如何在Java 8中CombineMerge两个Map of Map of Lists(Map<String, Map<Enum, List<String>>>)。

Question

给定:一张包含水果的地图

enum Food{
FRUITS, VEGGIES;
}

Map<String, Map<Food, List<String>>> fruitBasket= new HashMap<>();
fruitBasket.put("basket1", Collections.singletonMap(Food.FRUITS, Arrays.asList("apple","banana")));
fruitBasket.put("basket2", Collections.singletonMap(Food.FRUITS, Arrays.asList"orange", "kiwi")));
fruitBasket.put("basket3", Collections.singletonMap(Food.FRUITS, Arrays.asList("banana", "orange")));

fruitBasket:
[
basket1, [Food.FRUITS, {"apple", "banana"}],
basket2, [Food.FRUITS, {"orange", "kiwi"}],
basket3, [Food.FRUITS, {"banana", "orange"}]
]

同样，另一张包含蔬菜的地图

Map<String, Map<Food, List<String>>> veggieBasket= new HashMap<>();
veggieBasket.put("basket1", Collections.singletonMap(Food.VEGGIES, Arrays.asList("Tomato","Onion")));
veggieBasket.put("basket2", Collections.singletonMap(Food.VEGGIES, Arrays.asList("Onion", "Potato")));
veggieBasket.put("basket3", Collections.singletonMap(Food.VEGGIES, Arrays.asList("Potato", "Tomato")));

veggieBasket:
[
basket1, [Food.VEGGIES, {"Tomato","Onion"}],
basket2, [Food.VEGGIES, {"Onion", "Potato"}],
basket3, [Food.VEGGIES, {"Potato", "Tomato"}]
]

我正试图结合篮子 水果篮 和 素食篮

Final output: should look something like below

groceryBasket
[
basket1, [Food.FRUITS, {"apple", "banana"}, Food.VEGGIES, {"Tomato","Onion"}],
basket2, [Food.FRUITS, {"orange", "kiwi"}, Food.VEGGIES, {"Onion", "Potato"}],
basket3, [Food.FRUITS, {"banana", "orange"}, Food.VEGGIES, {"Potato", "Tomato"}]
]

我的解决方案。

Solution 1:
Map<String, Map<Food, List<String>>> groceryBasket= new HashMap<>();

grocery basket = Stream.concat(fruitBasket.entrySet().stream(), veggieBasket.entrySet().stream())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (fruitList, veggieList ) ->
                {
                    final List<String> groceryList = new ArrayList<>();
                    groceryList .addAll(fruitList);
                    groceryList .addAll(veggieList);
                    return groceryList;
                }));

Solution 2:
Map<String, Map<Food, List<String>>> groceryBasket= new HashMap<>();

grocery basket = Stream.concat(fruitBasket.entrySet().stream(), veggieBasket.entrySet().stream())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (fruitList, veggieList ) ->
                {
                    return Stream.of(fruitList, veggieList).flatMap(x -> x.stream()).collect(Collectors.toList());
                }));

我试过方案一和方案二，我想知道是否有更好的方法来处理这个问题？

Answer 1

你可以这样做。

Map<String, Map<Food, List<String>>> groceryBasket = 
   Stream.concat(fruitBasket.entrySet().stream(),veggieBasket.entrySet().stream())
       .collect(Collectors.toMap(Map.Entry::getKey, v -> v.getValue().entrySet().stream()
                   .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)),
                    (a, b) -> { a.putAll(b);return a; }
                   )
               );

Answer 2

为什么不试着把合并解析为：

Map<String, Map<Food, List<String>>> groceryBasket =
        Stream.concat(fruitBasket.entrySet().stream(), veggieBasket.entrySet().stream())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a, b) -> {
                    Map<Food, List<String>> innerMap = new HashMap<>(a);
                    innerMap.putAll(b);
                    return innerMap;
                }));

或者，如果内部地图是可变的，那么稍微方便一点，如

Map<String, Map<Food, List<String>>> groceryBasket = Stream.concat(fruitBasket.entrySet().stream(),
        veggieBasket.entrySet().stream())
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a, b) -> {
            a.putAll(b);
            return a;
        }));

Answer 3

你必须创建可修改的列表来合并重复的键。

Stream.concat(fruitBasket.entrySet().stream(), veggieBasket.entrySet().stream())
                .collect(Collectors.toMap(Map.Entry::getKey, e -> listOf(e.getValue()),
                        (left, right) -> {
                            left.addAll(right);
                            return left;
                        }));

创建可修改的列表。

public static List<Map<Food, List<String>>> listOf(Map<Food, List<String>> a) {
    final ArrayList<Map<Food, List<String>>> list = new ArrayList<>();
    list.add(a);
    return list;
}

输出。

{basket3=[{水果=[香蕉，橙子]}，{蔬菜=[土豆，西红柿]}]。

basket2=[{FRUITS=[橙子，猕猴桃]}，{VEGGIES=[洋葱，土豆]}]。

basket1=[{水果=[苹果、香蕉]}, {蔬菜=[番茄、洋葱]}]}。