在 Java 中合并两个集合

Question

I have two maps:

Map<String, Student> students1 = new HashMap<>();
students1.put("New York", new Student("John"));
students1.put("Canada", new Student("Robert"));

Map<String, Student> students2 = new HashMap<>();
students2.put("Chicago", new Student("Nick"));
students2.put("New York", new Student("Ann"));

As a result, I want to get this:

{Canada=Robert, New York=[John, Ann], Chicago=Nick}

I can easily do it like this:

Map<City, List<Student>> allStudents = new HashMap<>();

students1.forEach((currentCity, currentStudent) -> {
    allStudents.computeIfPresent(currentCity, (city, studentsInCity) -> {
        studentsInCity.add(currentStudent);
        return studentsInCity;
    });

    allStudents.putIfAbsent(currentCity, new ArrayList<Student>() {
        {
            add(currentStudent);
        }
    });
});

// then again for the second list

But is there any other way to merge many collections (two in this case)? Is there something like short lambda expression, or method from some of the integrated java libraries, etc...?

Answer 1

You could create a stream over any number of maps, then flat map over their entries. It is then as simple as grouping by the key of the entry, with the value of the entry mapped to a List as value:

Map<String, List<Student>> collect = Stream.of(students1, students2)
    .flatMap(map -> map.entrySet().stream())
    .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

With static import for readability:

Map<String, List<Student>> collect = Stream.of(students1, students2)
    .flatMap(map -> map.entrySet().stream())
    .collect(groupingBy(Entry::getKey, mapping(Entry::getValue, toList())));

Replace toList() with toSet() if a Set is more appropriate as value of the map.

Answer 2

I think the Stream version given by Magnilex is the most elegant way to do this. But I still want to give another choice.

static final Function<...> NEW_LIST = __ -> new ArrayList<>();

Map<City, List<Student>> allStudents = new HashMap<>();

students1.forEach((city, student) -> {
    allStudents.computeIfAbsent(city, NEW_LIST).add(student);
});

Answer 3

https://www.baeldung.com/java-merge-maps这是一个链接，应该会有所帮助