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At Google I/O 2017, Google announced first-class support for Kotlin on Android. Kotlin is generally associated with Android development and is also ideal for modern server side application. Official language for Android is Kotlin.
Kotlin is a JVM language. The bytecode generated by kotlin compiler is interpreted by JVM(Java Virtual Machine).Therefore, Java JDK should be installed on the machine. A least supported version of Java JDK should be Java 6.
Requirements: Any OS(Windows, Linux or Mac OS) capable of running the latest version of Java.
Filename extension: .kt,.kts

Defining local variables

In Kotlin, the assign-once or read-only variables are declared by the use of keyword val .In Java, it is equivalent to keyword final.
To declare a mutable variable or a variable whose value can be changed, we use keyword var.Kotlin is statically, strongly typed and uses type inference to determine the initial type of all your variables and constants.Therefore we don't need to specify the type it can automatically infer the type.If we want to specify then we can specify by writing the name followed by a colon.

val immutableX=3 // `Int` data type is automatically inferred
val y: Int=10 // immediate assignment
val z: Int // Type required when no initializer is provided
z=5 // deferred assignment

var x=5 // `Int` data type is automatically inferred
x +=1

Comments in Kotlin

Kotlin supports both single line and multiple line comments
// This is an single line comment
// This is also called end-of-line comments

/* This is a multiple line comment
or it is also called block comments. */
Block Comments can be nested.

String Templates in Kotlin

Using String templates we can put the value of one variable in another string. For example
var x=5
// simple name in template:
val s1="x is $a"
If we print the value of s1 then output will be
x is 5

We can even do computation by ${...}.For example
// between curly brackets we can call function or perform any expression:
val s2="${s1.replace("is", "was")}, but now is ${x+2}"
output if we print s2 is
x was 5,but now is 7


Defining functions in Kotlin

To define a fuction with two parameter of type Int and return type Int
fun multiply(x: Int, y: Int): Int{
    return x + y
}
To form a function with expression body and inferred return type
fun multiply(x: Int, y: Int)=x * y

To define a function returning no meaningful value
fun displayMultiply(x: Int, y: Int): Unit{
    println("multiplication of $x and $y is ${x + y}")
}
'Unit; can be omitted
fun displayMultiply(x: Int, y: Int){
    println("multiplication of $x and $y is ${x + y}")
}


Conditional Statement in Kotlin

Syntax to compare two variables
fun maxOf(x: Int, y: Int): Int{
    if (x > y){
        return x
    }else{
    return y
    }
}
We can also use '=' to write a shoter function as
fun maxOf(x: Int, y: Int)=if(x > y) x else y

switch case of java is equivalent to when statement in kotlin
when (a){
1 -> print("a==1")
2 -> print("a==2")
else ->{// block Statement
    print("a is neither 1 nor 2")
    }
}


For loop in Kotlin

For loop needs an iterator on a collection of items so that each item or object, it can create a loop. For example:
val arrayList=listOf("one", "two", "three","four","five")
for (item in arrayList){
    println(item)
}
We can also use while loop for iteration
val arrayList=listOf("zero","one", "two", "three","four","five")
var i=0
while (i < arrayList.size){
    println("$i is ${arrayList[index]}")
    index++
}