將List集合中相同屬性的對象合並

將List集合中相同屬性的對象合並

在實際需求中，可能遇到需要將List集合中的對象，按照某個(多個)屬性進行合並成一個對象，並將相同對象的某一個屬性（例如：數量）進行相加。

Lambda方式

首先創建實體對象

@Data
@AllArgsConstructor
@NoArgsConstructor
@SuppressWarnings("serial")
public class Student {
    private Integer uid;
    private String uname;
    private Double score;
}

合並代碼

public static void main(String[] args) {
        List<Student> list=new ArrayList<>();
        list.add(new Student(1,"張三",60.50));
        list.add(new Student(1,"張三",70.25));
        list.add(new Student(1,"張三",80.25));
        list.add(new Student(2,"李四",60D));

        List<Student> studentList=new ArrayList<>();
        list.parallelStream().collect(Collectors.groupingBy(o -> (o.getUid()+o.getUname()),Collectors.toList())).forEach((id,transfer)->{
            transfer.stream().reduce((a,b)->new Student(a.getUid(),a.getUname(),a.getScore()+b.getScore())).ifPresent(studentList::add);
        });
        for (Student student : studentList) {
            System.out.println(student);
        }
    }

Map方式

實體類重寫equals和hashCode方法並添加merge方法

@Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(uid, student.uid) &&
                Objects.equals(uname, student.uname);
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((uid == null) ? 0 : uid.hashCode());
        result = prime * result + ((uname == null) ? 0 : uname.hashCode());
        return result;
    }


    public static Student merge(Student s1, Student s2) {
        if (!s1.equals(s2)) {
            throw new IllegalArgumentException();
        }
        return new Student(s1.uid, s1.uname, s1.score + s2.score);
    }

測試方法

List<Student> list = new ArrayList<>();
list.add(new Student(1, "張三", 60.50));
list.add(new Student(1, "張三", 70.25));
list.add(new Student(1, "張三", 80.25));
list.add(new Student(2, "李四", 60D));

Map<Student, Student> map = new HashMap<Student, Student>();
        for (Student s : list) {
            if (map.containsKey(s)) {
                map.put(s, Student.merge(s, map.get(s)));
            } else {
                map.put(s, s);
            }
        }
Collection<Student> values = map.values();
values.forEach(System.out::println);

引用原文：
https://blog.csdn.net/weixin_55585048/article/details/116793654