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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.io.*;
  6. import java.util.ArrayList;
  7. import java.util.List;
  8. import java.util.Map;
  9. import java.util.Objects;
  10. import java.util.function.Function;
  11. import java.util.stream.Collectors;
  12. import java.util.stream.Stream;
  13.  
  14. /* Name of the class has to be "Main" only if the class is public. */
  15. class Ideone
  16. {
  17. 	public static void main (String[] args) throws java.lang.Exception
  18. 	{
  19. 		List<Student> list = new ArrayList<>(List.of(
  20.                 new Student(1, new ArrayList<>(List.of(new Marks(1, 10), new Marks(2, 10)))),
  21.                 new Student(1, new ArrayList<>(List.of(new Marks(1, 15), new Marks(3, 10)))),
  22.                 new Student(2, new ArrayList<>(List.of(new Marks(1, 5), new Marks(2, 10)))),
  23.                 new Student(2, new ArrayList<>(List.of(new Marks(2, 5), new Marks(3, 10))))
  24.         ));
  25.  
  26.         //Temporary map
  27.         Map<Integer, Student> mapTemp = list.stream()
  28.                 .collect(Collectors.toMap(Student::getStudentId,                                        //grouping by student id
  29.                         Function.identity(),                                                            //setting as value the student
  30.                         (s1, s2) -> new Student(s1.getStudentId(), new ArrayList<>(                     //Creating a new Student whose list of marks is given by the merged marks of the two colliding students
  31.                                 Stream.concat(s1.getMarkList().stream(), s2.getMarkList().stream())     //Chaining the two lists of marks into a single stream
  32.                                         .collect(Collectors.toMap(Marks::getSubjectId,                  //Grouping by the marks by the subject id
  33.                                                 Function.identity(),                                    //Setting as value the mark
  34.                                                 (m1, m2) -> new Marks(m1.getSubjectId(), m1.getMark() + m2.getMark())))  //Handling the colliding marks by summing them together
  35.                                         .values())))                                                    //Retrieving the collection of merged marks
  36.                 );
  37.  
  38.         //Result list with the merged students
  39.         List listRes = new ArrayList(mapTemp.values());
  40.         System.out.println(listRes);
  41. 	}
  42. }
  43.  
  44. class Student {
  45.     private int studentId;
  46.     private List<Marks> markList = new ArrayList<>();
  47.  
  48.     public Student(int studentId, List<Marks> markList) {
  49.         this.studentId = studentId;
  50.         this.markList = markList;
  51.     }
  52.  
  53.     public int getStudentId() {
  54.         return studentId;
  55.     }
  56.  
  57.     public List<Marks> getMarkList() {
  58.         return markList;
  59.     }
  60.  
  61.     @Override
  62.     public boolean equals(Object o) {
  63.         if (this == o) return true;
  64.         if (o == null || getClass() != o.getClass()) return false;
  65.         Student student = (Student) o;
  66.         return studentId == student.studentId;
  67.     }
  68.  
  69.     @Override
  70.     public int hashCode() {
  71.         return Objects.hash(studentId);
  72.     }
  73.  
  74.     @Override
  75.     public String toString() {
  76.         return String.format("[%d - %s]", studentId, markList);
  77.     }
  78. }
  79.  
  80. class Marks {
  81.     private Integer subjectId;
  82.     private Integer mark;
  83.  
  84.     public Marks(Integer subjectId, Integer mark) {
  85.         this.subjectId = subjectId;
  86.         this.mark = mark;
  87.     }
  88.  
  89.     public Integer getSubjectId() {
  90.         return subjectId;
  91.     }
  92.  
  93.     public Integer getMark() {
  94.         return mark;
  95.     }
  96.  
  97.     public void setSubjectId(Integer subjectId) {
  98.         this.subjectId = subjectId;
  99.     }
  100.  
  101.     public void setMark(Integer mark) {
  102.         this.mark = mark;
  103.     }
  104.  
  105.     @Override
  106.     public boolean equals(Object o) {
  107.         if (this == o) return true;
  108.         if (o == null || getClass() != o.getClass()) return false;
  109.         Marks marks = (Marks) o;
  110.         return Objects.equals(subjectId, marks.subjectId);
  111.     }
  112.  
  113.     @Override
  114.     public int hashCode() {
  115.         return Objects.hash(subjectId);
  116.     }
  117.  
  118.     @Override
  119.     public String toString() {
  120.         return String.format("[%d - %d]", subjectId, mark);
  121.     }
  122. }
  123.  
Success #stdin #stdout 0.12s 51244KB
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[[1 - [[1 - 25], [2 - 10], [3 - 10]]], [2 - [[1 - 5], [2 - 15], [3 - 10]]]]
https://ideone.com/0i6ZqG
Java 8 , Merge two or more nested list

https://stackoverflow.com/questions/72345314/java-8-merge-two-or-more-nested-list
language:
Java (HotSpot 12)
created:
3 years ago
visibility:
public

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