目录

list => map

map => map 数据处理

java.util.stream.Stream#flatMap

java.util.stream.Stream

list => map

Collectors.toMap，Collectors.groupingBy

public class TestData {
    private int id;
    private String name;
    private String sex;
    private String subject;
    private String score;
}

public class TestDataKey {
    private String name;
    private String sex;
}

1、获取每个人所学的科目列表：List => Map>

Map> idAndSujectListMap= list.stream().collect(Collectors.groupingBy(TestData::getId,
                Collectors.mapping(TestData::getSubject, Collectors.toList())));

//去重
Map> idAndSujectListMap= list.stream().collect(Collectors.groupingBy(TestData::getId,
                Collectors.mapping(TestData::getSubject, Collectors.toSet())));

2、获取每个人的总成绩：List => Map

Map idAndScoreSumMap= list.stream().collect(Collectors.groupingBy(TestData::getId, 
                Collectors.summingInt(TestData::getScore));

3、获取name+sex（唯一键）对应的所有信息：List => Map>

Map> keyAndListMap = list.stream().collect(Collectors.groupingBy(item -> 
                TestDataKey.builder().name(item.getName()).sex(item.getSex()).build()));

4、获取name+sex（唯一键）对应总成绩：List => Map

Map keyAndScoreSumMap = list.stream().collect(Collectors.groupingBy(item ->
                        TestDataKey.builder().name(item.getName()).sex(item.getSex()).build(),
                Collectors.summingInt(TestData::getScore)));

5、获取name+sex（唯一键）对应的ID：List => Map

Map keyAndIdMap = list.stream().collect(Collectors.toMap(item -> 
                      TestDataKey.builder().name(item.getName()).sex(item.getSex()).build(), 
                TestData::getId, (v1, v2) -> v1));

map => map 数据处理

Map 转 Map>

将map的value(逗号隔开的字符串)转为List

Map> resultMap = map.entrySet()
        .stream().collect(Collectors.toMap(Map.Entry::getKey,
                entry -> Arrays.asList(entry.getValue().split(","))));

java.util.stream.Stream#flatMap

使用场景：map不适用时，有多个元素时，考虑使用

1、获取所有学科：Map> idAndSubjectMap => List allSubject

//已有：Map> idAndSubjectMap
//之前
List subjects = Lists.newArrayList();
idAndSubjectMap.values().forEach(subjects::addAll);

//替换
List subjects = idAndSubjectMap.values().stream().flatMap(Collection::stream).collect(Collectors.toList())

2、获取所有学科：Map idAndSubjectMap => List allSubject

假设map的value学科是以逗号隔开的字符串

//已有：Map idAndSubjectMap
//之前
List subjects = Lists.newArrayList();
idAndSubjectMap.values().forEach(item -> subjects.addAll(Arrays.asList(item.split(","))));

//替换
List subjects = idAndSubjectMap.values().stream()
        .flatMap(value-> Arrays.stream(value.split(","))).collect(Collectors.toList());