Immutability is the opposite of mutation. Mutation is where we modify the state of an object. Mutation is error-prone and makes debugging difficult. Kotlin emphasizes immutability with val, which makes a variable immutable. Its collection types are also immutable by default. Once the val or collection is initialized, it is guaranteed to hold the same value. Immutability makes code easier to write and use and the internal state of the application will be more consistent.

Let's consider the following code sample:

object ImmutabilityTest {
    @JvmStatic
    fun main(args: Array<String>) {
        val msg = "Hello"
        msg = "hi"
        println(msg)
    }
}

In the preceding code, we created a immutable variable, msg, assigned a value. We then try to change its value.

If we compile this code, it gives us a compilation error that the value of msg cannot be changed:

[INFO] Changes detected - recompiling the module!
[INFO] Compiling 2 source files to ..Intermixing Java and KotlinmyJavaApp	argetclasses
[INFO]
[INFO] --- kotlin-maven-plugin:1.2.41:compile (compile) @ myJavaApp ---
[ERROR] ..Intermixing Java and KotlinmyJavaAppsrcmainjavaImmutabilityTest.kt: (5, 9) Val cannot be reassigned

Consider the following example as well:

object ImmutabilityTest {
    @JvmStatic
    fun main(args: Array<String>) {
        val msg: String
        msg = "Good Morning"
        msg = "Good Evening"

        println(msg)
    }
}

In the preceding code, we created a immutable variable, msg, which is of the String type. Note that we declared the type explicitly here and varied the initialization of msg. When we are varying the initialization, we have to declare the type explicitly. We then try to change its value. 

If we compile this code, it also gives us a compilation error that the value of msg cannot be changed:

[INFO] Changes detected - recompiling the module!
[INFO] Compiling 2 source files to ..Intermixing Java and KotlinmyJavaApp	argetclasses
[INFO]
[INFO] --- kotlin-maven-plugin:1.2.41:compile (compile) @ myJavaApp ---
[ERROR] ..Intermixing Java and KotlinmyJavaAppsrcmainjavaImmutabilityTest.kt: (6, 9) Val cannot be reassigned