1. What is Kotlin?

Kotlin is a modern, statically typed programming language that runs on the Java Virtual Machine (JVM) and is fully interoperable with Java. It is designed to be concise, expressive, and safe, addressing many of Java's limitations while improving productivity.

2. Installing Kotlin

Kotlin can be installed using the Kotlin website or through package managers. If you have IntelliJ IDEA or Android Studio, Kotlin is already integrated. Otherwise, you can install Kotlin via SDKMAN or Homebrew on macOS:

brew install kotlin

3. Kotlin Syntax

Kotlin has a concise and expressive syntax. Below is a simple Kotlin program:

fun main() {
    println("Hello, World!")
}

In this example, fun defines a function, and println prints the string to the console.

4. Variables

In Kotlin, you can define variables using the val keyword for immutable variables and var for mutable variables. Example:

val name: String = "John"   // Immutable
var age: Int = 25             // Mutable

val creates a constant, while var creates a variable whose value can be changed later.

5. Functions

Functions in Kotlin are declared using the fun keyword. Example:

fun greet(name: String): String {
    return "Hello, $name!"
}

fun main() {
    println(greet("Alice"))
}

The function greet takes a String parameter and returns a greeting message.

6. Classes and Objects

In Kotlin, you can define classes with the class keyword and create objects using the new keyword:

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

fun main() {
    val person = Person("John", 25)
    println(person.name)
}

7. Inheritance

Kotlin supports single inheritance. To create a class that inherits from another, use the : symbol. Example:

open class Animal(val name: String)

class Dog(name: String): Animal(name)

fun main() {
    val dog = Dog("Rex")
    println(dog.name)
}

open is used to allow the class to be inherited. The Dog class inherits from the Animal class.

8. Null Safety

Kotlin has built-in null safety to avoid NullPointerException. You can define nullable types by appending ? to the type. Example:

var name: String? = "John"
name = null  // Valid

You can also use the safe call operator (?.) to safely access properties of nullable objects:

val length = name?.length  // Returns null if name is null

9. Lambdas and Higher-Order Functions

Kotlin supports lambdas and higher-order functions, allowing you to pass functions as arguments. Example:

fun operate(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)
}

fun main() {
    val sum = operate(3, 4) { x, y -> x + y }
    println(sum)  // Output: 7
}

The operate function takes two integers and a function that defines the operation. Here, a lambda expression is passed to calculate the sum.

10. Coroutines

Coroutines in Kotlin simplify asynchronous programming by allowing you to write asynchronous code in a sequential manner. Example:

import kotlinx.coroutines.*

fun main() = runBlocking {
    launch {
        delay(1000L)
        println("World!")
    }
    println("Hello,")
}

The runBlocking function is used to run a coroutine in the main function. The launch function starts a new coroutine, and delay simulates a delay.