如何将多个映射与多个相同的键和列表作为值组合起来？

Question

public class Student {
    private String name;
    private String country;

    //Setters and Getters
}

public class MapAdder {
    static Map<String, List<Student>> studentMap =
            new LinkedHashMap<String, List<Student>>();

    public static void addToMap(String key, Student student) {
        studentMap.computeIfAbsent(key,
                k -> new LinkedList<Student>()).add(student);
    }

    public static Map<String, List<Student>> getStudentMap() {
        return studentMap;
    }

    public static void clearStudentMap() {
        studentMap.clear();
    }
}

主要方法

public static void main(String[] args) {
    Map<String, List<Student>> studentMap1;
    Map<String, List<Student>> studentMap2;
    Map<String, List<Student>> studentMap3;

    MapAdder.addToMap("India", new Student("Mounish", "India"));
    MapAdder.addToMap("USA", new Student("Zen", "USA"));
    MapAdder.addToMap("India", new Student("Ram", "India"));
    MapAdder.addToMap("USA", new Student("Ronon", "USA"));
    MapAdder.addToMap("UK", new Student("Tony", "UK"));

    studentMap1 = MapAdder.getStudentMap();
    MapAdder.clearStudentMap();

    MapAdder.addToMap("India", new Student("Rivar", "India"));
    MapAdder.addToMap("UK", new Student("Loki", "UK"));
    MapAdder.addToMap("UK", new Student("Imran", "UK"));
    MapAdder.addToMap("USA", new Student("ryan", "USA"));

    studentMap2 = MapAdder.getStudentMap();
    MapAdder.clearStudentMap();

    Map<String, List<Student>> map3 = Stream.of(studentMap1, studentMap2)
            .flatMap(map -> map.entrySet().stream())
            .collect(Collectors.toMap(
                    Entry::getKey,
                    Entry::getValue
            ));
}

但当我尝试合并这两个地图时，我得到的是空地图。实际上，我需要有一个地图与三个键（印度，英国，美国）和他们的值，从多个地图列表合并W.R.T键。

Answer 1

首先，从代码中删除以下调用：

MapAdder.clearStudentMap();

您正在清除StudentMap1和StudentMap2。

当您这样做时：

studentMap1 = MapAdder.getStudentMap();

studentMap.clear();

studentMap1 = MapAdder.getStudentMap();

Map<String, List<Student>> map3 = Stream.of(studentMap1, studentMap2)
        .flatMap(map -> map.entrySet().stream())
        .collect(Collectors.toMap(
                Map.Entry::getKey,
                e -> new ArrayList<>(e.getValue()),
                (left, right) -> { left.addAll(right); return left; }
        ));

public class MapAdder {
    public static void addToMap(Map<String, List<Student>> studentMap,
                                String key, Student student) {
        studentMap.computeIfAbsent(key,
                k -> new LinkedList<Student>()).add(student);
    }

    public static void main(String[] args) {
        Map<String, List<Student>> studentMap1 = new LinkedHashMap<>();
        Map<String, List<Student>> studentMap2 = new LinkedHashMap<>();
        Map<String, List<Student>> studentMap3;

        MapAdder.addToMap(studentMap1, "India", new Student("Mounish", "India"));
        MapAdder.addToMap(studentMap1, "USA", new Student("Zen", "USA"));
        MapAdder.addToMap(studentMap1, "India", new Student("Ram", "India"));
        MapAdder.addToMap(studentMap1, "USA", new Student("Ronon", "USA"));
        MapAdder.addToMap(studentMap1, "UK", new Student("Tony", "UK"));

        MapAdder.addToMap(studentMap2, "India", new Student("Rivar", "India"));
        MapAdder.addToMap(studentMap2, "UK", new Student("Loki", "UK"));
        MapAdder.addToMap(studentMap2, "UK", new Student("Imran", "UK"));
        MapAdder.addToMap(studentMap2, "USA", new Student("ryan", "USA"));

        Map<String, List<Student>> map3 = Stream.of(studentMap1, studentMap2)
                .flatMap(map -> map.entrySet().stream())
                .collect(Collectors.toMap(
                        Map.Entry::getKey,
                        e -> new ArrayList<>(e.getValue()),
                        (left, right) -> { left.addAll(right); return left; }
                ));
    }
}