In this case, you have to use a group of objects of the same type.

In addition to creating your own types, you often need to work with a large number of objects which are of the same type.įor example, if you’re writing an e-reader application, you’ll display and manage many books. Those available in most languages are Boolean, Int, Char, String and Object/Any types, along with several different number types.īut you won’t always work with only those types. This is true in Kotlin as well.įurthermore, types in object-oriented programming languages are often divided into primitive and object types.

If you’ve got some experience with programming, you should know there’s a small number of data types in every programming language. And on the right-hand side, you have buttons to run the code, change the version of Kotlin and share the code.īut that’s enough talk about the projects, time to start learning! What are Collections? Still, Kotlin does provide mechanisms for doing useful things with these immutable collections (like filtering values or creating new immutable collections from existing ones).On the left-hand side, you have the code editor, in which you’ll add all the snippets of code. Immutable collections are covariant, but they’re…well…immutable.But Kotlin strives to avoid all runtime failures, therefore, these mutable collections are invariant. Mutable collections can be read from and written to.Mutability and covariance cannot be achieved in the same Collections interface. If shapesList and rectanglesList didn’t have methods like add() (i.e., no mutability)Įven if type-checks were added to all the write-methods in the Mutable Collections, those type checks would produce their own runtime errors/exceptions…which goes against the Kotlin philosophy of having as much compile-time type safety as possible.If rectanglesList couldn’t be put into a variable with type MutableList in the first place (i.e., no covariance).There are two ways the above error could have been avoided: The last line would throw a runtime error! This is because the last line is trying to add a Circle to a MutableList.

Mutable list kotlin