Using iterator on a TreeSet

Question

SITUATION: I have a TreeSet of custom Objects and I have also used a custom Comparator. I have created an iterator to use on this TreeSet.

TreeSet<Custom> ts=new TreeSet<Custom>();
Iterator<Custom> itr=ts.iterator();
while(itr.hasNext()){
    Custom c=itr.next();
    //Code to add a new element to the TreeSet ts
}

QUESTION: Well I want to know that if I add a new element to the TreeSet within the while loop, then will that new element get sorted immediately. In other words, if I add a new element within the while loop and it is less than the one which I am currently holding in c, then in the next iteration will I be getting the same element in c as in the last iteration?(since after sorting, the newly added element will occupy a place somewhere before the current element).

Answer 1

If you add an element during your iteration, your next iterator call will likely throw a ConcurrentModificationException. See the fail-fast behavior in TreeSet docs.

To iterate and add elements, you could copy first to another set:

TreeSet<Custom> ts = ...
TreeSet<Custom> tsWithExtra = new TreeSet(ts);

for (Custom c : ts) {
  // possibly add to tsWithExtra
}

// continue, using tsWithExtra

or create a separate collection to be merged with ts after iteration, as Colin suggests.

Answer 2

You will get a java.util.ConcurrentModificationException if you add an element into the TreeSet inside while loop.

Set<String> ts=new TreeSet<String>();
ts.addAll(Arrays.asList(new String[]{"abb", "abd", "abg"}));
Iterator<String> itr=ts.iterator();
while(itr.hasNext()){
    String s = itr.next();
    System.out.println("s: " + s);
    if (s.equals("abd"))
        ts.add("abc");
}

Output

Exception in thread "main" java.util.ConcurrentModificationException