Java API Streams collecting stream in Map where value is a TreeSet

Question

There is a Student class which has name, surname, age fields and getters for them.

Given a stream of Student objects.

How to invoke a collect method such that it will return Map where keys are age of Student and values are TreeSet which contain surname of students with such age.

I wanted to use Collectors.toMap(), but got stuck.

I thought I could do like this and pass the third parameter to toMap method:

stream().collect(Collectors.toMap(Student::getAge, Student::getSurname, new TreeSet<String>()))`.

score 8 · Accepted Answer · answered Feb 04 '18 at 14:50

8

students.stream()
        .collect(Collectors.groupingBy(
                Student::getAge,
                Collectors.mapping(
                      Student::getSurname, 
                      Collectors.toCollection(TreeSet::new))           
))

answered Feb 04 '18 at 14:50

Eugene

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Ousmane D. · Answer 2 · 2018-02-04T15:41:00.603

1

Eugene has provided the best solution to what you want as it's the perfect job for the groupingBy collector.

Another solution using the toMap collector would be:

 Map<Integer, TreeSet<String>> collect = 
        students.stream()
                .collect(Collectors.toMap(Student::getAge,
                        s -> new TreeSet<>(Arrays.asList(s.getSurname())),
                        (l, l1) -> {
                            l.addAll(l1);
                            return l;
                        }));

edited Feb 04 '18 at 15:41

answered Feb 04 '18 at 15:31

Ousmane D.

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For sure, this solution works. However mutability would cause a thread safety issue during parallelism ? – zikzakjack Feb 04 '18 at 23:31
@zikzakjack nope, this is perfectly thread safe, each thread get its own container that are later merged. – Eugene Feb 05 '18 at 05:18
1

I’d replace `Arrays.asList` with `Collections.singleton`… – Holger Feb 05 '18 at 10:19

Java API Streams collecting stream in Map where value is a TreeSet

2 Answers2