Convert List<A> to Map<B,List<A>> - collections

Question

class Student{
String name;
String grade;
String vocationalSubject;
}
class Vocational{
String grade;
String vocationalSubject;
}
I want to convert List<Student> to Map<Vocational,List<Student>> using java 8 lambdas.
If I have Vocational as a member of Student, then I can do like :
students.stream().collect(Collectors.groupingBy(Student::getVocational))
As I do not have Vocational as a member of Student, I need to create it using Vocational::new But I am not sure how to set those two fields(grade and vocationalSubject) from Student in Vocational.
Trying to Do it using Java 8 Lambdas

Answer 1

Assuming you have implemented equals and hashCode methods correctly, then you can create Vocational objects in the groupingBy function.
Map<Vocational, List<Student>> mapping = students.stream()
.collect(
Collectors.groupingBy(s -> new Vocational(s.getGrade(), s.getVocationalSubject()))
);

Answer 2

May not be perfect but this should be close to what you are looking for. This will work even if you have not overridden "equals" method.
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
class Student
{
String name;
String grade;
String vocationalSubject;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getGrade() {
return grade;
}
public void setGrade(String grade) {
this.grade = grade;
}
public String getVocationalSubject() {
return vocationalSubject;
}
public void setVocationalSubject(String vocationalSubject) {
this.vocationalSubject = vocationalSubject;
}
Student(String name,String grade,String vocSub)
{
this.name=name;
this.grade=grade;
this.vocationalSubject=vocSub;
}
#Override
public String toString() {
return "Student [name=" + name + ", grade=" + grade + ", vocationalSubject=" + vocationalSubject + "]";
}
}
class Vocational
{
String grade;
String vocationalSubject;
public Vocational(String grade, String vocationalSubject) {
this.grade = grade;
this.vocationalSubject = vocationalSubject;
}
#Override
public String toString() {
return "Vocational [grade=" + grade + ", vocationalSubject=" + vocationalSubject + "]";
}
}
public class Temp
{
public static void main(String args[]){
Map<Vocational,List<Student>> map=new HashMap<Vocational,List<Student>>();
List<Student> listStu=new ArrayList<Student>();
listStu.add(new Student("Student1","100","aa"));
listStu.add(new Student("Student1","100","bb"));
listStu.add(new Student("Student2","50","aa"));
listStu.add(new Student("Student2","50","bb"));
listStu.add(new Student("Student3","100","aa"));
listStu.add(new Student("Student3","50","bb"));
listStu.add(new Student("Student4","50","aa"));
listStu.add(new Student("Student4","100","bb"));
listStu.stream().collect(Collectors.groupingBy(Student::getGrade,Collectors.groupingBy(Student::getVocationalSubject)))
.forEach((k,v)->v.forEach((x,y)->map.put(new Vocational(k, x), y)));
map.forEach((x,y)->System.out.println(x+":"+y));
}
}