Sort a Set in reverse order

Question

Apologies for the newbie question, but what's the proper way to get a Set (say LinkedHashSet) in reverse order? For Collections there's Collections.reverse(Collection c), but how does one do it for a Set with ordered elements (like a LinkedHashSet)?

Answer 1

Sets are not ordered in general, so to preserve the sorting, after sorting the set as a list, you would need to use a known iteration order implementation of Set, such as LinkedHashSet

List list = new ArrayList(set);
Collections.sort(list, Collections.reverseOrder());
Set resultSet = new LinkedHashSet(list);

You could also use TreeSet with a comparator, but that is not as fast as the ArrayList method above.

Answer 2

public class LargestArray {
public static void main(String[] args) {

    ArrayList<Integer> al = new ArrayList<>();
    Set<Integer> set = new TreeSet<>();
    set.add(10);
    set.add(20);
    set.add(7);
    set.add(4);
    set.add(1);
    set.add(2);
    set.add(3);
    set.add(4);
    System.out.println("after Sorting");
    for(int i : set) {
        System.out.print("  " + i);
    }
    al.addAll(set);
    set.clear();
    Collections.reverse(al);
    System.out.println();
    System.out.println("After Reverse");
    for (int i : al) {
        System.out.print(" " + i);
    }

}

}

output = after Sorting 1 2 3 4 7 10 20 After Reverse 20 10 7 4 3 2 1