If you use setFoo() and getFoo() then you are not using OOP at all.

Encapsulation is simple thing - just write setters and getters for all your internal data and you done. ;-)

PS.

«If the classes are immutable, there is no problem with this state of affairs. Circles satisfy all invariants of ellipses; and an (immutable) circle can be used in any context where an immutable ellipse is expected.»