Adivce on implementing boolean equals(Object)

Question

Given that SomeClass extends Object and is is not part of an overly complicated hierarchy where each subclass adds some data useful to identify its instances.
Which version is better and why?
public boolean equals(Object o){
    return (o != null
        && this.id != null
        && this.getClass().equals(o.getClass())
        && this.id.equals(((SomeClass) o).id))
    || super.equals();
public boolean equals(Object o){
    return (o != null
        && this.id != null
        && o instanceof SomeClass
        && this.id.equals(((SomeClass) o).id))
    || super.equals();
}

Answer 1

So your question is about instanceof vs. comparing the results of getClass, right?
It depends on the semantics of your hierarchy.
The contract of equals says it has to be symmetric. a.equals(b) must return the same result as b.equals(a).
class Super {
class Sub extends Super {
Super super = new Super();
Sub sub = new Sub();
boolean b1 = super.equals(sub);
boolean b2 = sub.equals(super); If you use instanceof, then super's equals must be final, or else Sub's equals must only check for instance of Super, not instanceof Sub. If Super checks for instanceof Super, and Sub checks for instanceof Sub, then b1 (super.equals(sub)) can be true (since sub is an instance of Super) but b2 (sub.equals(super)) can never be true.
If you look at the collections hierarchy, you'll see that the equals methods check for instanceof the interface. For instance (HA!), two Lists are equal iff they both implement List, and they have the same elements in the same order. So ArrayList, LinkedList, and any custom List that you might implement must check instanceof List, not instanceof ArrayList, etc.
The general rule is, if you're going to use instanceof, then to preserve symmetry, all classes in the subtree must check for instanceof the same class (or interface), and it has to be at the highest (closest to Object) level in the subtree of classes that could be equal to each other.
Does that make sense?
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