How to sort an ArrayList of ArrayLists (by one of the variables) in Java?

Question

(Java 8) I have an ArrayList of ArrayLists as follows:

[5, 10]

[2, 11]

[1, 12]

[8, 13]

I want these lists to be sorted by the first value in the list in either ascending or descending order. For example, with ascending order, the ArrayList would be ordered as:

[1, 12]

[2, 11]

[5, 10]

[8, 13]

How can I do this?

I am struggling with using the comparator class. Other documentation I have seen refers to when this data is represented by an array, (Arrays.sort then defining a comparator in the argument), but when it is a List of ArrayLists I cannot figure out the solution.

Answer 1

With Stream API (Java 8):

public static void main(String[] args) {
    List<List<Integer>> list = new ArrayList<>();

    list.add(asList(5, 10));
    list.add(asList(2, 11));
    list.add(asList(1, 12));
    list.add(asList(8, 15));

    System.out.println("sorted asc = " + list.stream()
            .sorted(Comparator.comparing(o -> o.get(0)))
            .collect(Collectors.toList()));

    System.out.println("sorted desc = " +  list.stream()
            .sorted((i, j) -> -i.get(0).compareTo(j.get(0)))
            .collect(Collectors.toList()));
}

private static List<Integer> asList(Integer... arr) {
    return Arrays.asList(arr);
}

sorted asc = [[1, 12], [2, 11], [5, 10], [8, 15]]

sorted desc = [[8, 15], [5, 10], [2, 11], [1, 12]]