What is the difference between "const" and "val"?

What is the difference between const and val?

In Kotlin, as like other programming languages, the properties can either be mutable(it can be changed) or immutable(it can't be changed). We use the keywords const and val to declare an immutable property . Developers often get confused between these terms i.e. when to use which one.

So, welcome to MindOrks, in this tutorial, we will see the difference between const and val in Kotlin.

Use of "const" in Kotlin

The const keyword is used to declare those properties which are immutable in nature i.e. these properties are read-only properties.

But, the value of these properties must be known at the compile-time only and this is the reason const is also known as Compile-time constant. So, no runtime assignment of values is allowed in const variables.

A property must satisfy the following to be a const property:

  • must be at top-level or member of object or member of a companion object
  • must be initialised with a String type or primitive type
  • no custom getter

So, you can't assign a const variable to a function or to some class because in this case the variable will be initialised at the runtime and not at compile-time.

Use of "val" in Kotlin

The val keyword is also used for read-only properties. But the main difference between the const and val is that the val properties can be initialised at the runtime also.

So, you can assign a val variable to a function or to some class.

Example 

const val companyName = "MindOrks" // this will work
val comapanyname = "MindOrks" // this will work

const val companyName = getCompanyName() // will not work
val companyName = getCompanyName() // this will work

In the above example, we are using companyName as the immutable variable. When using const , if you directly assign the value then it is fine but if you try to assign the value from some function getCompanyName , then you will get an error because here the value will be assigned at runtime and not at compile-time. But in the case of val , both the situations are ok.

Why use "const" when we can use "val"?

In the previous example, we saw that the value of the val variable is initialized at runtime and const at compile time. So, why to use const if we can use val ?

Let's find by taking another real-life Android example to understand the use-case of const and val : 

YourClassName {
    companion object {    
        const val FILE_EXTENSION = ".png"    
        val FILENAME: String
        get() = "Img_" + System.currentTimeMillis() + FILE_EXTENSION 
    }
}

In the above example, we are declaring the const variable named FILE_EXTENSION in the companion object and the FILENAME variable as val and initialise it with custom getter.

As the extension of the file will always be same, so it is declared as a const variable. But, the name of the file will be changed based on the logic that we use for the file name. Here, in our example, we are naming the file based on the current time. You can't give some value to it initially because the value is fetched at the runtime. So, we are using val here.

What happens after the code compilation is that wherever the const variables are used in the code, those variables are replaced by the value of that const variable but in case of val , the variables are kept as it is because we don't know the value of val at compile-time. So, if you decompile the above code, then you will see( learn how to covert Kotlin code into Java ): 

public final String getFILENAME() {
   return "Img_" + System.currentTimeMillis() + ".png";
}

Here, you can find that the variable FILE_EXTENSION has been replaced by its value i.e. ".png" i.e. the value has been inlined and hence there is no overhead to access that variable at the runtime. This is the advantage of using const over val .

This is all about the difference between const and val keyword in Kotlin.

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