Immutability

A cornerstone of functional programming is the concept of immutability. This dictates that an object remains unaltered from the time a function receives it as input until the function generates an output. You might wonder, how could the object change in the first place? To clarify, let’s examine a straightforward example:

fun <T> printAndClear(list: MutableList<T>) {
    for (e in list) {
        println(e)
        list.remove(e)
    }
}
printAndClear(mutableListOf("a", "b", "c"))

The code would initially output a, but then a ConcurrentModificationException would occur.

This happens because the for-each loop utilizes an iterator (a topic we discussed in the previous chapter). Modifying the list within the loop disrupts its operation. This leads us to ponder:

Wouldn’t it be wonderful to safeguard against such runtime exceptions from the get-go?

Next, we’ll discuss how immutable collections...