How to use Grouping and sorting together in JAVA 8

Question

I am trying to sort a list and then group the elements. Problem is that when I group the elements sorting order is going for a toss. Below is my piece of code

listOfRooms.stream().sorted(Comparator.comparing(DXARoom::getStayAverageRate))
        .collect(Collectors.groupingBy(DXARoom::getRoomInfoLabel));

This is how my output is coming for the above code

{
    "Presidential suite": [
        {
            "roomInfoLabel": "Presidential suite",
            "stayAverageRate": "1696.0",
       },
        {
            "roomInfoLabel": "Presidential suite",
            "stayAverageRate": "1729.0",
        },    
    "Premier king room": [
        {
            "roomInfoLabel": "Premier king room",
            "stayAverageRate": "370.0",
        },
        {
            "roomInfoLabel": "Premier king room",
            "stayAverageRate": "377.0",
        },
}

Basically, my requirement is to show cheapest room type first. So the output should be grouped by Premier king room first as it has the cheapest Room of 370£.

EXPECTED OUTPUT ->

   {
        "Premier king room": [
            {
                "roomInfoLabel": "Premier king room",
                "stayAverageRate": "370.0",
            },
            {
                "roomInfoLabel": "Premier king room",
                "stayAverageRate": "377.0",
            },
           "Presidential suite": [
            {
                "roomInfoLabel": "Presidential suite",
                "stayAverageRate": "1696.0",
           },
            {
                "roomInfoLabel": "Presidential suite",
                "stayAverageRate": "1729.0",
            }  

    }

Can you please suggest me any alternative method or whats wrong with above implementation, any new ideas are welcome.

https://stackoverflow.com/a/28765069/2711488 – Holger Oct 06 '17 at 12:36 — Holger, Oct 06 '17 at 12:36

Abhishek Galoda · Answer 1 · 2017-10-06T16:34:31.543

11

This solution worked for me

        .sorted(Comparator.comparing(DXARoom::getStayAverageRate))
        .collect(Collectors.groupingBy(DXARoom::getRateInfoLabel, LinkedHashMap::new, Collectors.toList()));

Basically first,

We need to sort the whole list
Group list in a new LinkedHashMap as it preserves the insertion order, which is the key

Thanks @Holger

edited Oct 06 '17 at 16:34

answered Oct 06 '17 at 14:42

Abhishek Galoda

2,753
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Eugene · Answer 2 · 2017-10-06T13:39:24.550

2

Seems like you are looking for this:

Collectors.groupingBy(
        DXARoom::getRoomInfoLabel,
        LinkedHashMap::new,
        Collectors.toList());

edited Oct 06 '17 at 13:39

answered Oct 06 '17 at 12:18

Eugene

117,005
15
201
306

I suppose this preserves the previous order. But how does it sort? – Arun Gowda Jul 08 '22 at 11:42

How to use Grouping and sorting together in JAVA 8

2 Answers2