Kotlin - First Program

Hello, World!

Every programming journey begins with "Hello, World!" - here's the Kotlin version:

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

Breaking It Down

Running Your Program

Method 1: Command Line

1. Create a file named Hello.kt
2. Add the code above
3. Compile and run:

# Compile to JAR
kotlinc Hello.kt -include-runtime -d Hello.jar

# Run the program
java -jar Hello.jar

Method 2: Direct Script Execution

# Compile and run in one step
kotlinc -script Hello.kt

Method 3: Using IDE

1. Create a new Kotlin project in IntelliJ IDEA
2. Replace the default code with our Hello World
3. Click the green "Run" arrow or press Ctrl+Shift+F10

Understanding Program Structure

Traditional Java vs Kotlin

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

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

Kotlin Advantages:

• No need for a class wrapper
• No public/static keywords required
• Shorter println function
• Less ceremony, more focus on logic

Expanding the Program

Adding Variables

fun main() {
    val name = "Kotlin"
    val version = 1.9
    println("Hello, $name $version!")
}

Understanding String Templates

The $ symbol allows you to embed variables directly in strings:

fun main() {
    val language = "Kotlin"
    val year = 2023
    
    println("I'm learning $language in $year")
    println("${language.length} letters in the name")
}

Multiple Statements

fun main() {
    println("Welcome to Kotlin!")
    println("This is line 2")
    println("This is line 3")
    
    val greeting = "Hello"
    val name = "Developer"
    println("$greeting, $name!")
}

Interactive Examples

Calculator Example

fun main() {
    val a = 10
    val b = 5
    
    println("$a + $b = ${a + b}")
    println("$a - $b = ${a - b}")
    println("$a * $b = ${a * b}")
    println("$a / $b = ${a / b}")
}

Personal Information

fun main() {
    val firstName = "John"
    val lastName = "Doe"
    val age = 25
    val city = "New York"
    
    println("Personal Information:")
    println("Name: $firstName $lastName")
    println("Age: $age years old")
    println("City: $city")
    println("Full info: $firstName $lastName, $age, from $city")
}

Fun with Numbers

fun main() {
    val number = 42
    
    println("The number is: $number")
    println("Double the number: ${number * 2}")
    println("Square of the number: ${number * number}")
    println("Is it even? ${number % 2 == 0}")
}

Common Beginner Mistakes

// Missing fun keyword
main() {
    println("Hello")
}

// Wrong file extension
// Save as Hello.java instead of Hello.kt

// Incorrect string syntax
fun main() {
    println('Hello, World!')  // Single quotes for strings
}

// Proper function declaration
fun main() {
    println("Hello, World!")
}

// Save as Hello.kt
// Use double quotes for strings

Program Variations

With Command Line Arguments

fun main(args: Array<String>) {
    if (args.isNotEmpty()) {
        println("Hello, ${args[0]}!")
    } else {
        println("Hello, World!")
    }
}

Run with: java -jar Hello.jar YourName

Multiple Functions

fun main() {
    greet("Kotlin")
    showMath(10, 5)
}

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

fun showMath(a: Int, b: Int) {
    println("$a + $b = ${a + b}")
}

Exploring Further

Comments in Code

fun main() {
    // Single line comment
    println("Hello, World!")
    
    /*
     * Multi-line comment
     * Can span multiple lines
     */
    val name = "Kotlin" // Comment at end of line
    println("Learning $name")
}

Different Data Types

fun main() {
    val text = "Hello"           // String
    val number = 42              // Int
    val decimal = 3.14           // Double
    val isTrue = true            // Boolean
    val letter = 'K'             // Char
    
    println("Text: $text")
    println("Number: $number")
    println("Decimal: $decimal")
    println("Boolean: $isTrue")
    println("Character: $letter")
}

Project Structure

Simple Project Layout

my-kotlin-project/
├── src/
│   └── main/
│       └── kotlin/
│           └── Main.kt
├── build.gradle.kts (if using Gradle)
└── README.md

Package Declaration

package com.example.myapp

fun main() {
    println("Hello from package!")
}

Best Practices for Beginners

1. Start Small: Begin with simple programs like Hello World
2. Use Meaningful Names: userName instead of x
3. Add Comments: Explain what your code does
4. One Concept at a Time: Don't try to learn everything at once
5. Practice Regularly: Write code every day, even if it's small
6. Experiment: Change values and see what happens