Variables (val & var)

Kotlin is a modern, concise language that gives you two ways to name data: val for read-only values and var for ones you can change.

Learn Variables (val & var) in our free Kotlin course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Kotlin course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll know exactly when to reach for each, how type inference saves you keystrokes, and how Kotlin's basic types — Int, Double, Boolean, Char, and String — work.

What You'll Learn in This Lesson

1️⃣ val vs var

Kotlin has exactly two keywords for declaring variables. val creates a read-only reference that can be assigned once. var creates a mutable reference you can reassign as many times as you like. The rule of thumb: prefer val , and switch to var only when you must.

The last two lines are the subtle part: val items can't point at a different list, but the list itself is mutable, so add still works. val freezes the reference , not the contents.

2️⃣ Types and Type Inference

Kotlin is statically typed — every variable has a type — but you rarely have to write it. The compiler infers the type from the value. When you do want to be explicit, add a colon and the type after the name. Kotlin's basic types include Int , Long , Double , Boolean , Char , and String .

Notice Kotlin won't silently widen numbers: to treat an Int as a Double you call .toDouble() . This prevents accidental precision loss.

Your turn. Fill in the TODO s, following the 👉 hints, then run and compare.

Mini-Challenge: A Shopping Total

Use what you've learned about val , var , and types to build a small total. Write it yourself and check it against the example output.

Common Errors (and the fix)

• "Val cannot be reassigned" — You tried to give a new value to a val . If it needs to change, declare it as var instead.
• "Type mismatch: Int vs Double" — You mixed number types without converting. Call .toDouble() (or the type you need) to convert explicitly.
• "Variable must be initialized" — A val or var needs a value or an explicit type. Either assign a value, or write the type so the compiler knows what it is.
• Using single quotes for text — 'hi' is a Char error; only one character fits in single quotes. Use "hi" with double quotes for a String.

Pro Tips

• 💡 Underscores in numbers: write 1_000_000 instead of 1000000 — the underscores are ignored but make big numbers readable.
• 💡 Compound assignment: score += 1 is shorthand for score = score + 1 (also -= , *= , /= ).
• 💡 Lint nudges: IntelliJ will suggest changing a var to a val if it's never reassigned. Take the hint.

📋 Quick Reference — Variables

Concept

Kotlin Syntax

Read-only variable

val x = 5

Mutable variable

var y = 5

Explicit type

val n: Long = 5

Convert a number

n.toDouble()

Compound assign

y += 1

Readable big number

1_000_000

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ val is read-only; var can be reassigned
• ✅ Idiomatic Kotlin prefers val
• ✅ Type inference means you rarely write the type
• ✅ Basic types: Int, Long, Double, Boolean, Char, String
• ✅ Number conversions are explicit: .toDouble()
• ✅ Next lesson: Null Safety — Kotlin's most famous feature

Practice quiz

Continue this course

• Previous: Introduction to Kotlin
• Next: Null Safety