Custom collector for a LinkedHashMap with values all set to Optional empty.

EmptyLinkedHashMapCollector.java

/**
     * Custom collector for a {@link LinkedHashMap} with values all set to {@link Optional#empty()}
     */
    private <T, R> Collector<T, ?, LinkedHashMap<T, Optional<R>>> toEmptyLinkedHashMap() {
        return Collectors.toMap(Function.identity(), f -> Optional.empty(), (x, y) -> y, LinkedHashMap::new);
    }