如何在Java8中进行分组和归约
因为我有主题的如何在Java8中进行分组和归约,java,arraylist,lambda,java-8,java-stream,Java,Arraylist,Lambda,Java 8,Java Stream,因为我有主题的列表 "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} ] 我想这样分组和减少结果 //Expected Result "Subjects
列表
"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}
]
我想这样分组和减少结果
//Expected Result
"Subjects":[
{"subject":"Math","grades":[1,2,3]},
{"subject":"Lab","grades":[10,12]}
]
我很好奇如何以java8风格映射和缩减对象。
我把过时的代码放在下面
我的主课
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 **/
}
publicstaticvoidmain(字符串[]args){
列表=新的ArrayList();
增加(新科目(“数学”,1));
增加(新科目(“数学”,2));
增加(新科目(“数学”,3));
增加(新科目(“数学”,3));
增加(新学科(“实验室”,10));
增加(新学科(“实验室”,12));
映射结果=新的HashMap();
list.stream().forEach(主题->{
if(result.get(subjects.getSubject())==null){
Set=newhashset();
set.add(subjects.getGrades());
result.put(subjects.getSubject(),set);
}否则{
Set=result.get(subjects.getSubject());
set.add(subjects.getGrades());
result.put(subjects.getSubject(),set);
}
});
结果。forEach((键,val)->{
System.out.println(“键:+KEY+”结果:+val);
});
}
公共课科目{
私有字符串主题;
私人整数职系;
公共科目(字符串科目,整数等级){
this.subject=主语;
这个。等级=等级;
}
/**准备好**/
}
您可以使用收集器.groupingBy(Subject::getSubject)
和收集器.mapping(Subject::getGrades,Collectors.toSet())
作为下游
list.stream()
.collect(Collectors.groupingBy(Subject::getSubject,
Collectors.mapping(Subject::getGrades,
Collectors.toSet())));
它会给你一张地图
注意:
result.put(subjects.getSubject(),set)代码>行在后一种情况下是多余的。甚至主题也被复制
{Lab=[10, 12], Math=[1, 2, 3]}