首頁猿問 Java8中如何進(jìn)行分組和縮減

Java8中如何進(jìn)行分組和縮減

Java

梵蒂岡之花 2023-06-28 15:37:55

正如我對List主題的了解"Subjects":[{"subject":"Math","grades":1},{"subject":"Math","grades":2},{"subject":"Math","grades":3},{"subject":"Math","grades":3},{"subject":"Lab","grades":10},{"subject":"Lab","grades":12}]我想像這樣分組和減少結(jié)果//Expected Result"Subjects":[{"subject":"Math","grades":[1,2,3]},{"subject":"Lab","grades":[10,12]}]我很好奇如何以 java8 風(fēng)格映射和減少對象。我在下面放置了過時的代碼。我的主課public static void main(String[] args) {List<Subject> list = new ArrayList<>();list.add(new Subject("Math",1));list.add(new Subject("Math",2));list.add(new Subject("Math",3));list.add(new Subject("Math",3));list.add(new Subject("Lab",10));list.add(new Subject("Lab",12)); Map<String, Set<Integer>> result = new HashMap<>();list.stream().forEach(subjects-> { if(result.get(subjects.getSubject())==null){ Set<Integer> set = new HashSet<>(); set.add(subjects.getGrades()); result.put(subjects.getSubject(),set ); }else{ Set<Integer> set =result.get(subjects.getSubject()); set.add(subjects.getGrades()); result.put(subjects.getSubject(), set); } }); result.forEach((key,val)->{ System.out.println("KEY:"+key + " RESULT :"+val); });} public class Subject {private String subject;private Integer grades;public Subject(String subject , Integer grade) {this.subject = subject;this.grades = grade;}/** get set **/}

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1 回答

白衣染霜花

TA貢獻(xiàn)1796條經(jīng)驗獲得超10個贊

您可以使用 aCollectors.groupingBy(Subject::getSubject)和 aCollectors.mapping(Subject::getGrades, Collectors.toSet())作為下游。

list.stream()
    .collect(Collectors.groupingBy(Subject::getSubject,
                 Collectors.mapping(Subject::getGrades, 
                     Collectors.toSet())));

它會給你一個Map<String, Set<Integer>>.