Kotlin First Program

Every Kotlin program starts with a main function. Think of the main function as the front door of your program — the computer always walks in through this door first when it runs your code.

The Simplest Kotlin Program

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

Run this code. The output is:

Hello, World!

Breaking Down Each Part

┌────────────────────────────────────────┐
│  fun main() {                          │
│  │   │     │                           │
│  │   │     └── Opening curly brace     │
│  │   │         starts the function     │
│  │   └── main: the function name       │
│  │       (always "main" to start)      │
│  └── fun: keyword meaning "function"   │
│                                        │
│      println("Hello, World!")          │
│      │       │              │          │
│      │       └── text to print         │
│      └── built-in function that        │
│          prints a line to the screen   │
│                                        │
│  }                                     │
│  └── Closing curly brace               │
│      ends the function                 │
└────────────────────────────────────────┘

println vs print

Kotlin gives you two ways to display text on the screen:

• println() — prints the text and moves to a new line after printing.
• print() — prints the text and stays on the same line.

fun main() {
    println("Line 1")
    println("Line 2")
    print("No ")
    print("new line here")
}

Output:

Line 1
Line 2
No new line here

Printing Numbers and Calculations

You can print numbers directly without quotes:

fun main() {
    println(42)
    println(3 + 7)
    println(10 * 5)
}

Output:

42
10
50

How the Program Runs

Your Code (.kt file)
       │
       ▼
Kotlin Compiler (kotlinc)
       │
       ▼
Bytecode (.class files)
       │
       ▼
JVM reads the bytecode
       │
       ▼
Looks for main() function
       │
       ▼
Executes line by line from top to bottom
       │
       ▼
Output appears on screen

The JVM always executes code inside main() from the first line to the last, one line at a time, in order.

Common Beginner Mistakes

Missing quotes around text

// Wrong
println(Hello World)

// Correct
println("Hello World")

Misspelling println

// Wrong
printLn("Hi")   // uppercase L causes an error

// Correct
println("Hi")

Missing curly braces

// Wrong
fun main()
    println("Hi")

// Correct
fun main() {
    println("Hi")
}

Your Turn

Modify the program to print your own name and city:

fun main() {
    println("My name is Alex.")
    println("I live in Berlin.")
    println("I am learning Kotlin.")
}

Each println call prints one line. You can add as many lines as you need.