How to collect to Map> to acheve that List is sorted?

Question

I have code like this:

final TreeMap<String, List<MyBean>> map= elements.stream()
                .filter(....)
                .collect(Collectors.groupingBy(MyBean::getName,
                        TreeMap::new,
                        Collectors.toList()
                ));

How to achieve that List is sorted by someStringField ?

score 2 · Accepted Answer · edited Mar 30 '20 at 14:33

2

You can add a Collectors#collectingAndThen to your Collectors#toList downstream:

final TreeMap<String, List<MyBean>> map = elements.stream()
      .collect(Collectors.groupingBy(MyBean::getName,
              TreeMap::new,
              Collectors.collectingAndThen(
                      Collectors.toList(),
                      l -> {
                          l.sort(Comparator.comparing(MyBean::getSomeStringField));
                          return l;
                      })
      ));

edited Mar 30 '20 at 14:33

gstackoverflow

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answered Mar 30 '20 at 13:44

Flown

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hmmm... it leads to compile error. Could you please check it ? – gstackoverflow Mar 30 '20 at 14:04
Error:(50, 21) java: incompatible types: inference variable RR has incompatible bounds equality constraints: java.util.List,D,V lower bounds: void – gstackoverflow Mar 30 '20 at 14:13
Looks like you missed **return l;** statement. – gstackoverflow Mar 30 '20 at 14:25
@gstackoverflow Thank you for the edit. I haven't tried it out before, it was just written on the fly. – Flown Mar 30 '20 at 15:34

score -1 · Answer 2 · answered Mar 30 '20 at 13:46

-1

I suggest you check the solution proposed in another thread: TreeMap sort by value

It seams you have to pass by SortedSet. TreeMap itself would not be sortable.

answered Mar 30 '20 at 13:46

André Pletschette

182
8

How to collect to Map> to acheve that List is sorted?

2 Answers2