val nullable: String? = "This is the wrong parent"
val nonnuable: String = null //Compiler error
val nullable: String? = null //OK
Nullability

10. Lambda + Inline
public inline fun measureTimeMillis(block: () -> Unit): Long {
val start = System.currentTimeMillis()
block()
return System.currentTimeMillis() - start
}

11. Lambda + Inline
public inline fun measureTimeMillis(block: () -> Unit): Long {
val start = System.currentTimeMillis()
block()
return System.currentTimeMillis() - start
}

12. Lambda + Inline
measureTimeMillis {
val jobs = List(1000) {
thread {
Thread.sleep(1000)
}
}
jobs.forEach { it.join() }
}

13. Lambda + Inline
measureTimeMillis {
val start = System.currentTimeMillis()
val jobs = List(1000) {
thread {
Thread.sleep(1000)
}
}
jobs.forEach { it.join() }
return System.currentTimeMillis() - start
}

14. Properties
class Image(val width: Int, val height: Int) {
val pixels: IntArray
init {
pixels = IntArray(this.width * this.height)
}
}

15. Properties
val image = Image(200, 200)
print(image.height) //200

16. inline fun AppCompatActivity.replaceFragment(
tag: String? = null,
animation: Boolean = false,
factory: (() -> Fragment)
) =
supportFragmentManager.beginTransaction().apply {
if (animation) {
setCustomAnimations(R.anim.fade_in, R.anim.fade_out)
}
replace(R.id.container, factory(), tag)
addToBackStack(null)
}.commit()
Extension

17. inline fun AppCompatActivity.replaceFragment(
tag: String? = null,
animation: Boolean = false,
factory: (() -> Fragment)
) =
supportFragmentManager.beginTransaction().apply {
if (animation) {
setCustomAnimations(R.anim.fade_in, R.anim.fade_out)
}
replace(R.id.container, factory(), tag)
addToBackStack(null)
}.commit()
Extension

18. override fun onCreate(savedInstanceState: Bundle?) {
...
replaceFragment {
MyFragment.newInstance()
}
}
Extension

19. Log.d(
TAG, "lineTo, from display point($x, $y), to internal point ($internalX, $internalY)"
)
String Template
Log.d(
TAG, "lineTo, from display point(" + x + "," + y + "), to internal point ( " +
internalX + " + "," + internalY + ")"
)

20. Operator Overloading
operator fun PointF.plus(pointB: PointF): PointF =
PointF(this.x + pointB.x, this.y + pointB.y)

21. Operator Overloading
operator fun PointF.plus(pointB: PointF): PointF =
PointF(this.x + pointB.x, this.y + pointB.y)

22. Operator Overloading
val p1 = controlPoint.add(prevPoint)

23. Operator Overloading
val p1 = controlPoint.add(prevPoint)
val p1 = controlPoint + prevPoint

24. Data class
data class Customer(var name: String, var email: String)
Neither need to create getter/setter manually, nor use the Lombok

25. Inner Function
fun dfs(graph: Graph) {
fun dfs(current: Vertex, visited: Set<Vertex>) {
if (!visited.add(current)) return
for (v in current.neighbors)
dfs(v, visited)
}
dfs(graph.vertices[0], HashSet())
}

26. Inner Function
fun dfs(graph: Graph) {
fun dfs(current: Vertex, visited: Set<Vertex>) {
if (!visited.add(current)) return
for (v in current.neighbors)
dfs(v, visited)
}
dfs(graph.vertices[0], HashSet())
}

27. No checked Exception
try {
FileReader file = new FileReader(file);
BufferedReader fileInput = new BufferedReader(file);
// ...
fileInput.readLine();
// ...
fileInput.close();
} catch (IOException e) {
// handle error
}

28. No checked Exception
try {
Data data = readData(filename);
} catch (IOException e) {
// handle error
}

29. No checked Exception
Data readData(String filename) throws IOException {
FileReader file = new FileReader(filename);
BufferedReader fileInput = new BufferedReader(file);
// ...
fileInput.readLine();
// ...
fileInput.close();
}

30. No checked Exception
Data readData(String filename) throws IOException {
FileReader file = new FileReader("test.txt");
BufferedReader fileInput = new BufferedReader(file);
// ...
fileInput.readLine();
// ...
fileInput.close();
}

31. No checked Exception
List<Result> results = fileNames.stream()
.map(filename -> readData(filename))
.collect(Collectors.toList());

32. No checked Exception
List<Result> results = fileNames.stream()
.map(filename -> readData(filename))
.collect(Collectors.toList());

33. No checked Exception
val results = filenames.map { filename -> readData(filename) }

34. Which projects are using Kotlin in LINE
• LINE android: 77% Java, 23% Kotlin
• Line Creator Studio (LINE拼貼): 100% Kotlin
• Clova SDK
• and others...

35. Summary
• If you don’t have any programming experience, learn Java first.
• If you are experienced Java developer, you should try Kotlin now.
• No doubt Kotlin will become more and more popular.

36. Q&A