如何对列表中两个不同类的字段进行排序？

Question

I have the below code where I need to sort the list based on fields from different classes.

Is it possible to sort data on fields from different classes and make it into one list?

import java.util.*;
import java.util.stream.Collectors;

public class Sample4 {

    private class Student {
        String name;
        Set<Marks> marks;

    }

    private class Marks {
        double points;
    }

    public static void main(String[] args) {
        System.out.println(new Sample4().test());
    }

    public List<Marks> test() {
        Set<Marks> marks = new LinkedHashSet<>(), marks1 = new LinkedHashSet<>(), marks2 = new LinkedHashSet<>();
        marks.add(new Marks(10.2));
        marks.add(new Marks(2.10));
        marks1.add(new Marks(20));
        marks1.add(new Marks(7));
        marks2.add(new Marks(13));
        marks2.add(new Marks(15));

        List<Student> stds = Arrays.asList(
                new Student("A", marks),
                new Student("C", marks1),
                new Student("B", marks2)
        );

        return stds.stream().sorted(Comparator.comparing(Student::getName)).flatMap(std -> std.getMarks().stream()).sorted(Comparator.comparing(Marks::getPoints)).collect(Collectors.toList());
    }
}

Please find the code above for reference. But the output I'm looking for is as below:

A -> 2.10, 10.2

B -> 13, 15

C -> 7, 20

Answer 1

If you want an output consisted of a link between a student name A and the marks such as 2.10 and 10.2 , then you need either the Map<String, List<Mark>> output or use the current List<Student that is just sorted as you need:

The output is List<Student> :

return stds.stream()
  .sorted(Comparator.comparing(Student::getName))              // sort Students by name
  .map(student -> new Student(                                 // with sorted marks
     student.getName(),                                        // - same name
     student.getMarks().stream()                               // - sorted marks
       .sorted(Comparator.comparingDouble(Marks::getPoints))   // -- comparing points
       .collect(Collectors.toCollection(LinkedHashSet::new)))) // -- as LinkedHashSet
  .collect(Collectors.toList());                               // as List<Student>

[Student(name=A, marks=[Marks(points=2.1), Marks(points=10.2)]),

Student(name=B, marks=[Fooarks(points=13.0), Marks(points=15.0)]),

Student(name=C, marks=[Marks(points=7.0), Marks(points=20.0)])]

---

If you find Map<String, List<Mark>> somehow more appropriate, then mind the TreeMap implementation that sorts the keys:

return stds.stream()
    .collect(Collectors.toMap(                           // Map<String, List>
         Student::getName,                               // the key
         student -> student.getMarks().stream()          // the value (sorted)
             .sorted(Comparator.comparingDouble(
                     Marks::getPoints))
             .collect(Collectors.toList()),
         (o1, o2) -> o1,                                 // merge BinaryOperator
         TreeMap::new));                                 // requested implementation

{A=[Foo.Marks(points=2.1), Foo.Marks(points=10.2)],

B=[Foo.Marks(points=13.0), Foo.Marks(points=15.0)],

C=[Foo.Marks(points=7.0), Foo.Marks(points=20.0)]}