Programming
What is the difference between "const" and "val"?
I have recently read about the const keyword, and I'm so confused! I can't find any difference between const and the val keyword, I mean we can use both of them to make an immutable variable, is there anything else that I'm missing?
Solution
consts are compile time constants. Meaning that their value has to be assigned during compile time, unlike vals, where it can be done at runtime.
This means, that consts can never be assigned to a function or any class constructor, but only to a String or primitive.
For example:
const val foo = complexFunctionCall() //Not okay
val fooVal = complexFunctionCall() //Okay
const val bar = "Hello world" //Also okay
Comments (5)
- +3 – What about something like this:
const val foo = "Hello world"andval bar = "Hello world"? Are they the same? — Jun 02, 2016 at 15:32 - +5 – @MathewHany, at least not in terms of bytecode, see: stackoverflow.com/questions/37482378/static-data-in-kotlin/… — Jun 02, 2016 at 15:34
- +5 – I think
constvalues will just be completely inlined during compilation. — Jun 02, 2016 at 15:35 - +0 – This begs another question: Why does Kotlin require
const valinstead of justconst? It seems to me thevalkeyword is totally superfluous in this context, sinceconst varwould be absurd on its face. — Jun 08, 2017 at 15:51 - +0 – @EricLloyd With
const val,constis a modifier onvalrather than a keyword. Modifiers > keywords. More examples of this same design are,annotation/enum/data class,private val,inline fun, etc. — Apr 02, 2018 at 19:36