overriding equals method when dealing with inheritance

Question

I have been reading about how best to override the equals method when dealing with subclasses and here I have found quite a few posts. They recommend different ways of implement

Answer 1

If you have equals both in ConcreteClassOne and ConcreteClassTwo then the symmetry of equals is broken:

Object c1 = new ConcreteClassOne(),
       c2 = new ConcreteClassTwo();
System.out.println("c1=c2? " + c1.equals(c2)");
System.out.println("c2=c1? " + c2.equals(c1)");

Now, if you implement equals the usual way, this is going to print

true
false

because in c2.equals you have instanceof ConcreteClassTwo, which fails for c1, but in the opposite case the analogous check passes.