按嵌套列表的元素对列表进行分组

Question

我有 Car 对象，其参数如下：

private String model;
private BigDecimal price;
private Color color;
private BigDecimal milleage;
private List<String> components;

我创建了汽车对象列表：

var cars = List.of(
    Car.create("FORD", BigDecimal.valueOf(120000), Color.RED, BigDecimal.valueOf(150000),
            List.of("AIR CONDITIONING", "VOICE SERVICE")),
    Car.create("FORD", BigDecimal.valueOf(160000), Color.RED, BigDecimal.valueOf(150000),
            List.of("AIR CONDITIONING", "VOICE SERVICE")),
    Car.create("AUDI", BigDecimal.valueOf(200000), Color.BLACK, BigDecimal.valueOf(195000),
            List.of("NAVIGATION", "AUTOMATIC GEARBOX")),
    Car.create("FIAT", BigDecimal.valueOf(70000), Color.BLUE, BigDecimal.valueOf(85000),
            List.of("AIR CONDITIONING", "MANUAL GEARBOX")));

现在我想创建 Map<String, List<Car>>，其中字符串是组件列表的元素，List<Car>> 是包含该组件的 Car 个对象的列表。

我根据一些类似的问题尝试过类似的方法，但真的不知道如何解决这个问题：

static Map<String, List<Car>> carsThatGotComponent(List<Car> cars) {
    return cars.stream()
               .flatMap(car -> car.getComponents()
                       .stream()
                       .map(component -> new AbstractMap.SimpleEntry<>(car, component)))
               .collect(Collectors.groupingBy(
                        Map.Entry::getValue,
                        Collectors.mapping(Map.Entry::getKey, Map.Entry::getValue)));
}

Answer 1

Collectors#mapping 需要下游 Collector 作为第二个参数，而不是映射函数。

public static <T,U,A,R> Collector<T,?,R> mapping(
    Function<? super T,? extends U> mapper, 
    Collector<? super U,A,R> downstream)

您想改用 Collectors.toList()：

return cars.stream()
    .flatMap(car -> car.getComponents()
                       .stream()
                       .map(component -> new AbstractMap.SimpleEntry<>(car, component)))
    .collect(Collectors.groupingBy(
            AbstractMap.SimpleEntry::getValue,
            Collectors.mapping(AbstractMap.SimpleEntry::getKey, Collectors.toList())));

只要你用java-10 or later, you can use simplify the whole Stream into one collector using Collectors#flatMapping as of java-9:

return cars.stream()
    .collect(Collectors.flatMapping(
             car -> car.getComponents()
                       .stream()
                       .map(component -> new AbstractMap.SimpleEntry<>(car, component)),
             Collectors.groupingBy(AbstractMap.SimpleEntry::getValue,
                     Collectors.mapping(AbstractMap.SimpleEntry::getKey, 
                             Collectors.toList()))));

按嵌套列表的元素对列表进行分组

Group list by elements of nested list

java

grouping

dictionary

java-stream