In Java, everything uses equals() where it can, but then it's not easy to get == behavior when it matters. Technically you can use a wrapper:
public final class Id
{
private Object x;
public Id( Object x ) { this.x = x; }
public int hashCode() { return System.identityHashCode( x ); }
public boolean equals( Object obj )
{ return obj instanceof Id && ((Id)obj).x == x; }
}
I'm digressing, but it would be so much nicer if every object kept a hard reference to its Id wrapper. That way the Id could be used as a WeakHashMap key.[1] Weak tables are one place where comparing keys for anything but reference identity makes no sense. XD
Back in terms of (iso id.a id.b), this would have the observable effect that (iso id.id.a id.id.a) would be true for all a.
[1] Thanks to the hard reference to the Id from its x, the Id itself wouldn't be collected until its x was unreachable too. A Id without such an anchor would potentially be garbage-collected much earlier than its x, and the WeakHashMap entry would be lost. ...Anyway, some of this could be solved by changing the design of WeakHashMap. :)
