KOtlin for java Developers a course by JetBrains

fun main(){ println("First ${foo()}, Second ${foo()}") }

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

output

calculating to.... calculating to.... First foo, Second foo

kotlin variables

    i val comes from value Read-Only correspondes final variable in java
    ii var comes from variable , mutable variable

modification of val

val language = mutableListOf("Java") language.add("kotlin")

kotlin functions

fun checksmall(a :Int, b:Int):Int{ return if (a<b) a else b }

transformed to

fun checksmall(a : Int, b : Int):Int = if (a < b) a else b

println(listof('a','b','c').joinToString( separator="", prefix = "(",posyfix = ")")))

abc

println(listof(1,2,3).joinToString(posyfix = "."))

1,2.3.

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

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

String description;

switch(colour){

case BLUE: description = "Cold";
break;

case ORANGE: description = "Mid";
break;

}

val description = when(colour){

BLUE -> "Cold" ORANGE -> "Mid"

}

val a = listOf("a","b","c")

for(s in a){

print(s) }

val a = listOf("a","b","c")

for((index,element) in a.withIndex()){

print("$index : $element") }

val map = mapOf( 1 to "One", 2 to "Two", 3 to "Three")

for((key , value) in map){

 println("$key = $value")

}

for(number in 1..10){

println(number)

}

for(number in 1 until 10){

println(number)

}

for( ch in "abc"){

print(ch + 1) }

for( number in 10 downTo1 step 2){

print(number)

}

public class Person {

private final String name;
private final int age;

public Person(String name,int age){
    this.name = name;
    this.age = age;
}

public String getname(){
    return name;
}

public int getage(){
    return age;
}

}

in Kotlin

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

Done, all the constructor and the getter all together

in java

public void updateweather(int degrees){

String description;
Colour colour;

if(degrees < 5){
    description = "Cold";
    colour = BLUE;
}else if(degrees < 23){
    description = "MID";
    colour = ORANGE;
}else{
    description = "Cold";
    colour = RED;
}

}

in kotlin use pair keyword

fun updateweather(degrees : Int){

val(description:String , colour:Colour)=
    if(degrees < 5){
        Pair("Cold",BLUE)
    } else if(degrees < 23){
        Pair("Mid",ORANGE)
    } else {
        Pair("HOt",RED)
    }

}

transformed using when keyword

fun updateweather(degrees : Int){

val(description:String , colour:Colour)= when{
    degrees < 5 -> Pair("Cold",BLUE)
    degrees < 23 -> Pair("Mid",ORABGE)
    else -> Pair("Hot",RED)
}

}

more short using to keyword

fun updateweather(degrees : Int){

val(description:String , colour:Colour)= when{
    degrees < 5 -> "Cold" to BLUE
    degrees < 23 -> "Mid" to ORABGE
    else -> "Hot" to RED
}

}