java 8 groupingBy - java-8

Question

I have a List of Student and each Student may register for a couple of subjects.
Therefore each Student will have a List of Subject. I would like to do a groupingBy on Subject using java 8 features.
I am not able to figure out a way. Any help will be appreciated.

Answer 1

This is just an example where groupBy is used. You can find many other examples on web. [This example is taken from here]
Group items by price: Collectors.groupingBy and Collectors.mapping example
public static void main(String[] args) {
//3 apple, 2 banana, others 1
List<Item> items = Arrays.asList(
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 20, new BigDecimal("19.99")),
new Item("orang", 10, new BigDecimal("29.99")),
new Item("watermelon", 10, new BigDecimal("29.99")),
new Item("papaya", 20, new BigDecimal("9.99")),
new Item("apple", 10, new BigDecimal("9.99")),
new Item("banana", 10, new BigDecimal("19.99")),
new Item("apple", 20, new BigDecimal("9.99"))
);
//group by price
Map<BigDecimal, List<Item>> groupByPriceMap =
items.stream().collect(Collectors.groupingBy(Item::getPrice));
System.out.println(groupByPriceMap);
// group by price, uses 'mapping' to convert List<Item> to Set<String>
Map<BigDecimal, Set<String>> result =
items.stream().collect(
Collectors.groupingBy(Item::getPrice,
Collectors.mapping(Item::getName, Collectors.toSet())
)
);
System.out.println(result);
}
Output
{
19.99=[
Item{name='banana', qty=20, price=19.99},
Item{name='banana', qty=10, price=19.99}
],
29.99=[
Item{name='orang', qty=10, price=29.99},
Item{name='watermelon', qty=10, price=29.99}
],
9.99=[
Item{name='apple', qty=10, price=9.99},
Item{name='papaya', qty=20, price=9.99},
Item{name='apple', qty=10, price=9.99},
Item{name='apple', qty=20, price=9.99}
]
}
//group by + mapping to Set
{
19.99=[banana],
29.99=[orang, watermelon],
9.99=[papaya, apple]
}

Answer 2

One of my friend suggested this solution and it worked fine
package grpBy;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
class Pair {
Subject sub1;
Student student;
public Pair(Student student, Subject sub1 ) {
this.sub1 = sub1;
this.student = student;
}
public String getSub1() {
return sub1.name;
}
public String getStudent() {
return student.name;
}
static Pair of(Student stu, Subject sub) {
return new Pair( stu, sub);
}
}
public class Test2 {
public static void main(String[] args) {
Subject maths = new Subject("maths", 1);
Subject chemi = new Subject("chemi", 1);
Subject phy = new Subject("phy", 1);
Subject bio = new Subject("bio", 1);
List<Subject> s1 = new ArrayList<>();
s1.add(maths);
s1.add(chemi);
List<Subject> s2 = new ArrayList<>();
s2.add(maths);
s2.add(phy);
List<Subject> s3 = new ArrayList<>();
s3.add(bio);
s3.add(phy);
Student jack = new Student(1, "jack", s1);
Student jil = new Student(2, "jil", s2);
Student john = new Student(3, "john", s3);
List<Student> students = new ArrayList();
students.add(jack);
students.add(jil);
students.add(john);
Map<String, List<String>> m = students.stream().
flatMap(student -> student.subjects.stream().map(subject -> Pair.of(student, subject))).
collect(Collectors.groupingBy(e -> e.getSub1(),
Collectors.mapping(e -> e.getStudent(),
Collectors.toList())));
System.out.println(m);
}
}