Not quite. Conceptually, you can have constness in your subtype-system (as long as you are sticking with interfaces (methods), as Liskov's subtyping model does

Not quite. Conceptually, you can have constness in your subtype-system (as long as you are sticking with interfaces (methods), as Liskov's subtyping model does, and aren't inherting potentially-mutable fields). MutableMap and ImmutableMap are both subtypes of a hypothetical ReadableMap. ImmutableMap is the same as ReadableMap, but has an informal contract that subclasses shouldn't add mutability.