CODEWITHSUNDEEP
java-8java-stream
Bick
Bick

Reputation: 18521

Java Stream - group by when key appears in a list

I am trying to group by a collection by a value that appears in my object as a list.

This is the model that I have 

public class Student {
  String stud_id;
  String stud_name;
  List<String> stud_location = new ArrayList<>();

  public Student(String stud_id, String stud_name, String... stud_location) {
      this.stud_id = stud_id;
      this.stud_name = stud_name;
      this.stud_location.addAll(Arrays.asList(stud_location));
  }
}

When I initialize it with the following : 

    List<Student> studlist = new ArrayList<Student>();
    studlist.add(new Student("1726", "John", "New York","California"));
    studlist.add(new Student("4321", "Max", "California"));
    studlist.add(new Student("2234", "Andrew", "Los Angeles","California"));
    studlist.add(new Student("5223", "Michael", "New York"));
    studlist.add(new Student("7765", "Sam", "California"));
    studlist.add(new Student("3442", "Mark", "New York"));

I want to get the following : 

California -> Student(1726),Student(4321),Student(2234),Student(7765)
New York -> Student(1726),Student(5223),Student(3442)
Los Angeles => Student(2234)

I try to write the following 

  Map<Student, List<String>> x = studlist.stream()
            .flatMap(student -> student.getStud_location().stream().map(loc -> new Tuple(loc, student)))
            .collect(Collectors.groupingBy(y->y.getLocation(), mapping(Entry::getValue, toList())));

But I am having trouble completing it - how do I keep the original student after the mapping?

Upvotes: 2

Views: 405

Answers (1)

Hank D
Hank D

Reputation: 6471

Summing up the comments above, the collected wisdom would suggest:

Map<String, List<Student>> x = studlist.stream()
            .flatMap(student -> student.getStud_location().stream().map(loc -> new AbstractMap.SimpleEntry<>(loc, student)))
            .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, toList())));

As an alternate approach, if you don't mind the Students in each list containing only that location, you might consider flattening the Student list to Students with only one location:

Map<String, List<Student>> x = studlist.stream()
        .flatMap( student ->
                student.stud_location.stream().map( loc ->
                        new Student(student.stud_id, student.stud_name, loc))
        ).collect(Collectors.groupingBy( student -> student.stud_location.get(0)));

Upvotes: 1

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