Hello World Program

The "Hello World" program is the traditional first program when learning a new language. This guide shows you how to write, compile, and run a Kotlin program from the command line.

Writing Your First Kotlin Program

Step 1: Create a new file named hello.kt using any text editor (Notepad++, VS Code, Sublime Text, etc.)

Step 2: Add the following code:

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

Understanding the Code

• ✅ fun main: Entry point of the Kotlin application
• ✅ println: Built-in function to print output to console
• ✅ No semicolons required (optional in Kotlin)
• ✅ .kt extension is used for all Kotlin source files

Compiling the Program

Step 1: Open terminal/command prompt and navigate to the directory containing hello.kt

cd path/to/your/file

Step 2: Compile the Kotlin file to a JAR:

kotlinc hello.kt -include-runtime -d hello.jar

Explanation of flags:

• ✅ kotlinc: Kotlin compiler command
• ✅ -include-runtime: Packages Kotlin runtime library in the JAR
• ✅ -d hello.jar: Output file name (destination)

Compilation Output

If successful, you'll see a hello.jar file created in the same directory. No output means compilation succeeded.

Common Compilation Errors

Error: "kotlinc is not recognized"

• ✅ Solution: Kotlin compiler is not in PATH. Verify installation.

Error: Syntax errors

• ✅ Check for typos in function name or missing braces
• ✅ Ensure proper code structure

Running the Program

Execute the compiled JAR file:

java -jar hello.jar

Expected Output:

Hello, World!

Alternative: Direct Execution (Kotlin Script)

You can also run Kotlin code directly without creating a JAR:

Create hello.kts (Kotlin Script):

println("Hello, World!")

Run directly:

kotlinc -script hello.kts

This is useful for quick testing and scripting.

Complete Workflow Summary

Enhanced Example: Custom Message

Let's create a more interactive program:

fun main() {
    val name = "Kotlin"
    println("Hello, $name!")
    println("Welcome to Kotlin programming.")
    println("Current year: ${2024}")
}

Output:

Hello, Kotlin!
Welcome to Kotlin programming.
Current year: 2024

Tips for Command Line Development

1. First compilation is slow: JVM warmup takes time
2. Use Kotlin daemon: Faster subsequent compilations

   kotlinc -daemon hello.kt

1. Clean build: Delete old JAR files before recompiling
2. Check version: Verify Kotlin version with kotlinc -version

Next Steps

• ✅ Learn about Kotlin program structure and concepts
• ✅ Set up an IDE for better development experience
• ✅ Explore variables, data types, and functions