Remove Elements from a HashSet while Iterating

hashmap, hashset, iteration, java

Solution

You can manually iterate over the elements of the set:

Iterator<Integer> iterator = set.iterator();
while (iterator.hasNext()) {
    Integer element = iterator.next();
    if (element % 2 == 0) {
        iterator.remove();
    }
}

You will often see this pattern using a `for` loop rather than a `while` loop:

for (Iterator<Integer> i = set.iterator(); i.hasNext();) {
    Integer element = i.next();
    if (element % 2 == 0) {
        i.remove();
    }
}

As people have pointed out, using a `for` loop is preferred because it keeps the iterator variable (`i` in this case) confined to a smaller scope.

Problem

So, if I try to remove elements from a Java HashSet while iterating, I get a ConcurrentModificationException. What is the best way to remove a subset of the elements from a HashSet as in the following example? ``` Set<Integer> set = new HashSet<Integer>(); for(int i = 0; i < 10; i++) set.add(i); // Throws ConcurrentModificationException for(Integer element : set) if(element % 2 == 0) set.remove(element); ``` Here is a solution, but I don't think it's very elegant: ``` Set<Integer> set = new HashSet<Integer>(); Collection<Integer> removeCandidates = new LinkedList<Integer>(); for(int i = 0; i < 10; i++) set.add(i); for(Integer element : set) if(element % 2 == 0) removeCandidates.add(element); set.removeAll(removeCandidates); ``` Thanks!

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