Kotlin - no_image

## Collection(Kotlin)

Collection usually contains a number of objects 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.

• Set is a collection of unique elements. It reflects the mathematical abstraction of set: a group of objects without repetitions.

• 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.

Collection Types

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.The read-only collection types are covariant( collection types have the same subtyping relationship as the element types. If a Rectangle class inherits from Shape, you can use a List<Rectangle> anywhere the List<Shape> is required.)

• A mutable interface that extends the corresponding read-only interface with write operations: adding, removing, and updating its elements.

The 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

Links

https://kotlinlang.org/docs/reference/collections-overview.html

credit goes to @swayangjit