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?
const
s are compile time constants. Meaning that their value has to be assigned during compile time, unlike val
s, where it can be done at runtime.
This means, that const
s 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
Just to add to Luka's answer:
Compile-Time Constants Properties the value of which is known at compile time can be marked as compile time constants using the const modifier. Such properties need to fulfill the following requirements: Top-level or member of an object declaration or a companion object. Initialized with a value of type String or a primitive type No custom getter Such properties can be used in annotations.
Source: Official documentation
You can transform the Kotlin to Java. Then you can see const has one more static modifier than val. The simple code like this.
Kotlin:
const val str = "hello"
class SimplePerson(val name: String, var age: Int)
To Java(Portion):
@NotNull
public static final String str = "hello";
public final class SimplePerson {
@NotNull
private final String name;
private int age;
@NotNull
public final String getName() {
return this.name;
}
public final int getAge() {
return this.age;
}
public final void setAge(int var1) {
this.age = var1;
}
public SimplePerson(@NotNull String name, int age) {
Intrinsics.checkParameterIsNotNull(name, "name");
super();
this.name = name;
this.age = age;
}
}
const
is removed, will it yield a different Java file?
SimplePerson
's private final String name;
which doesn't have the const and then is private as well, but that's because it's a member val instead of a top-level/package val and not because of the const
.
const kotlin to Java
const val Car_1 = "BUGATTI" // final static String Car_1 = "BUGATTI";
val kotlin to Java
val Car_1 = "BUGATTI" // final String Car_1 = "BUGATTI";
In simple Language
The value of the const variable is known at compile time. The value of val is used to define constants at run time.
Example 1-
const val Car_1 = "BUGATTI" ✔
val Car_2 = getCar() ✔
const val Car_3 = getCar() ❌
//Because the function will not get executed at the compile time so it will through error
fun getCar(): String {
return "BUGATTI"
}
This is because getCar() is evaluated at run time and assigns the value to Car.
Additionally -
val is read-only means immutable that is known at run-time var is mutable that is known at run-time const are immutable and variables that are known at compile-time
Both val
and const
are immutable.
const
is used to declare compile-time constants, whereas val
for run-time constants.
const val VENDOR_NAME = "Kifayat Pashteen" // Assignment done at compile-time
val PICon = getIP() // Assignment done at run-time
val
is not necessarily immutable.
val
Kotlin val
keyword is for read-only properties in comparison with Kotlin var
keyword. The other name for read-only
property is immutable
.
Kotlin code:
val variation: Long = 100L
Java equivalent looks like this:
final Long variation = 100L;
const val
We use const
keyword for immutable properties too. const
is used for properties that are known at compile-time. That's the difference. Take into consideration that const
property must be declared globally
.
Kotlin code (in playground):
const val WEBSITE_NAME: String = "Google"
fun main() {
println(WEBSITE_NAME)
}
Java code (in playground):
class Playground {
final static String WEBSITE_NAME = "Google";
public static void main(String[ ] args) {
System.out.println(WEBSITE_NAME);
}
}
val
that produces different results on multiple calls through a custom getter or because it’s a delegated property, or because it’s open and has a setter in a subclass
Because I read a lot, that "val" means immutable: This is definitely not the case, just see this example:
class Test {
var x: Int = 2
val y
get() = x
}
fun main(args: Array<String>) {
val test = Test()
println("test.y = ${test.y}") // prints 2
test.x = 4
println("test.y = ${test.y}") // prints 4
}
Sadly, true immutability you can currently only expect with const - but this only at compile time. At runtime you can't create true immutability.
val just means "readonly", you can't change this variable directly, only indirect like I have shown in the example above.
For those who are looking which is more appropriate or efficient between val
and const
:
If we are going to create String or any primitive data type then we must use const val
instead of val
. Because val
will be known at runtime, so when your app is running then it will process all the values. On other hand const val
will do this earlier at compile time. So performance wise const val
will give better result.
Success story sharing
const val foo = "Hello world"
andval bar = "Hello world"
? Are they the same?const
values will just be completely inlined during compilation.const val
instead of justconst
? It seems to me theval
keyword is totally superfluous in this context, sinceconst var
would be absurd on its face.const val
,const
is a modifier onval
rather than a keyword. Modifiers > keywords. More examples of this same design are,annotation/enum/data class
,private val
,inline fun
, etc.