The Kotlin Standard Library provides a comprehensive set of tools for managing collections – groups of a variable number of items (possibly zero) that are significant to the problem being solved and are commonly operated on.
Collections are a common concept for most programming languages, so if you're familiar with, for example, Java or Python collections, you can skip this introduction and proceed to the detailed sections.
A collection usually contains a number of objects (this number may also be zero) of the same type. Objects in a collection are called elements or items. For example, all the students in a department form a collection that can be used to calculate their average age.
The following collection types are relevant for Kotlin:
List is an ordered collection with access to elements by indices – integer numbers that reflect their position. Elements can occur more than once in a list. An example of a list is a telephone number: it's a group of digits, their order is important, and they can repeat.
Set is a collection of unique elements. It reflects the mathematical abstraction of set: a group of objects without repetitions. Generally, the order of set elements has no significance. For example, the numbers on lottery tickets form a set: they are unique, and their order is not important.
Map (or dictionary) is a set of key-value pairs. Keys are unique, and each of them maps to exactly one value. The values can be duplicates. Maps are useful for storing logical connections between objects, for example, an employee's ID and their position.
Kotlin lets you manipulate collections independently of the exact type of objects stored in them. In other words, you add a
String to a list of
Strings the same way as you would do with
Ints or a user-defined class. So, the Kotlin Standard Library offers generic interfaces, classes, and functions for creating, populating, and managing collections of any type.
The collection interfaces and related functions are located in the
kotlin.collections package. Let's get an overview of its contents.
The Kotlin Standard Library provides implementations for basic collection types: sets, lists, and maps. A pair of interfaces represent each collection type:
A read-only interface that provides operations for accessing collection elements.
A mutable interface that extends the corresponding read-only interface with write operations: adding, removing, and updating its elements.
Note that altering a mutable collection doesn't require it to be a
var: write operations modify the same mutable collection object, so the reference doesn't change. Although, if you try to reassign a
val collection, you'll get a compilation error.
The read-only collection types are covariant. This means that, if a
Rectangle class inherits from
Shape, you can use a
List<Rectangle> anywhere the
List<Shape> is required. In other words, the collection types have the same subtyping relationship as the element types. Maps are covariant on the value type, but not on the key type.
In turn, mutable collections aren't covariant; otherwise, this would lead to runtime failures. If
MutableList<Rectangle> was a subtype of
MutableList<Shape>, you could insert other
Shape inheritors (for example,
Circle) into it, thus violating its
Rectangle type argument.
Below is a diagram of the Kotlin collection interfaces:
Let's walk through the interfaces and their implementations. To learn about
Collection, read the section below. To learn about
Map, you can either read the corresponding sections or watch a video by Sebastian Aigner, Kotlin Developer Advocate:
Collection<T> is the root of the collection hierarchy. This interface represents the common behavior of a read-only collection: retrieving size, checking item membership, and so on.
Collection inherits from the
Iterable<T> interface that defines the operations for iterating elements. You can use
Collection as a parameter of a function that applies to different collection types. For more specific cases, use the
MutableCollection<T> is a
Collection with write operations, such as
List<T> stores elements in a specified order and provides indexed access to them. Indices start from zero – the index of the first element – and go to
lastIndex which is the
(list.size - 1).
List elements (including nulls) can duplicate: a list can contain any number of equal objects or occurrences of a single object. Two lists are considered equal if they have the same sizes and structurally equal elements at the same positions.
MutableList<T> is a
List with list-specific write operations, for example, to add or remove an element at a specific position.
As you see, in some aspects lists are very similar to arrays. However, there is one important difference: an array's size is defined upon initialization and is never changed; in turn, a list doesn't have a predefined size; a list's size can be changed as a result of write operations: adding, updating, or removing elements.
In Kotlin, the default implementation of
ArrayList which you can think of as a resizable array.
Set<T> stores unique elements; their order is generally undefined.
null elements are unique as well: a
Set can contain only one
null. Two sets are equal if they have the same size, and for each element of a set there is an equal element in the other set.
MutableSet is a
Set with write operations from
The default implementation of
LinkedHashSet – preserves the order of elements insertion. Hence, the functions that rely on the order, such as
last(), return predictable results on such sets.
An alternative implementation –
HashSet – says nothing about the elements order, so calling such functions on it returns unpredictable results. However,
HashSet requires less memory to store the same number of elements.
Map<K, V> is not an inheritor of the
Collection interface; however, it's a Kotlin collection type as well. A
Map stores key-value pairs (or entries); keys are unique, but different keys can be paired with equal values. The
Map interface provides specific functions, such as access to value by key, searching keys and values, and so on.
Two maps containing the equal pairs are equal regardless of the pair order.
MutableMap is a
Map with map write operations, for example, you can add a new key-value pair or update the value associated with the given key.
The default implementation of
LinkedHashMap – preserves the order of elements insertion when iterating the map. In turn, an alternative implementation –
HashMap – says nothing about the elements order.