Question

The old value/reference things. Im getting ConcurrentModificationException
for this adaptation of the Bron-Kerbosch.

public int[] bk(ArrayList<Integer> R, ArrayList<Integer> P, ArrayList<Integer> X) {
    int count[] = new int[n];
    int u=0, c = 0;

    ArrayList<Integer> tempPX = new ArrayList<Integer>();
    ArrayList<Integer> newP = P;
    ArrayList<Integer> newX = X;
    ArrayList<Integer> newR = R;

    if (P.isEmpty() && X.isEmpty()) {
        count[R.size()]++;
    } else {

        u = 0; c = 0; // find vertex with largest degree            
        tempPX.addAll(P); tempPX.addAll(X); // P ⋃ X
        for (Integer v : tempPX) {            
            if (neighbours[v].size() > neighbours[u].size()) {
                u = c; 
            }
            c++;
        } 

        P.removeAll(neighbours[u]); // P \ neighbours[u]
        for (Integer v : newP) {

            newR.add(v); // R ⋃ v

            newP.retainAll(neighbours[v]); // P â‹‚ neighbours[v]

            newX.retainAll(neighbours[v]); // X â‹‚ neighbours[v]

            bk(newR, newP, newX); 

            P.remove(v); // removing object
            X.add(v); // X ⋃ v
        }

    }

    return count;
}

The exception occurs at line for (Integer v : newP), and the recursive call in there.
I need to P.removeAll(neighbours[u]); then loop over that resulting list, inside doing the things in the comments, AND PASS COPIES in the recursive call so it wont complain and work not pass the references and keep modifying the same object P/X/R. So how and WHEN do i copy them?? Those first lines.. I’m making copies of the references aren’t i…
(yes i know i “modify” newP then loop over the old P, they just point to the same object it seems)

— new code after reading the replies –

   public int[] bk(List<Integer> r, List<Integer> p, List<Integer> x) {
    int count[] = new int[n];
    int u = 1;

    List<Integer> tempPX = new ArrayList<Integer>();
    List<Integer> newR, newP, newX;

    if (p.isEmpty() && x.isEmpty()) {
        count[r.size()]++;
    } else {

        // find vertex with largest degree in P U X        
        tempPX.addAll(p); 
        tempPX.addAll(x);
        for (Integer v : tempPX) {            
            if (neighbours[v].size() > neighbours[u].size()) {
                u = v; 
            }
        } 

        p.removeAll(neighbours[u]);  // P \ neighbours[u]
        newP = new ArrayList<Integer>(p); 
        for (Integer v : newP) {

            r.add(v); // R U  v
            newR = new ArrayList<Integer>(r);

            p.retainAll(neighbours[v]);  // P /\ neighbours[v]
            newP = new ArrayList<Integer>(p);

            x.retainAll(neighbours[v]); // X /\ neighbours[v]
            newX = new ArrayList<Integer>(x);

            bk(newR, newP, newX); 

            p.remove(v); // removing object
            x.add(v); // X U v
        }

    }

    return count;
}

Answer 1

As you’ve identified, you’re copying the reference, not the list. You need to instantiate a new ArrayList object with the entries in the old list.

e.g.

List<Integer> newList = new ArrayList<Integer>(oldList);

So this explicitly creates a new object containing references to the same elements.

Note as an aside that I pass around a reference to the interface List rather than the implementation – generally good practise since you’re not exposing implementation throughout the codebase.