Variables & Types

`val` vs `var`

Kotlin has two ways to declare a variable:

val name = "Kotlin"   // read-only (cannot be reassigned)
var count = 0          // mutable (can be reassigned)

count = 1              // OK
// name = "Java"       // compile error: val cannot be reassigned

Prefer val. Using read-only references by default makes code easier to reason about. Reach for var only when a value genuinely needs to change.

val makes the reference read-only, not the object. A val list can still have items added if the list itself is mutable. We cover that in Part 4.

Basic types

Type	Example	Notes
`Int`	`42`	32-bit integer
`Long`	`42L`	64-bit integer
`Double`	`3.14`	64-bit floating point (default for decimals)
`Float`	`3.14f`	32-bit floating point
`Boolean`	`true`	`true` / `false`
`Char`	`'K'`	a single character
`String`	`"hi"`	text

Everything is an object in Kotlin — there are no Java-style primitives in the language (the compiler optimizes them to primitives where possible).

Type inference

Kotlin infers the type from the initializer, so you rarely write it explicitly:

val message = "Hello"     // inferred String
val pi = 3.14159          // inferred Double
val isReady = true        // inferred Boolean

You can be explicit when you want to — for example to widen or to document:

val total: Long = 0
val ratio: Double = 1

Number literals

val million = 1_000_000     // underscores for readability
val hex = 0xFF              // hexadecimal
val binary = 0b1010         // binary
val big = 10_000_000_000L   // Long

No implicit conversions

Kotlin does not silently widen numbers. Convert explicitly:

val i = 10
val l: Long = i.toLong()   // explicit; `val l: Long = i` is an error
val d = i.toDouble()

Exercises

Declare a val for your name and a var for your age, then print a sentence using both.
Write a function circleArea(radius: Double): Double that returns the area of a circle (π × r²). Use Math.PI.

Solution

   fun circleArea(radius: Double): Double = Math.PI * radius * radius

</details>

This solution is in examples/core/part2/ and is tested by CI.

Next: Strings