How to sort by a field of class with its own comparator?

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I have a sample class, say Country:

class Country {
    public String name;
    public int population;
    public Flag flag;  
    ...                                                                        
}

I have this Flag class defined somewhere else

class Flag {
    String id;
    int pixel;
    ...
}

Now I create a separate comparator DefaultFlagRankingComparator() that can sort Flag by id. How can I sort a list of Country by Flag id, using this DefaultFlagRankingComparator()?

comparator object list sorting java
2 Answers

You can invoke the compare method of the Comparator with the flag field of each country.

DefaultFlagRankingComparator flagComparator =
        new DefaultFlagRankingComparator();
Collections.sort(countries, (a, b) ->
        flagComparator.compare(a.getFlag(), b.getFlag()));

You could also use Comparator.comparing to create a Comparator using a key extracting function and a Comparator that compares those keys (as suggested by Louis Wasserman).

DefaultFlagRankingComparator flagComparator =
        new DefaultFlagRankingComparator();
Collections.sort(countries,
        Comparator.comparing(Country::getFlag, flagComparator));
user_95131840

  1. You can compare String values of ids using predefined methods:

    Arrays.sort(countries, (c1, c2) -> Objects.compare(
        c1.getFlag().getId(), c2.getFlag().getId(),
        Comparator.naturalOrder()));

    Arrays.sort(countries, Comparator.comparing(country ->
        country.getFlag().getId(), Comparator.naturalOrder()));

  2. You can specify a custom Comparator:

    Arrays.sort(countries, Comparator
        .comparing(country -> country.getFlag(),
                new DefaultFlagRankingComparator()));

    static class DefaultFlagRankingComparator implements Comparator<Flag> {
    @Override
    public int compare(Flag o1, Flag o2) {
        return Objects.compare(o1.getId(), o2.getId(),
                Comparator.naturalOrder());
    }
}

  3. You can implement interface Comparable in the Flag class:

    Arrays.sort(countries, Comparator.comparing(Country::getFlag));

    static class Flag implements Comparable<Flag> {
    String id;

    public String getId() { return id; }

    @Override
    public int compareTo(Flag that) {
        return Objects.compare(this.getId(), that.getId(),
                Comparator.naturalOrder());
    }
}
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