Starting up with Kotlin

From Java to Kotlin

The Kotlin code is compiled to Java bytecode.
Java to Kotlin converter tool is available in IntelliJ
Adding a data modifier to a class automatically generates several methods for it:
- equals
- hashCode
- toString

Example of data class:

data class Person(val name: String, val age: Int)

No need to use new keyword in Kotlin

Example:

val person = Person("Alice", 27)
println(person.name)

Kotlin syntax helps reduce redundancy.

Example of updateWeather function in Kotlin:

fun updateWeather(degrees: Int){
    // Using Pair shortcut syntax
    val(description: String, color: Color) =
        if(degrees < 10){
            Pair("cold", BLUE)
        } else if( degrees < 25) {
            Pair("mild", ORANGE)
        } else {
            Pair("hot", RED)
        }
    ...
}

Could simplify it further by removing the data types because Kotlin can infer it.

fun updateWeather(degrees: Int){
  // Using Pair shortcut syntax
  val(description, color) =
      if(degrees < 10){
          Pair("cold", BLUE)
      } else if( degrees < 25) {
          Pair("mild", ORANGE)
      } else {
          Pair("hot", RED)
      }
  ...
}

Could simplify it further by using when instead of if:

  fun updateWeather(degrees: Int){
      // Using Pair shortcut syntax and when clause
      val(description, color) = when {
          degrees < 10 -> Pair("cold", BLUE)
          degrees < 25 -> Pair("mild", ORANGE)
          else -> Pair("hot", RED)
      }
      ...
  }

Could alternatively use to instead of Pair:

  fun updateWeather(degrees: Int){
      // Using Pair shortcut syntax and when clause
      val(description, color) = when {
          degrees < 10 -> "cold" to BLUE
          degrees < 25 -> "mild" to ORANGE
          else -> "hot" to RED
      }
      ...
  }

Does the automatic Java to Kotin Converter always produce the idiomatic Kotlin code?
- No, it doesn't
- The Java code likely will look very "Java" in the Kotlin version.
- You need to be immersed in Kotlin syntax to recognize how to manually reduce redundancies or to use "prettier" Kotlin syntax.

Basics

"Hello, world" Example

Hello World:

package intro

fun main() {
    val name = "Kotlin"
    println("Hello, $name!")
}

info

The main function can go with or without arguments
- Example with argument:
```
  fun main(args: Array<String>)
```

main with argument Example:

package intro

fun main(args: Array<String>){
    val name = if(args.size > 0) args[0] else "Kotlin"
    println("Hello, $name!")
}

note

An Array<String> in kotlin is the same array as in java.
In Kotlin, if else clauses are expressions which can be assigned to a variable or returned in a function as shown above.
String templates utilize the $ sign next to a variable name to insert a value inside a string literal.

String templates work for variables and functions:
- "String Text Here: $variable"
- "String Text Here: ${functionCall(...)}"

What will be printed if no arguments are passed?

fun main(args: Array<String>) {
    println("Hello, ${args.getOrNull(0)}!")
}

Hello, null! will be printed

What will be printed?

fun foo(): String {
    println("Calculating foo...")
    return "foo"
}

fun main(args: Array<String>){
    println("First ${foo()}, second ${foo()}")
}

This will be printed:

  Calculating foo...
  Calculating foo...
  First foo, second foo

Variables

An assigned-once and read-only variable will be denoted with val
- This is similar to a final variable in java.
A mutable variable will be denoted with var
Types are inferred in Kotlin:
- val greeting: String = "Hi!" does not need to explicitly say it is a String.
- val greeting = "Hi!" knows it is a String type.
Why doesn't the following code compile?
```
var string = 1;
string = "abc";
```
- Because you can't assign a string literal to a variable of Int type.
Is it possible to modify an object stored in val?
- Yes
- There are no additional constraints.
- You can easily modify an object stored in val as in Java.
- val is an immutable reference which doesn't say anything about the content that is stored.
Example to demonstrate val is a read-only reference and not an object.
```
val languages = mutableListOf("Java")
languages.add("Kotlin")
```
Why doesn't the following code compile?
```
val list = listOf("Java")
list.add("Kotlin")
```
- Because you can't modify a read-only list.

tip

Don't omit types
Specify the types of a variable explicitly to improve clarity.

Functions

fun max(a: Int, b: Int): Int {
  return if(a > b) a else b
}

is equivalent to:

fun max(a: Int, b: Int) = if (a > b) a else b

Unit is very similar to void
- You don't really need to specify Unit as the return type if you don't want to.

fun displayMax(a: Int, b: Int): Unit {
  println(max(a,b))
}

Is it possible to call a top-level function from Java? If yes, how?
- You call it as a static function of the class, which name corresponds to the file name.

Calling a top-level function from Java

MyFile.kt

package intro

fun foo() = 0

UsingFood.java

package other;

import intro.MyFileKt;

public class UsingFoo {
  public static void main(String[] args){
    MyFileKt.foo();
  }
}

Named & default arguments

What will be printed?

println(
  listOf("a", "b", "c").joinToString(
    separator = "",
    prefix = "(",
    postfix = ")"
  )
);

Output: (abc)

Default Values

fun displaySeperator(character: Char = '*', size: Int = 10){
  ...
}

Could assign a value inside the function parameters to specify the default values.

Named arguments

Suppose you only want to specify the second parameter and keep the first one at its current default value.

fun displaySeperator(character: Char = '*', size: Int = 10){
  ...
}

...

displaySepartor(size = 5)

The output will be: *****

What will be printed?

fun displaySeperator(character: Char = '*', size: Int = 10){
  repeat(size) {
    print(character)
  }
}

displaySepartor(3, '5')

Answer: the code won't compile.

Control Structures

Conditionals: if & when

Ternary operator does not exist in Kotlin
Just use the if expression
Example:

val max = if(a > b) a else b

when expression:

enum class Color {
  BLUE, ORANGE, RED
}

fun getDescription(color: Color): String =
  when(color){
    BLUE -> "cold"
    ORANGE -> "mild"
    RED -> "hot"
  }

Checking multiple values at once Example:

fun respondToInput(input: String) = when(input) {
  // you can check mulitple values at once
  "y", "yes" -> "Good Outcome"
  "n", "no" -> "Bad Outcome"
  else -> "I don't understand you"
}

Any expression can be used as a branch Condition
Example:

fun mix(c1: Color, c2: Color) =
  when (setOf(c1, c2)) {
    setOf(RED, YELLOW) -> ORANGE
    setOf(YELLOW, BLUE) -> GREEN
    setOf(BLUE, VIOLET) -> INDIGO
    else -> throw Exception("Dirty color")
  }

Checking types
Example:

when(pet) {
  is Cat -> pet.meow()
  is Dog -> pet.woof()
}

Capturing when subject in a variable
Example:

when(val pet = getMyFavoritePet()) {
  is Cat -> pet.meow()
  is Dog -> pet.woof()
}

You should always replace if with when?
- False

Loops

While loops:

while(condition){
  ...
}

Do-while loops:

do{
  ...
} while(condition)

for loop:

val list = listOf("a", "b", "c")
for (s in list){
  print(s)
}

Iterating over map

val map = mapOf (1 to "one", 2 to "two", 3 to "three")

for((key, value) in map){
  println("$key = $value")
}

Iterating over range

// including upper bound
for (i in 1..9){
  print(i)
}

// excluding upper bound
for (i in 1 until 9){
  print(i)
}

Iterating over String
Example:
- The output will be: bcd

for (ch in "abc") {
  print(ch + 1);
}

Rewrite the following java code into Kotlin:

for(char c = '0'; c < '9'; c++){
  System.out.print(c);
}

Kotlin version:

for(c in '0' until '9'){
  print(c)
}

'in' checks & ranges

in could be used to check if an item belongs in something.
Example: c in 'a'..'z'
note
Both the lower and upper bound is checked.
Not in a range Example:
```
c !in '0'..'9'
```

in as when-condition Example:

fun recognize(c: Char) = when(c) {
  in '0'..'9' -> "It's a digit!"
  in 'a'..'z', in 'A'..'Z' -> "It's a letter!"
  else -> "I don't know..."
}

What will be printed?

println("Kotin" in "Java".."Scala")
println("Kotlin" in setOf("Java", "Scala"))

Answer: true false
- First one checks in alphabetical order. So, a word that starts with the letter 'K' like "Kotlin" lies between the words "Java" and "Scala" in alphabetical order.

Exceptions

throw is an expression

val percentage =
  if(number in 0..100)
    number
  else
    throw IllegalArgumentException(
      "A percentage value must be" + "between 0 and 100: $number")

try is an expression:

val number = try {
  Integer.parseInt(string)
} catch (e: NumberFormatException){
  return
}

@Throws annotation

@Throws(IOException::class)
fun foo() {
  throw IOException()
}

Is there any difference between calling foo and bar from Java?

fun foo() {
  throw IOException()
}

@Throws(IOException::class)
fun bar() {
  throw IOException()
}

Answer: The code calling bar compiles, but the code calling foo doesn't compile.

Extensions

Extension Functions

The time that the function extends is called a Receiver.
- In the example below, String is the receiver of the lastChar function.
- In the body of this function, we can access the receiver by this reference.
- this refers to String.

fun String.lastChar() = this.get(this.length - 1)

info

this can actually be omitted, resulting in this alternative:

fun String.lastChar() = get(length - 1)

note

You need to import the extension function to use it.

import com.example.util.lastChar
val c: Char = "abc".lastChar()

Calling Extension Functions from Java code

Extension functions are like static functions in java
Example java code:

import static StringExtensionKt.lastChar;
char c = lastChar("abc");

How many arguments does the repeat function have if you call it from Java?

fun String.repeat(n: Int): String {
  val sb = StringBuilder(n * length) for (i in 1..n) {
    sb.append(this)
  }
  return sb.toString()
}

Answer: 2
Is it possible to call a private member of String inside an extension function to String?

fun String.lastChar() = get(length - 1)

Answer: no
- You can't call a private member from a static function of another class in java.
- Kotlin extension functions are regular static functions defined in a separate auxiliary class.
- You cannot call private members from extensions.

Examples from the Standard Library

No Kotlin SDK
- Just JDK + Extensions
  - Benefits:
    - small runtime jar
    - easy Java interop

An example of an Extension function: until

infix fun Int.until(to: Int): IntRange

//to call it, two different syntaxes:
1.until(10)
1 until 10

Formatting multi-line strings

Indentations can be caught in multi-line strings
- The code below uses the trimMargin extension to address that.

val q = """To Code,
    | or not to code?..""".trimMargin()

Thus, the output becomes:

To Code,
or not to code?..

Conversion to numbers:
- Example: "123".toInt()
- "1e-10".toDouble()

Custom extension functions

You can create your own extension functions:

fun <T> T.eq(other: T?): Boolean {
    return this == other
}

val a = "Hello"
val b = "Hello"
val c = "World"

println(a.eq(b)) // true
println(a.eq(c)) // false

What is the type of 'a' to 1.0?
- Answer: Pair<Char, Double>
Write Sum as an extension function.
Answer:

fun List<Int>.sum(): Int {
  var total = 0
  for(i in this){
    total += i
  }
  return total
}

fun main(args: Array<String>){
  val ls = listOf(1, 2, 3)
  val output = ls.sum()
  println(output)
}

Calling Extensions

What will be printed?

open class Parent
class Child: Parent()

fun Parent.foo() = "parent"
fun Child.foo() = "child"

fun main(args: Array<String>) {
  val parent: Parent = Child()
  println(parent.foo())
}

Answer: parent
What will be printed?

fun String.get(index: Int) = '*'

fun main(args: Array<String>) {
  println("abc".get(1))
}

Answer: b

From Java to Kotlin​

Basics​

"Hello, world" Example​

Variables​

Functions​

Calling a top-level function from Java​

Named & default arguments​

Default Values​

Named arguments​

What will be printed?​

Control Structures​

Conditionals: if & when​

Loops​

'in' checks & ranges​

Exceptions​

Extensions​

Extension Functions​

Calling Extension Functions from Java code​

Examples from the Standard Library​

Formatting multi-line strings​

Custom extension functions​

Calling Extensions​

From Java to Kotlin

Basics

"Hello, world" Example

Variables

Functions

Calling a top-level function from Java

Named & default arguments

Default Values

Named arguments

What will be printed?

Control Structures

Conditionals: if & when

Loops

'in' checks & ranges

Exceptions

Extensions

Extension Functions

Calling Extension Functions from Java code

Examples from the Standard Library

Formatting multi-line strings

Custom extension functions

Calling Extensions