Java 8 : grouping field values by field names

Question

我正在尝试寻找一种比以下方法更优雅的方法来创建 map ，该 map 使用 Java 8 按字段名称对字段值进行分组:

@Test
public void groupFieldValuesByFieldNames() {
    Person lawrence = aPerson().withFirstName("Lawrence").withLastName("Warren").born();
    Person gracie = aPerson().withFirstName("Gracie").withLastName("Ness").born();

    Map<String, List<String>> valuesByFieldNames = new HashMap<>();
    Stream.of(lawrence, gracie).forEach(person -> {
        valuesByFieldNames.computeIfAbsent("lastName", s -> new ArrayList<>()).add(person.getLastName());
        valuesByFieldNames.computeIfAbsent("firstName", s -> new ArrayList<>()).add(person.getFirstName());
    });

    assertThat(valuesByFieldNames, hasEntry("lastName", asList("Warren", "Ness")));
    assertThat(valuesByFieldNames, hasEntry("firstName", asList("Lawrence", "Gracie")));
}

Answer 1

试试这个。

Map<String, List<String>> valuesByFieldNames = Stream.of(lawrence, gracie)
    .flatMap(p -> Stream.of(new String[]{"firstName", p.getFirstName()},
                            new String[]{"lastName", p.getLastName()}))
    .collect(Collectors.groupingBy(a -> a[0],
             Collectors.mapping(a -> a[1], Collectors.toList())));

或者更一般地

Map<String, List<String>> valuesByFieldNames = Stream.of(lawrence, gracie)
    .flatMap(p -> Stream.of(new AbstractMap.SimpleEntry<>("firstName", p.getFirstName()),
                            new AbstractMap.SimpleEntry<>("lastName", p.getLastName())))
    .collect(Collectors.groupingBy(e -> e.getKey(),
             Collectors.mapping(e -> e.getValue(), Collectors.toList())));