by lazy

Learn lazy initialization with by lazy.

by lazy

With by lazy you compute a val's value on first access. This lets an expensive operation run only if it is really needed.

lazy is a delegate; the block runs the first time the variable is accessed and the result is stored, later accesses return the same result.

val bigData by lazy {
    println("Computing")
    1 + 2 + 3
}

fun main() {
    println(bigData)   // Computing, 6
    println(bigData)   // 6 (not recomputed)
}

Exercise Try by lazy

At the top level, declare val result by lazy { 2 * 3 }. Inside fun main(), print with println(result).

Show example solution

val result by lazy { 2 * 3 }

fun main() {
    println(result)
}

by lazy vs lateinit

by lazy and lateinit both provide deferred initialization but in different ways. by lazy is for val only and the value is computed from a block; it runs automatically on first access.

lateinit is for var only and you assign the value manually; if it is not assigned before access, it throws. by lazy is thread-safe, lateinit is not.

Test yourself

by lazy: fill in the missing part

val result ____ lazy { 1 + 2 }

Show answer

val result by lazy { 1 + 2 }

When does by lazy compute?
- At compile time
- On every access
- On first access
- Never
Show answer

On first access — by lazy computes only on first access and caches.
by lazy: match the terms

by lazy

On first access
lateinit

Assigned later
const val

Compile-time
val

Assigned once

Next lesson: Summary - Variables

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies