Create Map<String, List<Integer>> from String[][] using java 8 streams

Question

I can create with enhanced for loop and with map's computeIfAbsent as below.

String [][] students = {{"David","50"},{"Sherif","70"},{"Bhavya","85"},{"Bhavya","95"}};
Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();
for(String student[] : students) {
    map.computeIfAbsent(student[0], (k)->new ArrayList<Integer>()).add(Integer.parseInt(student[1]));
}

Is there any way I can use stream with collectors api to build map as above?

Map<String, List<Integer>> m = Arrays.stream(students)
        .collect(Collectors.?);

Ravindra Ranwala · Accepted Answer · 2018-04-23 02:33:05Z

1

Try this out.

String[][] students = { { "David", "50" }, { "Sherif", "70" }, { "Bhavya", "85" }, { "Bhavya", "95" } };
Map<String, List<Integer>> studentsByName = Stream.of(students).collect(Collectors.groupingBy(kv -> kv[0],
        Collectors.mapping(kv -> Integer.valueOf(kv[1]), Collectors.toList())));
System.out.println(studentsByName);

edited Apr 23, 2018 at 2:33

answered Apr 23, 2018 at 2:27

Ravindra Ranwala

21.2k7 gold badges50 silver badges65 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Hadi · Accepted Answer · 2018-04-23 04:30:18Z

1

You can try like this

map = Arrays.stream(students)
         .map(array->new Pair<String,Integer>(array[0],Integer.valueOf(array[1])))
         .collect(Collectors.groupingBy(p->p.getKey(), Collectors.mapping(p->p.getValue(),
                                        Collectors.toList())));

edited Apr 23, 2018 at 4:30

answered Apr 23, 2018 at 4:22

Hadi

17.3k4 gold badges41 silver badges63 bronze badges

Comments

Ousmane D. · Accepted Answer · 2018-04-23 20:21:03Z

1

Using groupingBy:

Arrays.stream(students)
      .map(a -> new AbstractMap.SimpleEntry<>(a[0], Integer.valueOf(a[1])))
      .collect(groupingBy(AbstractMap.SimpleEntry::getKey,
                     mapping(AbstractMap.SimpleEntry::getValue,  
                                    toList())));

Using toMap:

Arrays.stream(students)    
      .map(a -> new AbstractMap.SimpleEntry<>(a[0], Integer.valueOf(a[1])))
      .collect(toMap(AbstractMap.SimpleEntry::getKey, 
                    k -> new ArrayList<>(Collections.singletonList(k.getValue())), 
                             (left, right) -> {left.addAll(right);return left;}));

edited Apr 23, 2018 at 20:21

answered Apr 23, 2018 at 7:40

Ousmane D.

56.6k8 gold badges101 silver badges134 bronze badges

Collectives™ on Stack Overflow

Create Map<String, List<Integer>> from String[][] using java 8 streams

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related