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From: Andreas Leitgeb <avl@logic.at>
Newsgroups: comp.lang.java.programmer
Subject: Re: How to assign an object to another in it method?
Date: Wed, 12 Jul 2017 16:23:50 -0000 (UTC)
Organization: A noiseless patient Spider
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Jerry Stuckle <jstucklex@attglobal.net> wrote:
> Nope.  In the strictest sense is is NOT a valid derived class.  The
> rules given for Employee were that wage must be >= minimum wage.  The
> rules for Manager do not include that restriction.  Therefore it is an
> invalid inheritance - no matter what you name them.

Welcome to the topic of covariant versus contravariant types.

If I have a square (e.g. a unit square), then this square *is*
also *a* rectangle.

If I have a class Rectangle and a class Square, then Square
is a subclass of Rectangle (as every square is obviously also 
a rectangle). But then  Rectange  may have a method to *set*
both side lengths. But Square would not allow to set two
different lengths.

Question: is a Square a Rectangle, or is a Rectange a Square?
Answer: depends...

PS: that's also why Java generics offer <T super S> for certain uses.