Why Java does not support Equality comparison on the Lines of Comparator?

Question

Java provides way to define comparison of object outside scope Object using Comparator.

Now my questions is why java does not allow do same for equals() and hashcode().

Now each collection contains() method can easily use this external equality provider to check objects are equal.

Answer 1

Guava has the Equivalence class, which does pretty much what you are asking for.

You can even wrap an Object in an Equivalence to decorate an Object with a better hashCode() equals() implementation (e.g. if you want to use an Object with a bad equals() hashCode() as a Map key but don't have access to the sources)

Here's an example: arrays don't have proper implementations of equals() and hashCode(), but here's an Equivalence for char arrays:

private static final Equivalence<char[]> CHAR_ARRAY_EQUIV = new Equivalence<char[]>(){

    @Override
    protected boolean doEquivalent(char[] a, char[] b) {
        return Arrays.equals(a, b);
    }

    @Override
    protected int doHash(char[] chars) {
        return Arrays.hashCode(chars);
    }
};

Sample code:

final char[] first ={'a','b'};
final char[] second ={'a','b'};

Assert.assertFalse(first.equals(second));
Assert.assertFalse(first.hashCode() == second.hashCode());

final Wrapper<char[]> firstWrapped = CHAR_ARRAY_EQUIV.wrap(first);
final Wrapper<char[]> secondWrapped = CHAR_ARRAY_EQUIV.wrap(second);

Assert.assertTrue(firstWrapped.equals(secondWrapped));
Assert.assertTrue(firstWrapped.hashCode() == secondWrapped.hashCode());