TreeMultiSet clear is not working

I have a TreeMultiSet as a member of the singleton object. When I clear the set using clear(), it doesn’t clear any.

class Campaign {
   private static Campaign singleton;
   private TreeMultiset<Item> items;
  
   public Campaign() {
        items = TreeMultiSet.create();
   }
   public void destroy() {
      items.clear();
   }
   public void setItem(Item item) {
      items.add(item);
   }
   public static Campaign getSingleton() {
     if (singleton == null) {
            synchronized (Campaign.class) {
                if (singleton == null) {
                    singleton = new Campaign();
                }
            }
        }
        return singleton;
   }
}

Item item = new Item();
Campaign.getSingleton().setItem(item);
Campaign.getSingleton().destroy(); //its not clearing the set. no errors either.

[jbduncan]

Hi @rohit5ram, there are two things I can think of which may allow you to improve your code.

1. I’d consider moving your current Item.compareTo implementation into it’s own Ordering comparator, like so:

private static final Ordering<Item> ITEM_BY_PRIORITY_REVERSE_ORDERING =
  new Ordering<Item>() {
    @Override
    public int compare(Item left, Item right) {
      Preconditions.checkNotNull(left, "left");
      Preconditions.checkNotNull(right, "right");
      return Ints.compare(left.priority, right.priority);
    }
  }.reverse();

The advantage of this is that you can implement Item.compareTo separately in terms of all its fields rather than just priority, which is what future readers of your code would expect compareTo to do, I imagine.

1. I’d then consider replacing ArrayList<CampaignFileItem> campaignFileItems = ... /* your unsorted items */; Collections.sort(campaignFileItems); with

ImmutableList<Item> campaignFileItems = ITEM_BY_PRIORITY_REVERSE_ORDERING.immutableSortedCopy(... /* your unsorted items */);

The advantage of this is you end up with an immutable data structure, which is easier to reason about as you can never accidentally change it (assuming Item itself is immutable, i.e. all its fields are final and they are of primitive types and/or object types which themselves cannot be mutated e.g. String).

[lowasser]

To answer your other question, yes, your contract violating comparator is absolutely the reason clear() is not working.