Kotlin Basics: A Quick Guide

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Content

println() v print()
val & var
1. Strings and Booleans
2. Numbers
3. Displaying Variables
Operators
1. Numbers
2. Assignments
3. Comparisons
4. Logical
Strings
1. Access a String
2. String Length
3. String Functions
4. Comparing Strings
5. Finding a String in a String
6. Quotes Inside a String
7. Escape Characters
8. String Concatenation
Control Flow / Conditions
1. If .. Else
2. Ternary
3. When
4. While
5. Do while
Arrays
1. Access
2. Change
3. Length / Size
4. Exist
5. For
6. Range
Assertion Methods
Functions
1. Parameters
2. return x
3. return x + y
4. eturn x + y (short)
Objected Oriented Programming
1. Class
2. Create Object from class
3. Access the properties and add some values
4. Constructor Method
5. Superclass
6. Subclass
7. Create an object of MyChildClass and call myFunction

1. println() v print()

println("Hello World!")

println("I am learning Kotlin.")

println("It is awesome!")

println(3 + 3)

println("Hello" + " - " + "World") // This is a comment
print("Hello World! ")

print("I am learning Kotlin. ")

print("It is awesome!")

println(""" println("Hello World!") println("I am learning Kotlin.") println("It is awesome!") println(3 + 3) println("Hello" + " - " + "World") // This is a comment """)

2. val & var

Data Types, Define Variables, & Update Variables
camelCase: variables, functions, properties. val studentName = "Alice"
PascalCase: classes, objects. class StudentProfile { }, object NetworkManager { }
UPPER_SNAKE_CASE: constants. const val MAX_USERS = 100
lowercase: packages. package com.example.myapp
Types: Int, Double, Char, Boolean, Strings, Arrays
Convert: toByte(), toShort(), toInt(), toLong(), toFloat(), toDouble(), toChar()

val myNum = 5             // Int

val myDoubleNum = 5.99    // Double

val myLetter = 'D'        // Char

val myBoolean = true      // Boolean

val myText = "Hello"      // String

val myNum: Int = 5 // Int val myDoubleNum: Double = 5.99 // Double val myLetter: Char = 'D' // Char val myBoolean: Boolean = true // Boolean val myText: String = "Hello" // String

— Strings and Booleans



    var name = "John"      // String (text)

    val birthyear = 1975   // Int (number)

    println(name)          // Print the value of name

    println(birthyear)     // Print the value of birthyear
    var name: String = "John"  // String

    val birthyear: Int = 1975  // Int

    println(name)

    println(birthyear)
    val isKotlinFun: Boolean = true

    val isFishTasty: Boolean = false

    println(isKotlinFun)   // Outputs true

    println(isFishTasty)
    // declare without assigning value

    var ming: String

    ming = "John"

    println(ming)

/* Will not work: var name name = "John" println(name) */

— Numbers

Int: -2,147,483,648 to 2,147,483,647
Byte: -128 to 127
Short: 32768 to 32767
Long: -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 OR End with L
Float: 6 decimals Or End with F
Double: 15 decimals / 1.7976931348623157E308

val myNum: Int = 5

— Displaying Variables

println("Hello " + name)

val lastName = "Doe" val fullName = name + ' ' + lastName println(fullName)

3. Operators

— Numbers

+	Addition             x + y

-	Subtraction          x - y

*	Multiplication	     x * y

/	Division	     x / y

%	Modulus / remainder  x % y

++	Increment            ++x

--	Decrement	     --x

— Assignments

=	  x = 5	        x = 5

+=	  x += 3	x = x + 3

-=	  x -= 3	x = x - 3

*=	  x *= 3	x = x * 3

/=	  x /= 3	x = x / 3

%=	  x %= 3	x = x % 3

— Comparisons

==	Equal to	            x == y

!=	Not equal	            x != y

>	Greater than	            x > y

<	Less than	            x < y

>=	Greater than or equal to    x >= y

<=	Less than or equal to	    x <= y

— Logical

&& 	and

|| 	or

!	not / reverse

4. Strings

var greeting = "Hello"

var greeting: String = "Hello"

var name: String

name = "John"

println(name)

— Access a String

var txt = "Hello World"

println(txt[0]) // first element (H)

println(txt[2]) // third element (l)

— String Length

var txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

println("The length of the txt string is: " + txt.length)

— String Functions

var txt = "Hello World"

println(txt.toUpperCase())   // Outputs "HELLO WORLD"

println(txt.toLowerCase())   // Outputs "hello world"

— Comparing Strings

var txt1 = "Hello World"

var txt2 = "Hello World"

println(txt1.compareTo(txt2))  // Outputs 0 (they are equal)

— Finding a String in a String

var txt = "Please locate where 'locate' occurs!"

println(txt.indexOf("locate")) // Outputs 7

— Quotes Inside a String

var txt1 = "It's alright"

var txt2 = "That's great"

— Escape Characters

var txt = "We are the so-called \"Vikings\" from the north."

println(txt)

— String Concatenation

var firstName = "John"

var lastName = "Doe"

println(firstName + " " + lastName)

var firstName = "John " var lastName = "Doe" println(firstName.plus(lastName))

5. Control Flow / Conditions

— If .. Else

val time = 20

val greeting = if (time < 18) {
  "Good day."
} else {
  "Good evening."
}
println(greeting)

— Ternary

val time = 20

val greeting = if (time < 18) "Good day." else "Good evening."
println(greeting)

— When

val day = 4
val result = when (day) {

  1 -> "Monday"

  2 -> "Tuesday"

  3 -> "Wednesday"

  4 -> "Thursday"

  5 -> "Friday"

  6 -> "Saturday"

  7 -> "Sunday"

  else -> "Invalid day."

}

println(result)

// Outputs "Thursday" (day 4)

— While

Checks the condition first.

var i = 0

while (i < 10) {
  println(i)
  i++
  if (i == 4) {
    break         // break
  }
}

var i = 0
while (i < 10) {
  if (i == 4) {
    i++
    continue      // continue
  }
  println(i)
  i++
}

— Do while

Execute at least once

var i = 0

do {

  println(i)

  i++

}

while (i < 5)

6. Arrays

— Access

val cars = arrayOf("Volvo", "BMW", "Ford", "Mazda")

println(cars[0])

// Outputs Volvo

— Change

val cars = arrayOf("Volvo", "BMW", "Ford", "Mazda")

cars[0] = "Opel"

println(cars[0])

// Now outputs Opel instead of Volvo

— Length / Size

val cars = arrayOf("Volvo", "BMW", "Ford", "Mazda")

println(cars.size)

// Outputs 4 

— Exist

val cars = arrayOf("Volvo", "BMW", "Ford", "Mazda")

if ("Volvo" in cars) {

  println("It exists!")

} else {

  println("It does not exist.")

}

val nums = arrayOf(2, 4, 6, 8) if (2 in nums) { println("It exists!") } else { println("It does not exist.") }

— For

i.e. loop

val cars = arrayOf("Volvo", "BMW", "Ford", "Mazda")

for (x in cars) {

  println(x)

}

— Range

e.g Range

for (chars in 'a'..'x') {

  println(chars)

}
for (nums in 5..15) {

  println(nums)

} 
for (nums in 5..15) {

  if (nums == 10) {

    break               // break

  }

  println(nums)

}

for (nums in 5..15) { if (nums == 10) { continue // continue } println(nums) }

7. Assertion Method

assertEquals(expected, actual) → check equality
assertNotEquals(unexpected, actual) → check not equal
assertTrue(condition) → condition must be true
assertFalse(condition) → condition must be false
assertNull(value) → value must be null
assertNotNull(value) → value must not be null
assertSame(expected, actual) → they must be the same object
assertNotSame(unexpected, actual) → not the same object
fail("message") → force a failure (useful if some code should not be reached)

class ExampleUnitTest {  // Object Oriented Programming: Class (car), Object (open, proton, volvo)

    @Test

    fun addition_isCorrect() {

        assertEquals(4, 2 + 2)

    }

}
class ModifiedUnitTest {

    @Test

    fun addition_isIncorrect() {

        assertEquals(4, 1 + 2)

    }

}

fun addition_isCorrect() { assertEquals(4, 2 + 2) }

8. Functions

fun myFunction() {

  println("I just got executed!")

}  
fun main() {

  myFunction()

  myFunction()

  myFunction()

}

// I just got executed! // I just got executed! // I just got executed!

— Parameters

fun myFunction(fname: String) {

  println(fname + " Doe")

}
fun main() {

  myFunction("John")

  myFunction("Jane")

  myFunction("George")

}
// John Doe

// Jane Doe

// George Doe 
fun myFunction(fname: String, age: Int) {

  println(fname + " is " + age)

}
fun main() {

  myFunction("John", 35)

  myFunction("Jane", 32)

  myFunction("George", 15)

}

// John is 35 // Jane is 32 // George is 15

— return x

fun myFunction(x: Int): Int {

  return (x + 5)

}
fun main() {

  var result = myFunction(3)

  println(result)

}

// 8 (3 + 5)

— return x + y

fun myFunction(x: Int, y: Int): Int {

  return (x + y)

}
fun main() {

  var result = myFunction(3, 5)

  println(result)

}

// 8 (3 + 5)

— return x + y (short)

fun myFunction(x: Int, y: Int) = x + y
fun main() {

  var result = myFunction(3, 5)

  println(result)

}

// 8 (3 + 5)

9. Objected Oriented Programming

— Class

class Car {

  var brand = ""

  var model = ""

  var year = 0

}

— Create Object from class

class Car {

  var brand = ""

  var model = ""

  var year = 0

}

val c1 = Car()

— Access the properties and add some values

c1.brand = "Ford"

c1.model = "Mustang"

c1.year = 1969
println(c1.brand)   // Outputs Ford

println(c1.model)   // Outputs Mustang

println(c1.year)    // Outputs 1969 
val c1 = Car()

c1.brand = "Ford"

c1.model = "Mustang"

c1.year = 1969
val c2 = Car()

c2.brand = "BMW"

c2.model = "X5"

c2.year = 1999

println(c1.brand) // Ford println(c2.brand) // BMW

— Constructor Method

class Car(var brand: String, var model: String, var year: Int)

fun main() { val c1 = Car("Ford", "Mustang", 1969) val c2 = Car("BMW", "X5", 1999) val c3 = Car("Tesla", "Model S", 2020) }

— Superclass

open class MyParentClass {

  val x = 5

}

— Subclass

class MyChildClass: MyParentClass() {

  fun myFunction() {

    println(x) // x is now inherited from the superclass

  }

}

— Create an object of MyChildClass and call myFunction

fun main() {

  val myObj = MyChildClass()

  myObj.myFunction()

}

Note: Most code examples are adapted from W3Schools for educational purposes.