Java - Sorting List by child list size

Question

I'm trying to order the list below of workers by the quantity of this days . But I can't get the right way to do that. I can order them by ID for example on this way:

workers.stream().sorted(Comparator.comparingInt(Worker::getId)).collect(Collectors.toList());

But I can't find the way to use the Worker.getDays().size() to order them....

    "workers": [
        {
          "id": 1,
          "days": ["Monday", "Wednesday", "Friday"]
        },
        {
          "id": 2,
          "days": ["Tuesday", "Thursday"]
        },
        {
          "id": 3,
          "days": ["Monday", "Tuesday", "Friday"]
        },
        {
          "id": 4,
          "days": ["Thursday"]
        },   
]

Hope I could find a nice solution, thanks in advance :)

Could you provide a Minimal, Complete, and Verifiable example? — lealceldeiro
– lealceldeiro, Commented Oct 2, 2018 at 21:35

LuCio · Accepted Answer · 2018-10-04 08:23:45Z

3

Just use a lambda-expression to get the days of a worker and the size..

  List<Worker> workersSortedByDaysSize = workers.stream()
    .sorted(Comparator.comparingInt(w -> w.getDays().size()))
    .collect(Collectors.toList());

This assumes days is not null.

edited Oct 4, 2018 at 8:23

answered Oct 2, 2018 at 21:37

LuCio

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2 Comments

GabriMagic Over a year ago

It works perfectly, I didn't how to build the lambda, thanks a lot

LuCio Over a year ago

You're welcome. I hope the linked documentation will help you to understand how to write lambda-expressions.

superz600 · Accepted Answer · 2018-10-02 21:42:38Z

0

Try:

List<Worker> sorted = workers
    .stream()
    .sorted(Comparator.comparingInt(w -> w.getDays().size()))
    .collect(Collectors.toList());

answered Oct 2, 2018 at 21:42

superz600

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Comments

mavriksc · Accepted Answer · 2018-10-02 21:42:50Z

0

workers.stream().sorted((w1, w2) -> w1.getDays().size() - w2.getDays().size()).collect(Collectors.toList());
or do w2-w1 to reverse the list

answered Oct 2, 2018 at 21:42

mavriksc

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