A quick and fast intro to Kotlin

KOTLIN
● Primary target JVM
● Javascript
● Compiled in Java byte code
● Created for industry
Core goals is 100% Java interoperability.

KOTLIN main features
Concise
Drastically reduce the amount of
boilerplate code you need to write.
Safe
Avoid entire classes of errors such
as null pointer exceptions.
Interoperable
Leverage existing frameworks and
libraries of the JVM with 100% Java
Interoperability.

data class Person(
var name: String,
var surname: String,
var age: Int)
Create a POJO with:
● Getters
● Setters
● equals()
● hashCode()
● toString()
● copy()
Concise
public class Person {
final String firstName;
final String lastName;
public Person(...) {
...
}
// Getters
...
// Hashcode / equals
...
// Tostring
...
// Egh...
}

KOTLIN lambdas
● must always appear between curly brackets
● if there is a single parameter then it can be referred to by it
Concise
val list = (0..49).toList()
val filtered = list
.filter({ x -> x % 2 == 0 })
val list = (0..49).toList()
val filtered = list
.filter { it % 2 == 0 }

// ERROR
SAFE CALL
NULL safety

Extend existing classes functionality
Ability to extend a class with new functionality without having to inherit from the class
● does not modify classes
● are resolved statically

Extend existing classes functionality
fun String.lastChar() = this.charAt(this.length() - 1)
// this can be omitted
fun String.lastChar() = charAt(length() - 1)
fun use(){
Val c: Char = "abc".lastChar()
}

Everything is an expression
val max = if (a > b) a else b
val hasPrefix = when(x) {
is String -> x.startsWith("prefix")
else -> false
}
when(x) {
in 1..10 -> ...
102 -> ...
else -> ...
}
boolean hasPrefix;
if (x instanceof String)
hasPrefix = x.startsWith("prefix");
else
hasPrefix = false;
switch (month) {
case 1: ... break
case 7: ... break
default: ...
}

Default Parameters
fun foo( i :Int, s: String = "", b: Boolean = true) {}
fun usage(){
foo( 1, b = false)
}

for loop
can iterate over any type that provides an iterator implementing next() and hasNext()
for (item in collection)
print(item)
for ((index, value) in array.withIndex()) {
println("the element at $index is $value")
}

Collections
● Made easy
● distinguishes between immutable and mutable collections
val numbers: MutableList = mutableListOf(1, 2, 3)
val readOnlyNumbers: List = numbers
numbers.add(4)
println(readOnlyView) // prints "[1, 2, 3, 4]"
readOnlyNumbers.clear() // -> does not compile

// Using R.layout.activity_main from the main source set
import kotlinx.android.synthetic.main.activity_main.*
class MyActivity : Activity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
textView.setText("Hello, world!")
}
}
public class MyActivity extends Activity() {
@override
void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView textView = (TextView) findViewById(R.id.textView);
textView.setText("Hello, world!");
}
}
Kotlin Android Extensions

Extension functions
fun Fragment.toast(message: CharSequence, duration: Int = Toast.LENGTH_SHORT) {
Toast.makeText(getActivity(), message, duration).show()
}
fragment.toast("Hello world!")

Activities
startActivity( intentFor< NewActivity > ("Answer" to 42) )
Intent intent = new Intent(this, NewActivity.class);
intent.putExtra("Answer", 42);
startActivity(intent);

Functional support (Lambdas)
view.setOnClickListener { toast("Hello world!") }
View view = (View) findViewById(R.id.view);
view.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(this, "asdf", Toast.LENGTH_LONG)
.show();
}
});

Dynamic Layout
scrollView {
linearLayout(LinearLayout.VERTICAL) {
val label = textView("?")
button("Click me!") {
label.setText("Clicked!")
}
editText("Edit me!")
// codice koltin
// ...
}
}.style(...)
?
Click me!
Edit me!

Easily mixed with Java
● Do not have to convert everything at once
● You can convert little portions
● Write kotlin code over the existing Java code
Everything still works

Kotlin costs nothing to adopt
● It’s open source
● There’s a high quality, one-click Java to Kotlin converter tool
● Can use all existing Java frameworks and libraries
● It integrates with Maven, Gradle and other build systems.
● Great for Android, compiles for java 6 byte code
● Very small runtime library 924 KB

Kotlin usefull links
● A very well done documentation : Tutorial, Videos
● Kotlin Koans online
● Constantly updating in Github, kotlin-projects
● Talks: Kotlin in Action, Kotlin on Android

What Java has that Kotlin does not
https://kotlinlang.org/docs/reference/comparison-to-java.html

Primitive Types
Everything is an object
we can call member functions and properties on any variable.
val a: Int? = 1
val b: Long? = a
print(a == b)

Primitive Types
val a: Int? = 1
val b: Long? = a
print(a == b) // FALSE //

Primitive Types
val b: Byte = 1
val i: Int = b
val i: Int = b.toInt()
val a: Int? = 1
val b: Long? = a

Primitive Types
val b: Byte = 1
val i: Int = b // ERROR //
val i: Int = b.toInt() // OK //
val a: Int? = 1
val b: Long? = a

Static Members
class MyClass {
companion object Factory {
fun create(): MyClass = MyClass()
}
}
val instance = MyClass.create()
Singleton
object MyClass {
// ....
}

Gradle Goes Kotlin
https://kotlinlang.org/docs/reference/using-gradle.html

Thank You
Erinda Jaupaj
@ErindaJaupi

A quick and fast intro to Kotlin

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