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

The simplest approach is to extend equals() in both the concrete and the abstract classes.

public class ConcreteClassTwo extends ConcreteClassOne {
    public boolean equals(Object other) {
        boolean rv = super.equals( other );
        if ( other instanceof ConcreteClassTwo ) {
           rv = rv && (this.trackingNo == ((ConcreteClassTwo) other).trackingNo);
        }
        return rv;
    }
}