Java 8:过滤数组(NxM)以创建Map <String,HashSet <String >>

Question

我想将NxN数组映射到Java 8中的Map.

这个想法是每个[i] [0]元素都是一个键,每个[i] [j] j> 0都是映射中每个键的值列表.

谢谢你的帮助.:)

这是我的班级:

public class GroupingDishes {

    public static void main(String[] args) {

        String[][] dishes = {
                {"Salad", "Tomato", "Cucumber", "Salad", "Sauce"},
                {"Pizza", "Tomato", "Sausage", "Sauce", "Dough"},
                {"Quesadilla", "Chicken", "Cheese", "Sauce"},
                {"Sandwich", "Salad", "Bread", "Tomato", "Cheese"}
        };

        Map<String, HashSet<String>> groupDishes = groupingDishes(dishes);
    }

    public static Map<String, HashSet<String>> groupingDishes(String[][] dishes) {

        Map<String, HashSet<String>> mapFood = new HashMap<>();

        for (int i = 0; i < dishes.length; i++) {

            String food = dishes[i][0];

            for (int j = 0; j < dishes[i].length; j++) {

                if (mapFood.containsKey(food)) {

                    HashSet<String> existingIngredients = mapFood.get(dishes[i][0]);
                    existingIngredients.add(dishes[i][j]);
                    mapFood.put(food, existingIngredients);

                } else {

                    HashSet<String> newIngredient = new HashSet<>();
                    mapFood.put(food, newIngredient);

                }
            }
        }
        return mapFood;
    }
}

Answer 1

您可以使用第一项作为键转换String[][]为流String[],然后收集到地图,String[]其余作为集合的值.

public static Map<String, HashSet<String>> groupingDishes2(String[][] dishes) {
    return Arrays.stream(dishes)
        .collect(Collectors.toMap(
            arr -> arr[0],
            arr -> Arrays.stream(arr).skip(1).collect(Collectors.toCollection(HashSet::new))));
}

顺便说一句,我怀疑你真的需要一个Map<String, HashSet<String>>.将类型更改为更好Map<String, Set<String>>,然后实现也可以更简单.

public static Map<String, Set<String>> groupingDishes(String[][] dishes) {
    return Arrays.stream(dishes)
        .collect(Collectors.toMap(
            arr -> arr[0],
            arr -> Arrays.stream(arr).skip(1).collect(Collectors.toSet())));
}

或者甚至更好,正如@Holger建议的那样,一个更好的选择,因为"带有skip和limit不能很好地执行的流,也Collectors没有得到任何结果的初始容量提示":

public static Map<String, Set<String>> groupingDishes(String[][] dishes) {
    return Arrays.stream(dishes)
        .collect(Collectors.toMap(
            arr -> arr[0],
            arr -> new HashSet<>(Arrays.asList(arr).subList(1, arr.length))));
}