The "extends" keyword gives it away that inheritance in Java was not positioned to support the restrictive cases you have in mind (Square : Rectangle, NegativeN

The "extends" keyword gives it away that inheritance in Java was not positioned to support the restrictive cases you have in mind (Square : Rectangle, NegativeNumber : Number, TenCharacterString : String). What issues do you see with wrapping? TenCharacterString eg. could use char[] as its backing store and implement CharSequence if you want to get it to speak a common language with String.