Kotlin - Essentials

 // Package definition and imports 

package my.demo
import kotlin.text.*   


// Program entry point 
fun main(){
    val a = 2 // immutable
    var b = 3 // mutable 
    println("Hi Kotlin!")
}

// Function 
fun sum(a: Int, b: Int): Int {
    return a+b
}

// Classes 
open class House 

class Room(var number: Int, var size: RoomSize){
    
}

data class RoomSize(
    var height : Double, 
    var weight : Double 
)

fun main(){
    var str = "Ram is Young"
    var str2 = str.replace("is", "was")
    println(str)
    println(str2)
}

// Conditional Expressions
fun maxOf(a: Int, b: Int): Int =
    if (a > b) a else b

// Loop

fun printAllItemsInList(){
    val items = listOf<String>("Ronaldo", "Messi", "Rooney", "Gerad")
    for ( i in items) println(i)
    // forEach is used to iterate with Collections because it returns the object
    items.forEach { items -> println(items) }

    // while loop
    var index = 0
    while (index < items.size ) println(items[index])

}

// When Expression
fun describe(obj: Any): String{
    return when(obj) {
        1 -> "One"
        "Hello" -> "Greeting"
        is Long -> "Long"
        !is String -> "Not a String"
        else -> "Unknown"
    }
}

// Check number is within range
fun checkRange(number: Int): Boolean =
    if (number in 1..100) true else false

// COLLECTIONS
fun checkIfCollectionContainsObject(){
    val items = setOf("apple", "banana", "mango")
    // sample start
    when {
        "orange" in items -> {
            // do something
            println("Orange is Orange")
        }
        "apple" in items -> println("apple is red")
    }
    // sample end
}

// Lambda expression to map collections
fun filterTheCollections(){
    val fruits = listOf("apple", "banana", "mango", "peach")
    fruits.filter { it.startsWith("a") }
        .sortedBy { it }
        .map { it.uppercase() }
        .forEach {
            print(it)
        }
}