Iterating over hashset and removing elements from it

Question

I have a while(whiteSet.size()>0) check in my code and inside the while block I call dfs method. The method modifies the HashSet. Why does the while loop never gives an error?

public class CycleInDG {
    Set<Vertex> whiteSet = new HashSet<>();
    Set<Vertex> greySet = new HashSet<>();
    Set<Vertex> blackSet = new HashSet<>();
    List<Integer> cyclePath = new ArrayList<>();
    Map<Vertex, Vertex> parentMap = new HashMap<>();
    Graph g = new Graph();

public boolean hasCycle() {
    Map<Integer, Vertex> map = g.getGraphMap();
    Collection<Vertex> vertices = map.values();
    for (Vertex vertex : vertices) {
        whiteSet.add(vertex);
    }
    while(whiteSet.size()>0) {
        parentMap.clear();
        if(dfs(whiteSet.iterator().next(), null)) {
            return true;
        }
    }
    return false;
}
public boolean dfs(Vertex current, Vertex parent) {
    moveVertex(current, whiteSet, greySet);
    for (Vertex vertex : current.getNeighbors()) {
        if(blackSet.contains(vertex)) {
            continue;
        }
        if(greySet.contains(vertex)) {
            parentMap.put(vertex, current);
            return true;
        }
        parentMap.put(vertex, current);
        boolean result = dfs(vertex, current);
        if(result) {
            return result;
        }
    }
    moveVertex(current, greySet, blackSet);
    return false;
}

public void moveVertex(Vertex v, Set<Vertex> src, Set<Vertex> dest) {
    src.remove(v);
    dest.add(v);
}
}

CME is thrown if you modify a collection while iterating using an Iterator object. There's no way for a collection to see `while(whiteSet.size()>0)`. — shmosel, Jul 10 '17 at 00:51
and I have `if(dfs(whiteSet.iterator().next(), null)) {` line inside the while loop. Am I not doing that? — curiousengineer, Jul 10 '17 at 00:51
Why? You don't keep the iterator around. CME only happens if you keep using the same iterator after the modification. — Louis Wasserman, Jul 10 '17 at 00:52
suppose you should look at this [post](https://stackoverflow.com/questions/18448671/how-to-avoid-concurrentmodificationexception-while-removing-elements-from-arr) — Rajith Pemabandu, Jul 10 '17 at 00:52
You're discarding that iterator as soon as you call `next()`. You would have to use the same `Iterator` object before and after modifying to get an exception. — shmosel, Jul 10 '17 at 00:52
So if did `Iterator it = whiteSet.iterator()` and used `"it"` for the loop and modified, I would have gotten the exception is what I understand? — curiousengineer, Jul 10 '17 at 00:54

Iterating over hashset and removing elements from it

0 Answers0