EasyMock Generic(ish) Collection Matcher

I wanted to check that a collection is basically the same as another without knowing the collection type. I can only really check that the same elements are present, like a basic set.

The code below is based on the code here but with generics where possible (the matches method is not generic in EasyMock 2.5):

private static class CollectionMatcher<T> implements IArgumentMatcher {
    private Collection<T> collection;
    private String notFound = "";

    public CollectionMatcher(Collection<T> collection) { this.collection = collection; }

    public static <T> Collection<T> collectionEq(Collection<T> collection) {
        reportMatcher(new CollectionMatcher<T>(collection));
        return null;
    }

    public void appendTo(StringBuffer buffer) {
        buffer.append(notFound).append(" not found in {");
        String comma = "";
        for (Object o : collection) {
            buffer.append(comma).append(o.toString());
            comma = ",";
        }
        buffer.append("}");
    }

    public boolean matches(Object otherCollection) {
        if (!(otherCollection instanceof Collection)) return false;

        Collection<T> other = (Collection<T>) otherCollection;
        if (other.size() != collection.size()) return false; // Optionally check sizes
        for (T o : other) {
            if (!collection.contains(o)) {
                notFound = o.toString();
                return false;
            }
        }
        return true;
    }
}