Java - 优化比较器操作对 POJO 列表进行排序

Question

I'm writing a Comparator for one of my DTO.

Each DTO object MyDTO contains a Set of Themes .

class MyDTO {
    Long id;
    Set<Theme> themes;
}

class ThemeDTO {
    Long id;
    String title;
}

We've a given list of themeIDs as input, say Set<Long> inputThemes against which we've to make a comparison and sort list of MyDTOs.

Logic for comparison is:

1. If number of matching themes in myDto1 is more than in myDto2, then, myDto1 will come first. (AND will precede OR).
2. If number of matches are same, objects will be arranged alphabetically as per theme titles.

I've written below comparator, but feels like it can be optimised further.

myDTOList.sort(new Comparator<MyDTO>() {
    @Override
    public int compare(MyDTO o1, MyDTO o2) {

        Set<Long> o1ThemeSet = o1.getThemes().stream().map(ThemeDTO::getId).collect(toSet());
        Set<Long> o2ThemeSet = o2.getThemes().stream().map(ThemeDTO::getId).collect(toSet());

        // Filtering w.r.t to themes coming as input
        Set<Long> inputThemeSet = inputThemes.stream().collect(toSet());
        o1ThemeSet.retainAll(inputThemeSet);
        o2ThemeSet.retainAll(inputThemeSet);

        if (o1ThemeSet.size() == o2ThemeSet.size()) {

            // If 2 MyDTO have same number of theme matches, sorting is done alphabetically.
            return o1.getTitle().toLowerCase().compareTo(o2.getTitle().toLowerCase());

        } else if (o1ThemeSet.size() < o2ThemeSet.size()) {
            return 1;
        } else {
            return -1;
        }
    }
});

Answer 1

Create a map for MyDTO list where store MyDTO#id as key and matched themes as value

Set<Long> inputThemeSet = inputThemes.stream().collect(toSet());
Map<Long, Integer> map = new HashMap<>();
myDTOList.forEach(e -> {
    Integer cnt = (int)e.getThemes().stream().filter(k -> inputThemeSet.contains(k.getId())).count();
    map.put(e.getId(), cnt);
});

Then you can sort, here optimization is for same MyDTO you don't need to do upper operation for every two comparison MyDTO object. Number of comparison is huge bigger than list size.

myDTOList.sort(new Comparator<MyDTO>() {
    @Override
    public int compare(MyDTO o1, MyDTO o2) {
        if (map.get(o1.getId()) == map.get(o2.getId())) {
            return o1.getTitle().toLowerCase().compareTo(o2.getTitle().toLowerCase());
        } 
        return (map.get(o1.getId()) < map.get(o2.getId())) ? 1 : -1;
    }
});

Note: The code is untested hope you get the idea