删除arraylists的arraylist中的索引

Question

I am trying to remove an arraylist within a list of arraylists but it does not seem to be working. In context of my code, I have multiple routes which are in an arraylist. Each route is a arraylist of places.

public static void removeBadRoutes(ArrayList<ArrayList<Place>> routes, Expedition e) { 
    for(int i = 0; i < routes.size(); i++) {
        if(!isGoodRoute(routes.get(i), e)) {
            routes.remove(routes.get(i));
        }
    }
}

I also tried routes.remove(i) which didnt do anything. Edit: By "not working" I mean that nothing is being removed, it still displays routes which should have been removed based on my condition.

Answer 1

To remove elements from a Collection while iterating over it, you should use its Iterator ; otherwise, you'll end up running into a ConcurrentModificationException (which I expect is what you're referring to by "not working"):

public static void removeBadRoutes(ArrayList<ArrayList<Place>> routes, Expedition e) { 
    for (Iterator<ArrayList<Place>> it = routes.iterator(); it.hasNext();) {
        if (!isGoodRoute(it.next(), e)) {
            it.remove();
        }
    }
}

Also, with Java 8, you can use List#removeIf :

public static void removeBadRoutes(ArrayList<ArrayList<Place>> routes, Expedition e) {
    routes.removeIf(route -> !isGoodRoute(route, e));
}