Java 使用 Stream 对列表进行排序和分组

Question

我有一个List<Records> =

[
    {"studentId": "001", "date": "20180705", "courseId": "CS220", "score": "80"},
    {"studentId": "001", "date": "20180702", "courseId": "CS320", "score": "75"},
    {"studentId": "001", "date": "20180704", "courseId": "CS330", "score": "90"},

    {"studentId": "002", "date": "20180703", "courseId": "CS640", "score": "95"},
    {"studentId": "002", "date": "20180628", "courseId": "CS530", "score": "80"},
    {"studentId": "002", "date": "20180701", "courseId": "CS545", "score": "90"},

    {"studentId": "100", "date": "20180708", "courseId": "CS542", "score": "80"},
    {"studentId": "100", "date": "20180629", "courseId": "CS240", "score": "97"},

    ... 
]

如何按相同的 studentId 对对象进行分组并只保留列表中得分最高的那个？像下面这样: List<Records> =

[
    {"studentId": "001", "date": "20180704", "courseId": "CS330", "score": "90"},

    {"studentId": "002", "date": "20180703", "courseId": "CS640", "score": "95"},

    {"studentId": "100", "date": "20180629", "courseId": "CS240", "score": "97"},

    ... 
]

Answer 1

使用Stream来收集值。您可以在 Baeldung -- Guide to Java 8 groupingBy Collector 上找到示例

2.8. Getting the Maximum or Minimum from Grouped Results

这可以像这样使用:

records.stream()
    .collect(
         Collectors.groupingBy(
             Record::getStudentId, 
             Collectors.maxBy(Compartor.comparingInt(Record::getPoint))
         )
    );

返回的映射将以每个 studentId 作为键以及具有最大值的实例。使用 POJO 的示例:

class POJO {
    String name;
    int value;

    //getters
    //toString //POJO [%name% %value%]
}

快速测试:

Map<String, Optional<POJO>> map = Stream.of(
            new POJO("A", 1), new POJO("A", 2), new POJO("A", 10),
            new POJO("B", 8), new POJO("B", 4),
            new POJO("C", 4),
            new POJO("D", 4), new POJO("D", 1), new POJO("D", 2)
            ).collect(
                    Collectors.groupingBy(
                            POJO::getName, 
                            Collectors.maxBy(
                                    Comparator.comparingInt(POJO::getValue)
                            )
                    )
            );

    System.out.println(map);

{A=Optional[POJO [A 10]],
B=Optional[POJO [B 8]],
C=Optional[POJO [C 4]],
D=Optional[POJO [D 4]]}