Sort a list in reverse order and remove duplicates

Question

I am trying to sort a list in reverse order and then removing duplicates. I am using the following code.

 public static void main(String[] args) {
     List<Integer> list = new ArrayList<>();
     list.add(3);
     list.add(3);
     list.add(2);
     list.add(3);
     list.add(2);
     Collections.sort(list, Collections.reverseOrder());
     System.out.println(list);
     Set<Integer> set = new HashSet<>(list);
     System.out.println(set);
 }

It works fine but the order is again changed when converted to a Set. What to do to preserve the order

HashSet does **not** preserve insertion order. It is that simple. — GhostCat, May 10 '17 at 14:39

score 2 · Answer 1 · answered May 10 '17 at 14:37

Use a LinkedHashSet instead of a HashSet.

Set<Integer> set = new LinkedHashSet<>(list);

Unlike normal sets which have no guarantee of order, LinkedHashSets enforce insertion order. Since you've already sorted the list, you don't require any of the extra benefits which a TreeSet would provide (which orders based on natural ordering or an ordering of your own choosing).

score 1 · Answer 2 · answered May 10 '17 at 14:37

1

Instead of HashSet use TreeSet , the access time is less O(log(n)) instead of O(1) but it keeps the order of the keys

https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html

answered May 10 '17 at 14:37

Amer Qarabsa

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Sort a list in reverse order and remove duplicates

2 Answers2