Writing Your First Kotlin Program

Writing Your First Kotlin Program: A Step-by-Step Guide

Kotlin, developed by JetBrains, has become a popular programming language due to its conciseness, expressiveness, and interoperability with Java. It is widely used for Android app dev

elopment, but its application extends far beyond that, from backend development to even web and desktop applications. Whether you’re coming from a Java background or are completely new to programming, writing your first Kotlin program is straightforward. This guide will walk you through setting up your environment and writing your first Kotlin application.

Writing Your First Kotlin Program

Now that you have everything set up, it’s time to write your first Kotlin program—a simple “Hello, World!” application.

Create a Kotlin File

In your newly created project, you’ll see a src folder where all your Kotlin code will reside.

• Right-click on the src folder, then choose New > Kotlin File/Class.
• Name the file Main.kt.

Writing the “Hello, World!” Program

Open the newly created Main.kt file and write the following Kotlin code:

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

Explanation of the Code:

• fun: This keyword is used to declare a function in Kotlin. In this case, we are defining the main function, which is the entry point of the application.
• main(): The main function is special in Kotlin. It is where the program starts execution, similar to Java’s public static void main.
• println(): This function prints text to the console. In this case, it prints "Hello, World!".

Running Your Program

• To run the program, right-click on the file in the project window and choose Run Main.kt.
• You should see Hello, World! printed in the console at the bottom of the IDE.

Congratulations! You’ve just written and executed your first Kotlin program.

Understanding Kotlin Basics

Let’s explore some basic Kotlin concepts that will help you in writing more complex programs.

Variables

Kotlin supports two types of variables:

• val: Immutable variable (like final in Java). Once assigned, its value cannot be changed.

val name = "Kotlin" // Immutable variable

• var: Mutable variable. You can change its value.

var age = 25 // Mutable variable

Functions

Functions in Kotlin are declared using the fun keyword. Here’s an example of a function that adds two numbers:

fun addNumbers(a: Int, b: Int): Int {
    return a + b
}

• fun: Declares a function.
• a: Int, b: Int: These are the function parameters, both of type Int.
• : Int: Specifies the return type of the function.
• return: Returns the sum of a and b.

Null Safety

Kotlin’s type system is designed to eliminate the danger of null references. By default, variables cannot hold null values. If you need a variable to hold a null value, you must explicitly mark it as nullable using ?.

var name: String? = null // Nullable variable

Kotlin forces you to handle potential null values safely using techniques like:

• Safe Call (?.): If the variable is null, the call returns null instead of throwing a NullPointerException.

name?.length // Returns the length if name is not null, else returns null

Elvis Operator (?:): Provides a default value when the variable is null.

val length = name?.length ?: 0 // Returns 0 if name is null

Adding More to the Program

Let’s modify the initial “Hello, World!” program to accept user input and perform some basic operations.

fun main() {
    println("Enter your name:")
    val name = readLine()
    println("Hello, $name!")
}

Explanation:

• readLine(): This function reads input from the user. The input is stored in the name variable.
• String Interpolation: Kotlin allows you to embed variables directly into strings using the $ symbol. In this case, $name inserts the value of the name variable into the string.

When you run this modified program, it will ask for your name and greet you personally.

Using Conditional Statements

In Kotlin, you can use traditional conditional statements like if, else, and when.

Example of if-else:

fun main() {
    println("Enter your age:")
    val age = readLine()?.toIntOrNull() ?: 0
    if (age >= 18) {
        println("You are an adult.")
    } else {
        println("You are not an adult.")
    }
}

• The toIntOrNull() function converts the user input to an integer. If the input is invalid (e.g., not a number), it returns null.
• The Elvis operator (?:) ensures that if the input is invalid, the default value of 0 is used.

Example of when:

fun main() {
    val dayOfWeek = 3
    when(dayOfWeek) {
        1 -> println("Monday")
        2 -> println("Tuesday")
        3 -> println("Wednesday")
        4 -> println("Thursday")
        5 -> println("Friday")
        6 -> println("Saturday")
        7 -> println("Sunday")
        else -> println("Invalid day")
    }
}