This would be overengineering, but the following six types would solve the problem: * ImmutableSquare * ImmutableRectangle * MutableSquare * MutableRectangl

This would be overengineering, but the following six types would solve the problem: * ImmutableSquare * ImmutableRectangle * MutableSquare * MutableRectangle * Square * Rectangle The "Mutable..." classes are mutable, the "Immutable..." classes are immutable, and the "Square" and "Rectangle" classes mean that you can read the values, but there is no guarantee about either mutability or immutability. In this system, "ImmutableSquare" and "MutableSquare" are subtypes of "Square"; "ImmutableRectangle" and "MutableRectangle" are subtypes of "Rectangle"; and also "ImmutableSquare" is a subtype of "ImmutableRectangle" (and therefore "Rectangle"). But if you go this way, don't be surprised when you end up with millions of classes.