Describe the mechanism of collections in Kotlin: differences between List, MutableList, Set, MutableSet, Map, MutableMap. What pitfalls can arise from their use? Provide code examples.

Answer.

Kotlin offers two types of collections: immutable and mutable.

• List — an immutable list. Elements cannot be added/removed.
• MutableList — a mutable list: elements can be added and removed.
• Set — an immutable set (unique elements).
• MutableSet — a mutable set.
• Map — an associative array (key-value), immutable.
• MutableMap — a mutable associative array.

Code example:

val fruits: List<String> = listOf("Apple", "Banana", "Cherry")
val mutableFruits: MutableList<String> = mutableListOf("Apple", "Banana")
mutableFruits.add("Cherry")

val fruitSet: Set<String> = setOf("Apple", "Banana") // Uniqueness of elements
val mutableFruitSet: MutableSet<String> = mutableSetOf("Apple", "Banana")
mutableFruitSet.add("Cherry")

val scores: Map<String, Int> = mapOf("Tom" to 80, "Jane" to 90)
val mutableScores: MutableMap<String, Int> = mutableMapOf("Tom" to 80)
mutableScores["Jane"] = 90

Nuances:

• Collections are immutable by default.
• Many functions returning lists from the standard library return List (not MutableList).
• When working with collections, it is important not to modify those that were not intended for changes.

Trick question.

If you have a function that takes List<T> as a parameter, can you somehow modify this list inside the function?

Answer: No, if the parameter is of type List<T>, you cannot add/remove elements inside the function. But if a MutableList<T> is actually passed, it can be cast to this type (which introduces risk):

fun modifyList(list: List<String>) {
    if (list is MutableList) {
        list.add("Another") // Works if the original collection is mutable
    }
}

This is not recommended, as it violates the contract (List should be immutable!).

Examples of real errors due to a lack of knowledge of the nuances of the topic.

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Story

In a large project, a function that took List<Customer> was used, and an attempt was made to add a new element inside the function. The compilation error (add/clear/remove are not defined for List) was only discovered during review, as a mutable list was required for data transformation.

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Story

A collection was declared as MutableList, but was passed across several threads without synchronization. As a result, competing modifications occurred, leading to "ConcurrentModificationException" and corrupting the state of the list. The error was sought for a long time, as thread-safety was not considered.

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Story

In one of the services dealing with database query results, mutableListOf was used, and then a reference was given outside. External code mistakenly cleared the list. As a result, empty data reached the user, as references were not cloned and mutable collections were modified outside the business logic layer.