maintaining TreeSet sort as object changes value

I've got a object that defines a 'natural sort order' using Comparable<>. These are being stored in TreeSets.

Other than removing and re-adding the object, is there another way to update the sort when the members that are used to define the sort order are updated?

As others have noted, there is no in-built way. But you can always subclass that TreeSet, with your constructor(s) of choice, and add in the required functionality:

public class UpdateableTreeSet<T extends Updateable> extends TreeSet<T> {

    // definition of updateable
    interface Updateable{ void update(Object value); }

    // constructors here
    ...

    // 'update' method; returns false if removal fails or duplicate after update
    public boolean update(T e, Object value) {
       if (remove(e)) {
           e.update(value);
           return add(e);
       } else { 
           return false;
       }
    }
}

From then on, you will have to call ((UpdateableTreeSet)mySet).update(anElement, aValue) to update the sorting value and the sorting itself. This does require you to implement an additional update() method in your data object.

I had a similar problem, found this thread and tucuxi's answer (thanks!) based on which I implemented my own UpdateableTreeSet. My version provides means to

• iterate over such a set,
• schedule (deferred) element updates/removals from within the loop
• without having to create a temporary copy of the set and finally
• do all the updates/removals as a bulk operation after the loop has ended.

UpdateableTreeSet hides a lot of the complexity from the user. In addition to deferred bulk updates/removals, single-element update/removal as shown by tucuxi still remains available in the class.

Update 2012-08-07: The class is available in a little GitHub repository including an introductory README with schematic sample code as well as unit tests showing how (not) to use it in more detail.

If you really need to use a Set, then you're out of luck, I think.

I'm going to throw in a wildcard, though - if your situation is flexible enough to work with a List instead of a Set, then you can use Collections.sort() to re-sort the List on demand. This should be performant, if the List order doesn't have to be changed much.