Set vs List in Kotlin: Key Differences and Usage Guide

In Kotlin, a List is an ordered collection that allows duplicate elements, while a Set is an unordered collection that stores unique elements only. Use List when order matters and duplicates are allowed, and Set when you need to ensure no duplicates and order is not important.

Quick Comparison

Here is a quick side-by-side comparison of Set and List in Kotlin based on key factors.

Factor	List	Set
Order	Maintains insertion order	No guaranteed order
Duplicates	Allows duplicates	No duplicates allowed
Access by index	Yes, supports indexing	No indexing support
Common use case	When order and duplicates matter	When uniqueness matters
Implementation examples	ArrayList, LinkedList	HashSet, LinkedHashSet, TreeSet

Key Differences

List in Kotlin is an ordered collection. This means the elements keep the order you add them in, and you can access elements by their position using an index. Lists allow duplicate elements, so the same value can appear multiple times.

On the other hand, Set is a collection that stores unique elements only. It does not allow duplicates, so if you try to add an element that already exists, it will be ignored. Sets do not guarantee the order of elements, except for some implementations like LinkedHashSet which preserve insertion order.

Because of these differences, List is best when you care about the order and want to keep duplicates, such as a list of tasks or names. Set is ideal when you want to ensure no duplicates, like a collection of unique IDs or tags.

Code Comparison

Here is how you create and use a List in Kotlin to store some names, including duplicates, and access elements by index.

kotlin

fun main() {
    val names: List<String> = listOf("Alice", "Bob", "Alice", "Charlie")
    println("List contents: $names")
    println("First element: ${names[0]}")
    println("Contains 'Bob'? ${names.contains("Bob")}")
}

Output

List contents: [Alice, Bob, Alice, Charlie]
First element: Alice
Contains 'Bob'? true

Set Equivalent

Here is the equivalent code using a Set to store unique names. Notice duplicates are removed automatically and there is no index access.

kotlin

fun main() {
    val uniqueNames: Set<String> = setOf("Alice", "Bob", "Alice", "Charlie")
    println("Set contents: $uniqueNames")
    println("Contains 'Bob'? ${uniqueNames.contains("Bob")}")
}

Output

Set contents: [Alice, Bob, Charlie]
Contains 'Bob'? true

When to Use Which

Choose List when you need to keep the order of elements and allow duplicates, such as storing user inputs, logs, or ordered tasks. Use Set when you want to ensure all elements are unique and order does not matter, like storing unique IDs, tags, or categories.

If you want uniqueness but also care about order, consider using LinkedHashSet which combines both features.

Key Takeaways

Use List for ordered collections that allow duplicates and support indexing.

Use Set for collections that require unique elements without duplicates.

List maintains insertion order; Set does not guarantee order unless using LinkedHashSet.

Access elements by index only with List, not with Set.

Choose the collection type based on whether order or uniqueness is more important for your data.