Difference between "*" and "Any" in Kotlin generics

generics, kotlin

Solution

It may be helpful to think of the star projection as a way to represent not just any type, but some fixed type which you don't know what is exactly.

For example, the type `MutableList<*>` represents the list of something (you don't know what exactly). So if you try to add something to this list, you won't succeed. It may be a list of `String`s, or a list of `Int`s, or a list of something else. The compiler won't allow to put any object in this list at all because it cannot verify that the list accepts objects of this type. However, if you attempt to get an element out of such list, you'll surely get an object of type `Any?`, because all objects in Kotlin inherit from `Any`.

From asco comment below:

Additionally `List<*>` can contain objects of any type, but only that type, so it can contain Strings (but only Strings), while `List<Any>` can contain Strings and Integers and whatnot, all in the same list.

Problem

Original source