java - Remove elements from HashSet on iteration -

suppose have hashset:

[1, 2, 3, 4, 5, 6] 

i want iterate on in such way, given sum, 6, while iterating on elements, if find 2 elements in set having sum = 6, want remove other one. e. g., if iterating on 1, should remove 5. trying this:

hashset<integer> hs = new hashset(arr); int sum = 6; for(int num : hs) {     if(hs.contains(sum - num)) {         hs.remove(sum - num);     } } 

obviously throws java.util.concurrentmodificationexception. approach use iterator removes current element , not take other element parameter. else can use?

update: know techniques use additional set , all. wanted optimal solution without increasing time , space complexity, if that's possible.

keep running set of numbers have found.

this allow have one-pass solution.

start empty running set, , iterate through set of numbers. each element iterate through, if sum-compliment in set, remove iterator. otherwise, add running set.

hashset<integer> hs = new hashset(arr); hashset<integer> running = new hashset(); int sum = 6; iterator<integer> iter = hs.iterator(); while (iter.hasnext()) {     int num = iter.next();     if (running.contains(sum - num)) {         iter.remove();     } else {         running.add(num);     } } 

this code modify original hashset, , both hashsets contain same contents @ end of code block. in case, might better use running set @ end of code , not modify original. make code far more flexible , reusable.