如何 - 按键对列表进行分组并使用流拆分值 Java 8

Question

我需要对ABC List进行分组，同时我还想拆分值。输入：

List<ABC> list = new ArrayList<>();

ABC A1 = new ABC();
A1.setName("OO");
A1.setVal("OO111,OO222,OO333"); // split this while grouping
list.add(A1);

ABC A2 = new ABC();
A2.setName("OO");
A2.setVal("OO1");
list.add(A2);

ABC A3 = new ABC();
A3.setName("YY");
A3.setVal("1333,11444");
list.add(A3);

Output:

```java
Map<String, List<String>> postsPerType;
//Group by Name: ex: OO & YY. Split the Value by "," and add it to list
     Key: 00 Values: List: "OO111","OO222","OO333","OO1"
     Key: YY Values: List: "1333","11444"

求推荐，谢谢

Answer 1

我认为您可以将 reducing 收集器链接到 groupingBy 收集器以获得您需要的东西：

Map<String, List<String>> postsPerType =
    list.stream()
        .collect(Collectors.groupingBy(ABC::getName,
                                       Collectors.reducing(new ArrayList<String>(),
                                                           a -> Arrays.asList(a.getVal().split(",")),
                                                           (l1,l2)-> {
                                                             List<String> l = new ArrayList<>(); 
                                                             l.addAll(l1);
                                                             l.addAll (l2);
                                                             return l;
                                                           })));

这会产生以下结果 Map：

{YY=[1333, 11444], OO=[OO111, OO222, OO333, OO1]}

Answer 2

如果您使用 Java 9+，您可以使用 Collectors.flatMapping() 作为 groupingBy 的下游：

list.stream().collect(Collectors.groupingBy(ABC::getName,
                    Collectors.flatMapping(a -> Arrays.stream(a.getVal().split(",")), 
                                           Collectors.toList())));

这应该会产生您想要的结果。

Answer 3

另一种方法是使用 groupingBy 和 collectingAndThen 收集器。

list.stream()
            .collect(Collectors.groupingBy(
                    ABC::getName,
                    Collectors.collectingAndThen(
                            Collectors.mapping(ABC::getVal, toList()),
                            strings -> strings.stream()
                                     .flatMap(s -> Arrays.stream(s.split(",")))
                                     .collect(toList()))
            ));

Answer 4

试试这个。

record ABC(String name, String val) {
    public String getName() { return name; }
    public String getVal() { return val; }
}

public static void main(String[] args) {
    List<ABC> list = List.of(
        new ABC("00", "00111,00222,00333"),
        new ABC("00", "001"),
        new ABC("YY", "1333,11444"));
    Map<String, List<String>> postsPerType = new HashMap<>();
    for (ABC e : list)
        postsPerType.computeIfAbsent(e.getName(), k -> new ArrayList<>())
            .addAll(Arrays.asList(e.getVal().split(",")));
    System.out.println(postsPerType);
}

输出：

{YY=[1333, 11444], 00=[00111, 00222, 00333, 001]}

如何 - 按键对列表进行分组并使用流拆分值 Java 8

How to - grouping a list by key and split the values using streams Java 8

java

group-by

java-stream