The problem with what Java calls a `List` is that it really isn't a list. It's a mutable cell, whose contents at any given point in time is a list. The methods

The problem with what Java calls a `List` is that it really isn't a list. It's a mutable cell, whose contents at any given point in time is a list. The methods of the mutable cell let you replace the list with a new one, which happens to be partially based on the old one. Also, because Java's data structures are usually ephemeral, the new list can be created in the same place where the old list was. A type constructor of lists (which Java's standard library doesn't have to the best of my knowledge) is covariant on its element type, but a type constructor of mutable cells is invariant on its content type.