A `Door` is not an agent, so no, it can't open itself (unless it's a game and your doors have agency). When you say a door can be opened, you are making the as

A `Door` is not an agent, so no, it can't open itself (unless it's a game and your doors have agency). When you say a door can be opened, you are making the assumption that the door is installed in some sort of building. But then, the door is a component of that building, not just a stand-alone door. It can only be opened in that context. So I would suggest something like this: class Door(Material, Size) class Room(List<RoomConnection>, Size) enum DoorState { Closed, Open, Locked } class RoomConnection(Room, Room, Optional<(Door, DoorState)>, Size) To open the door, you don't need a method in any of these "data" classes, you just need to update the door state given there's a door in a room connection you're interested in. The room connection may or may not have a door, and that could change over time. If it does have a door, then the door must have a state (but not all doors have a state, only the ones that are in a Room Connection, possibly others). Notice that to update the room connection, you don't need mutable state locally... you just need a way to provide a new RoomConnection to code where opening a door may be possible (normally, you're given a RoomConnection as an input, and you might give a new one, basic FP style but OOP can also operate in that way). So, I don't think it's that hard if you know what you're doing. I don't want to even comment on the suggestion to have a `Man.open(Door)` method :) that's really not good.