If you want to modify a mutable using another object, you can modify the class to have a function that accepts the other object and modifies itself (if you control the mutable class), add an extension to the mutable method that takes the object and modifies itself (if you don't control the mutable) or create a method that takes both objects and produces a result of the two objects.

Which one you choose all depends on your code style.

val

is like

final

in java and means you can't reassign it