EqualsEquivalence.java

final class EqualsEquivalence<A>
implements Equivalence<A>
{
    @Override
    public boolean equals(A a, A b)
    {
        return a.equals(b);
    }

    @Override
    public int hashCode(A obj)
    {
        return obj.hashCode();
    }

    <B> Equivalence<B> map(final Function<? super B, ? extends A> f)
    {
        return new Equivalence<B>()
        {
            @Override
            public boolean equals(B a, B b)
            {
                return EqualsEquivalence.this.equals(f.apply(a), f.apply(b));
            }

            @Override
            public int hashCode(B obj)
            {
                return EqualsEquivalence.this.hashCode(f.apply(obj));
            }
        };
    }
}

Equivalence.java

interface Equivalence<T>
{
    boolean equals(T a, T b);

    int hashCode(T obj);
}

EquivalenceWrapper.java

final class EquivalenceWrapper<T>
{
    final T value;
    final Equivalence<T> equivalence;

    EquivalenceWrapper(
        T value,
        Equivalence<T> equivalence
    )
    {
        this.value = value;
        this.equivalence = equivalence;
    }

    @Override
    @SuppressWarnings("unchecked")
    public boolean equals(Object obj)
    {
        try
        {
            return equivalence.equals(
                value,
                ((EquivalenceWrapper<T>) obj).value
            );
        }
        catch (ClassCastException e)
        {
            return false;
        }
    }

    @Override
    public int hashCode()
    {
        return equivalence.hashCode(value);
    }
}