To practice Kotlin

1. Open Android studio
2. Choose a new Empty Activity > Next > Name the project > Choose Kotlin Language > Finish
3. Click on tools  > Kotlin > Kotlin REPL
4. The REPL will appear in the window's bottom
5. Press Ctrl + Enter to execute any code

Main Function in Kotlin

    I have covered how to write a function in Kotlin in this Article. 

    To run a kotlin program, instead of using the interpreter or the repl, we need a main function as it is the first thing to be executed. We open a new kotlin file class and its extension is .kt in Android Studio or IntelliJ IDEA.  Every kotlin function return something and when nothing is specified, it returns "unit", which mean no value. We write a main function as follows:

 // Calling a print line function from main function where it takes an array of strings as its argument

fun main (args: Array<String>){
    println("What is your name?")
}

        

Then click on the green run button next to fun main, the run log window will open on the bottom and the output will be as follows:

What is your name?

Main function's arguments in Kotlin

 // Calling a print line function from main function with arguments and using a string template 

// We get the first element of args which is "World"

// We use $ and {} for an expression

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

Hello, World

//A variable num with value 20

fun main (args: Array<String>){
    val num = 20
    println("number = $num")
}

//The output

number = 20

//A variable name with value Leila

fun main (args: Array<String>){
    val name = "Leila"
    println("My name is ${name.length} letters")
}

//The output

My name is 5 letters

In Kotlin, everything has a value

you passed!

import java.util.*
fun main (args: Array<String>){
    val candy = candyType()
    println(candy)
}
fun candyType(): String {
    val candy = listOf("Toffee", "caramels", "gummies", "chocolate")
    return candy[Random().nextInt(4)]
}

Quiz

1. Not designing the App before starting coding (Both UI/UX).
2. Not deciding about the features of the App from the beginning. 
3. Using more activities than needed. Since the activities have the same views but different content. 
4. Not using classes from the beginning. 
5. Not having the final UI elements before coding. 
6. Not using a style resource file from the start. You will change only one line to change a view’s design or style.
7. Not being patient to learn new things.
8. Wanting to fix a problem right away.
9. Giving up too soon when trying to fix a bug.
10. I haven’t been convinced that it is normal not to know how to do things at the start and it is ok to either search or ask someone.

• Testing Testing Testing is more important than you think. Several people testing a feature is a must not just one or two.

• You have to simulate a normal user’s behavior or monitor the users while using your application to understand their behavior more & update accordingly

• Don’t Don’t Don’t depend on your own assumptions when testing your application. The users probably have different behaviors from your own.

//Concatenation 

val firstName = "Leila"

val lastName = "Kim"

firstName  + lastName

res0: kotlin.String = LeilaKim

"Leila" + "Kim"

res1: kotlin.String = LeilaKim

//Building a String and Replacing text by its value using the dollar sign

"My name is $firstName"

res2: kotlin.String = My name is Leila

//We can use any type even an Integer

val age = 20

"$firstName is $age years old."

res3: kotlin.String = Leila is 20 years old.

//Integers can get added first before being displayed in the result

val brother = 1

val sister = 1 

"$firstName has ${brother + sister} siblings"

res4: kotlin.String = Leila has 2 siblings

// A variable of type Boolean with value true

val isEmpty: Boolean = true

// A variable of type Boolean with value false

val isFull: Boolean = false

//Two variables of value 21 and 18

val leilaAge = 21

val sisterAge = 18

//Equal comparison "use a double equal sign"

//Is 21 equal to 18? No, so the output is false

leilaAge == sisterAge

res0: kotlin.Boolean = false

//Not equal comparison

//Is 21 not equal to 18? Yes, so the output is true

leilaAge != sisterAge

res2: kotlin.Boolean = true

//Greater than comparison

leilaAge >  sisterAge

res3: kotlin.Boolean = true

//Greater than or equal comparison

leilaAge >= sisterAge

res4: kotlin.Boolean = true

//Less than comparison

leilaAge < sisterAge

res5: kotlin.Boolean = false

//Less than or equal comparison

leilaAge <=sisterAge

res6: kotlin.Boolean = false

//If - else  statement 

// if the condition is true, the program executes a certain block of code, if the condition is false, the program executes the block of code after "else"

if(sisterAge>=16) println("she can drive")

 else println("She can not drive ")

Output: she can drive

//Kotlin lets use range of values inside the condition

if(sisterAge in 13..19) println("Still a teenager")

Output: Still a teenager

//when is like switch for other languages

when(sisterAge){

     6 -> println("A child")

     18 -> println("A teenager")

     21 -> println("An adult")

 }

Output: A teenager

To practice Kotlin

1. Open Android studio
2. Choose a new Empty Activity > Next > Name the project > Choose Kotlin Language > Finish
3. Click on tools  > Kotlin > Kotlin REPL
4. The REPL will appear in the window's bottom
5. Press Ctrl + Enter to execute any code

Basic Types in Kotlin

    Kotlin treats everything as if they are objects, so primitive types such as numbers, booleans and characters are objects. This means that we can call methods on them. For example, we can call the following methods on numbers: toLong(), toByte(), toInt() and more. 

    Numbers have a supertype called Number. Supertype is "a generic entity type that has a relationship with one or more subtypes" as Oracle explains it. 

    Kotlin allows us to use object wrappers, which is a class that encases a primitive data type or another object to either convert it to a class or make it compatible with a newer version. It is called boxing. This will be explained later when needed.

Operators in Kotlin

 //Addition

2 + 2                                                          

res0: kotlin.Int = 4

 //Subtraction 

99 - 12                                                       

res1: kotlin.Int = 87

 //Division

20 / 5                                                          

res2: kotlin.Int = 4

3 / 6

//Integer divided by Integer returns Integer

res3: kotlin.Int = 0                                

3.0 / 6.0

// Float divided by float returns float

res4: kotlin.Double = 0.5   

 //Multiplication                 

6 * 30                                                           

res5: kotlin.Int = 180

 //Multiplication 

3.times(6)                                                 

res0: kotlin.Int = 18

//Division

18.div(2)                                                           

res1: kotlin.Int = 9

 //Addition

9.plus(2)                                                           

res2: kotlin.Int = 11

 //Subtraction 

15.minus(2)                                                      

res3: kotlin.Int = 13 reres: kotlin.Int = 9s1: kotlin.Int = 9

 //A variable of type string called firstName of value Leila

val firstName: String = "Leila"     

 //A variable of type integer called age of value 20

var  age: Int = 20     

//The type, Int,  is inferred in the next line of code

//Once the type of the variable is determined, you cannot change it later. 

var age = 20         

//The type is needed in the next line of code, since it is not initialized

var age: Int

Basic Types in Kotlin Cont.

Nullability in Kotlin

Quiz