java.util.ConcurrentModificationException But I am not removing

Question

My below recursive function throws a ConcurrentModificationException on the 'continue' statement. I looked at a few posts on ConcurrentModificationException and all of the problems seem to be with removing an element from an element, but I am not removing any element in my function.

My function is below :

    public static void getRootedTreeHelper(Node n, boolean[] marked, String spacing){ 

        System.out.println(spacing + "in helper with " + n.getId());
        marked[n.getId()] = true;
        if(n.children.isEmpty())
            return;
        else{
            for(Node child : n.children){
                if(marked[child.getId()])
                    continue; // ConcurrentModificationException is thrown here. 
                else{
                    n.addChild(child);
                    spacing = spacing + "\t";
                    getRootedTreeHelper(child, marked, spacing);
                }
            }
        }
    }

As requested : The relevant portions of Node Class are shown below

public class Node {

    private int id;
    ArrayList<Node> children;

    public Node(int id) {
        this.id = id;
        children = new ArrayList<Node>();
    }

    /**
     * add node n to this node's children
     * @param n
     */
    public void addChild(Node n) {
        children.add(n);
    }

    // getters and setters()
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
}

Does anyone have any ideas?

Edit solved : Rather than using a for each loop to iterate through all the children, I use a for loop.

Answer 1

From ConcurrentModificationException Javadoc:

Note that this exception does not always indicate that an object has been concurrently modified by a different thread. (...) For
example, if a thread modifies a collection directly while it is
iterating over the collection with a fail-fast iterator, the iterator will throw this exception.

The bug is adding a child into the collection while also iterating over it.

The fault is only detected by the iterator, when it is incremented in the for loop.