How to sort an ArrayList of Strings with seasons and years?

Question

I have an unsorted ArrayList of Strings of seasons and years like the following:

["Summer 2020", "Autumn 2018", "Spring 2019", "Autumn 2019", "Winter 2018", "Summer 2018", "Winter 2020", "Spring 2018"]

The correct sorting of this list is the following:

["Winter 2018", "Spring 2018", "Summer 2018", "Autumn 2018", "Spring 2019", "Autumn 2019", "Winter 2020", "Summer 2020"]

How can I achieve this order? The order of seasons in a year is like Winter 2021, Spring 2021, Summer 2021, Autumn 2021, and all the seasons for every year will not necessarily exist in the list.

Answer 1

You'll need to implement a custom Comparator that sorts by the year first and then by the seasons. An enum can be used to specify the order of the seasons easily.

private static class SeasonComparator implements Comparator<String> {
    
    private enum Season {
        WINTER, SPRING, SUMMER, AUTUMN
    }

    @Override
    public int compare(String s1, String s2) {
        String[] arr1 = s1.split(" ");
        String[] arr2 = s2.split(" ");
        
        int year1 = Integer.parseInt(arr1[1]);
        int year2 = Integer.parseInt(arr2[1]);
        
        if (year1 != year2) {
            return year1 - year2;
        } else {
            Season season1 = Season.valueOf(arr1[0].toUpperCase());
            Season season2 = Season.valueOf(arr2[0].toUpperCase());
            return season1.ordinal() - season2.ordinal();
        }
    }
}

public static void main(String[] args) {
    List<String> seasons = new ArrayList<>(Arrays.asList("Summer 2020", "Autumn 2018",
            "Spring 2019", "Autumn 2019", "Winter 2018", "Summer 2018", "Winter 2020", "Spring 2018"));

    System.out.println("Unordered list:");
    System.out.println(seasons);
    
    seasons.sort(new SeasonComparator());
    
    System.out.println("Sorted list:");
    System.out.println(seasons);
}

Output

Unordered list:
[Summer 2020, Autumn 2018, Spring 2019, Autumn 2019, Winter 2018, Summer 2018, Winter 2020, Spring 2018]

Sorted list:
[Winter 2018, Spring 2018, Summer 2018, Autumn 2018, Spring 2019, Autumn 2019, Winter 2020, Summer 2020]