mapOfItemsToSuppliersByStream

gistfile1.java

    static class Tuple<F, S> {
        private F first;
        private S second;

        public static <F, S> Tuple<F, S> pair(F f, S s) {
            return new Tuple<F, S>(f, s);
        }

        private Tuple(F first, S second) {
            this.first = first;
            this.second = second;
        }

        public F getFirst() {
            return first;
        }

        public S getSecond() {
            return second;
        }
    }

    @Test
    public void mapOfItemsToSuppliersByStream()
    {
        final Map<String, List<Supplier>> itemsToSuppliersStream
                = Stream.of(this.company.getSuppliers())
                .flatMap(supplier -> 
                    Arrays.stream(supplier.getItemNames())
                    .map(itemName -> Tuple.pair(itemName, supplier)))
                .collect(Collectors.groupingBy(t -> t.getFirst(),
                        Collectors.mapping(t -> t.getSecond(),
                            Collectors.toList())));

        Verify.assertIterableSize(
                "should be 2 suppliers for sofa",
                2,
                itemsToSuppliersStream.get("sofa"));
    }