How can I sort a list in java by another list of ids

Question

I have a list of objects in java that looks something like this:

List<Video> videos = new ArrayList<Video>();

and my video object looks like this:

public class Video {
    private String nameId;
    private Integer id;
    ...
}

I have another list that just has my nameId strings:

List<String> nameIdList = ArrayList<String>();

How can I write a compareto method that doesn't sort my list of videos by comparing each video and instead uses another list? My approach so far has been to use a simple bubble sort but that will be very inefficient as my list gets bigger

Example:

I could have video objects with nameIds : "apple", "bannana", "orange"

and my list of strings to sort them by could be: "bannana", "apple", "orange"

so my videos I would want to return to my client should be in the order of: "bannana", "apple", "orange"

Could you post a sample input / output? I'm not sure I understand the question. — Tunaki, Mar 24 '16 at 21:50
Are you defining an order based on "another list", e.g. {"A New Hope", "The Empire Strikes Back", "Return of the Jedi"}? — rgettman, Mar 24 '16 at 21:51
I've added an example to my question. @rgettman yes I'm defining the order based on another list. sorry for the confusion — DanielD, Mar 24 '16 at 21:54
What happens when you encounter "pear" which isn't in the sort order definition? — rgettman, Mar 24 '16 at 21:56
if "pear" was in my list of strings, I would want to just skip that since there doesn't exist a video with a nameId of "pear" — DanielD, Mar 24 '16 at 22:00

score 30 · Answer 1 · answered Aug 15 '17 at 18:32

30

Java 8:

videos.sort(Comparator.comparing(v->nameIdList.indexOf(v.getNameId())));

this is smaller than this

answered Aug 15 '17 at 18:32

Tokala Sai Teja

650
8
9

score 5 · Answer 2 · answered Sep 13 '17 at 19:54

5

A simple and clean solution, which surprisingly has not been mentioned:

videos.sort(Ordering.explicit(nameIdList).onResultOf(Video::getNameId));

Ordering.explicit is part of Guava.

answered Sep 13 '17 at 19:54

amurka

645
1
6
14

score 2 · Answer 3 · answered Mar 24 '16 at 21:55

You could use a custom Comparator

List<String> nameIdList = new ArrayList<>();
Comparator<Video> compare = new Comparator<Video>() {
     public int compare(Video v1, Video v2) {
         return nameIdList.indexOf(v1.getNameId()) - nameIdList.indexOf(v2.getNameId());
     }
};

To make this more efficient you could have a Map<String, Integer> so you can look up the desired order more efficiently.

score 2 · Answer 4 · edited May 23 '17 at 11:47

Unless the lists involved are very long, I would use this solution, but here is an alternative solution that has time complexity O(n). Any solution using a Comparator together with Collections.sort or Arrays.sort has time complexity no better than O(n log n).

The method sorts the list videos based on the other list, and clears away any Video whose nameId is not in the list.

public static void sort(List<Video> videos, List<String> list) {
    Map<String, List<Video>> map = new HashMap<>();
    for (String string : list)
        map.put(string, new ArrayList<>());
    for (Video video : videos) {
        List<Video> value = map.get(video.getNameId());
        if (value != null)
            value.add(video);
    }
    videos.clear();
    for (String string : list)
        for (Video video : map.get(string))
            videos.add(video);
}

Above code works only if `List list` is sorted. – Arpit Aggarwal Feb 22 '17 at 18:42 — Arpit Aggarwal, Feb 22 '17 at 18:42

score 2 · Answer 5 · answered Jan 17 '17 at 09:12

Java 8. Convert to map and pick by hash:

Map<String, Video> videoMap = videos.stream()
         .collect(Collectors.toMap(Video::getNameId, v -> v));

return videos.stream()
         .map(v -> videoMap.get(v))
         .collect(Collectors.toList());

MA-Dev · Answer 6 · 2021-07-29T13:32:42.643

1

You can use Java 8 stream

List<Video> orderedList=
              nameIdList.stream()
                  .map(
                      s ->
                          videos.stream()
                              .filter(v -> v.get_id().equals(s))
                              .findFirst()
                              .orElse(null))
                  .collect(Collectors.toList());

edited Jul 29 '21 at 13:32

answered Jul 29 '21 at 13:27

MA-Dev

101
2
11

How can I sort a list in java by another list of ids

6 Answers6

Linked