java-8 - Java Map：按键的属性和最大值分组

Question

我有一个Map<Reference, Double>挑战的实例是键对象可能包含对同一对象的引用，我需要返回相同类型的“输入”映射，但按属性键分组并保留最大值。

我尝试使用groupingBy，maxBy但我被卡住了。

private void run () {
    Map<Reference, Double> vote = new HashMap<>();

    Student s1 = new Student(12L);
    vote.put(new Reference(s1), 66.5);
    vote.put(new Reference(s1), 71.71);
    Student s2 = new Student(44L);
    vote.put(new Reference(s2), 59.75);
    vote.put(new Reference(s2), 64.00);

    // I need to have a Collection of Reference objs related to the max value of the "vote" map
    Collection<Reference> maxVote = vote.entrySet().stream().collect(groupingBy(Map.Entry.<Reference, Double>comparingByKey(new Comparator<Reference>() {
        @Override
        public int compare(Reference r1, Reference r2) {
            return r1.getObjId().compareTo(r2.getObjId());
        }
    }), maxBy(Comparator.comparingDouble(Map.Entry::getValue))));
}

class Reference {
    private final Student student;

    public Reference(Student s) {
        this.student = s;
    }
    public Long getObjId() {
        return this.student.getId();
    }
}

class Student {
    private final  Long id;
    public Student (Long id) {
        this.id = id;
    }

    public Long getId() {
        return id;
    }
}

maxBy我的论点有一个错误：Comparator.comparingDouble(Map.Entry::getValue)我不知道如何解决它。有没有办法达到预期的结果？

标签： java-8java-stream

Answer 1

您可以使用Collectors.toMap来获取集合Map.Entry<Reference, Double>

Collection<Map.Entry<Reference, Double>> result = vote.entrySet().stream()
        .collect(Collectors.toMap( a -> a.getKey().getObjId(), Function.identity(),
            BinaryOperator.maxBy(Comparator.comparingDouble(Map.Entry::getValue)))).values();

然后再次流过以获得List<Reference>

List<Reference> result = vote.entrySet().stream()
        .collect(Collectors.toMap(a -> a.getKey().getObjId(), Function.identity(),
            BinaryOperator.maxBy(Comparator.comparingDouble(Map.Entry::getValue))))
        .values().stream().map(e -> e.getKey()).collect(Collectors.toList());