In other words, the wildcard with an extends-bound ( upper bound) makes the type covariant.

In return for this limitation, you get the desired behavior: Collection is a subtype of Collection. This means that you can safely read E's from items (elements of this collection are instances of a subclass of E), but cannot write to it as you don't know what objects comply with that unknown subtype of E. To access an element in the list you may use the traditional get() method as well as index operator, You may check if a value exists in the collection using the contains() method println('first's $įor more tech blogs, please visit Knoldus Blogs.The wildcard type argument ? extends E indicates that this method accepts a collection of objects of E or a subtype of E, not just E itself. In the following code the reference fruits is of this interface type, specialized to String for the parametric type: val fruits: List = listOf('Apple', 'Banana', 'Grape') The function listOf() returns a reference to an interface. Mutable List – mutableListOf(),arrayListOf() and ArrayList.ListĪs a first step in creating a list, Kotlin wants you to declare your intent-immutable or mutable. The collection interfaces and related functions are located in the llectionspackage. Maps are useful for storing logical connections between objects, for example, an employee’s ID and their position. Keys are unique, and each of them maps to exactly one value. Map (or dictionary) is a set of key-value pairs.For example, the numbers on lottery tickets form a set: they are unique, and their order is not important. Generally, the order of set elements has no significance. It reflects the mathematical abstraction of set: a group of objects without repetitions.

Set is a collection of unique elements.

An example of a list is a telephone number: it’s a group of digits, their order is important, and they can repeat. Elements can occur more than once in a list.

List is an ordered collection with access to elements by indices – integer numbers that reflect their position.

The following collection types are relevant for Kotlin: The Kotlin Standard Library provides a comprehensive set of tools for managing collections – groups of a variable number of items that are significant to the problem being solved and are commonly operated on.Ĭollections are a common concept for most programming languages, usually containing a number of objects (elements,items) of the same type.

Kotlin collection