Comments on: Implementing equals()

By: Henrik Jernevad

Henrik Jernevad — Mon, 19 Apr 2010 19:25:47 +0000

Hi Daniel, thanks for commenting! (And sorry for a late response.)

That's a very good example you provided, and I agree with you.

It reminds me of discussions about what type of matrix multiplication should be used by the * operator (for languages which support operator overloading). One solutions is simply not to use the * operator at all, but provide different methods for different types of multiplication. Maybe the same principle could be used here?

Or maybe Set and SortedSet simply should be two different classes? Say, Set and List. Each with their own definition of equality.

By: Daniel

Daniel — Sat, 10 Apr 2010 01:17:04 +0000

And of course one, somewhat awkward, way of achieving pluggable equality checks that is useful in some cases is to use a wrapper that defines equals/hashcode appropriately.

By: Daniel

Daniel — Sat, 10 Apr 2010 01:10:28 +0000

Your argument makes sense, but there are still issues. For example, consider

Comparator desc = ... // Sort descending SortedSet ascending = new TreeSet (); ascending.addAll (Arrays.asList("a","b")); SortedSet descending = new TreeSet (desc); descending.addAll (ascending); System.out.println (ascending.equals (descending));

These sets are equal, as required by the contract of the Set interface, but this may not be what you want when comparing SortedSets.

Sometimes it would be useful to have pluggable equality checks (analogously to what Comparator does for comparisons).

By: Loriel

Loriel — Sun, 28 May 2006 05:16:12 +0000

Du övertygade mig nästan! =) Men jag har fortfarande känslan av att det borde finnas undantag… Jag ska fundera på det mer någon gång när jag får tid…