Immutability

If the state of an object can't be changed after it is initialized, then we consider this object immutable. Immutability allows you to write simple and reliable code. If the state of an object can't be changed, we can safely use it as a key for a map function, as in the following example:

fun main(vars: Array<String>) {
    data class ImmutableKey(val name: String? = null)
    val map = HashMap<ImmutableKey, Int>()
    map[ImmutableKey("someName")] = 2
    print(map[ImmutableKey("someName")])
}

We can consider the ImmutableKey class immutable because all its variables are marked with the val keyword and are initialized using a primary constructor.

Another significant thing about this example is that the ImmutableKey class is marked with the data modifier.