But is it though? If your objects are immutable I agree but if you have setHeight and setWidth I don't.

If you have set width and height, then you don’t need the Square class at all.

It can be mutable if setHeight sets width automatically, and vice-versa.

This is a terrible idea. A rectangle by definition has width and height independent of each other. A square does not fit that description and thus is not a rectangle.

Even if you did something like implementing the square's size as an average of the underlying rectangle's width and height, which would apparently support both classes correctly, you will get in trouble with the semantics or with other, less "fakeable" properties of these objects.

> A rectangle by definition has width and height independent of each other.

That's not really true though. Definition is a quadrilateral w/ 4 right angles. Width and height can be dependent or not

I've never come across this definition. I've always heard that squares are a subset of rectangles, just as rectangles are a subset of parallelograms.