考虑以下WorkExperience
类:
public class WorkExperience {
private int year;
private List<Skills> skill;
public WorkExperience(int year, List<Skills> skill) {
this.year = year;
this.skill = skill;
}
//getter setter
}
public class Skills {
private String skills;
public Skills(String skills) {
this.skills = skills;
}
@Override
public String toString() {
return "Skills [skills=" + skills + "]";
}
}
让我们说我希望按年份按照我的技能进行分组,这就是我们每年groupBy
的做法:
public static void main(String[] args) {
List<Skills> skillSet1 = new ArrayList<>();
skillSet1.add(new Skills("Skill-1"));
skillSet1.add(new Skills("Skill-2"));
skillSet1.add(new Skills("Skill-3"));
List<Skills> skillSet2 = new ArrayList<>();
skillSet2.add(new Skills("Skill-1"));
skillSet2.add(new Skills("Skill-4"));
skillSet2.add(new Skills("Skill-2"));
List<Skills> skillSet3 = new ArrayList<>();
skillSet3.add(new Skills("Skill-1"));
skillSet3.add(new Skills("Skill-9"));
skillSet3.add(new Skills("Skill-2"));
List<WorkExperience> workExperienceList = new ArrayList<>();
workExperienceList.add(new WorkExperience(2017,skillSet1));
workExperienceList.add(new WorkExperience(2017,skillSet2));
workExperienceList.add(new WorkExperience(2018,skillSet3));
Map<Integer, Set<List<Skills>>> collect = workExperienceList.stream().collect(
Collectors.groupingBy(
WorkExperience::getYear,
Collectors.mapping(WorkExperience::getSkill, Collectors.toSet())
)
);
}
groupBy
正在返回:Map<Integer, Set<List<Skills>>>
但我需要的是:Map<Integer, Set<Skills>>
如何将列表流转换为单个容器?</ strong>
答案 0 :(得分:14)
仅使用Java 8功能的flatMapping
替代
Map<Integer, Set<Skills>> map = workExperienceList.stream()
.collect(Collectors.toMap(
WorkExperience::getYear,
we -> new HashSet<>(we.getSkill()),
(s1, s2)-> { s1.addAll(s2); return s1; }));
你可以稍微优化一下
Map<Integer, Set<Skills>> map = workExperienceList.stream()
.collect(Collectors.toMap(
WorkExperience::getYear,
we -> new HashSet<>(we.getSkill()),
(s1, s2) -> {
if(s1.size() > s2.size()) { s1.addAll(s2); return s1; }
else { s2.addAll(s1); return s2; }
}));
答案 1 :(得分:12)
我们可以使用 Java-9中添加的Collectors.flatMapping
收藏家。使用flatMapping
,我们可以将中间列表展平为一个容器。 flatMapping
可用于原始流的元素可转换为流的情况。
workExperienceList.stream().collect(Collectors.groupingBy(
WorkExperience::getYear,
Collectors.flatMapping(workexp -> workexp.getSkill().stream(),
Collectors.toSet())));
API说明 :
flatMapping()收集器在多级缩减中使用时非常有用,例如groupingBy或partitioningBy的下游。
答案 2 :(得分:1)
实现目标的另一种方法是使用静态工厂方法Collector.of()
来实现自己的收集器:
Map<Integer, Set<Skills>> collect = workExperienceList.stream()
.collect(Collector.of(
HashMap::new,
( map, e ) -> map.computeIfAbsent(e.getYear(), k -> new HashSet<>()).addAll(e.getSkill()),
( left, right ) -> {
right.forEach(( year, set ) -> left.computeIfAbsent(year, k -> new HashSet<>()).addAll(set));
return left;
})
);
与其他答案相比,这是相当混乱和臃肿的。
答案 3 :(得分:0)
尝试更加面向对象。所以我想创建一个新的小对象
public static class AsGroup {
private final Integer year;
private final Collection<Skill> skillSet;
public AsGroup(Integer year, Collection<Skill> skillSet) {
this.year = year;
this.skillSet = skillSet;
}
public AsGroup addSkills(AsGroup asGroupSkills) {
this.skillSet.addAll(asGroupSkills.skillSet);
return this;
}
}
然后你可以用以下方法解决问题:
Map<Integer, Optional<com.company.Main.AsGroup>> groupedByYear = workExperienceList.stream()
.map(workExperience ->
new AsGroup(workExperience.getYear(), new HashSet<>(workExperience.getSkill()))
).collect(groupingBy((asGroup) -> asGroup.year,
reducing((group1, group2) -> (group1.addSkills(group2))))
);
您可以按照以下方式使用它:
groupedByYear.forEach(((year, groupedSkills) -> System.out.println(year + " " + groupedSkills.get().skillSet)));
它打印如下:
2017 [Skill [skill=Skill-1], Skill [skill=Skill-1], Skill [skill=Skill-4], Skill [skill=Skill-2], Skill [skill=Skill-2], Skill [skill=Skill-3]]
2018 [Skill [skill=Skill-1], Skill [skill=Skill-2], Skill [skill=Skill-9]]