Java Community | Help. Code. Learn.•3y ago

4 lines of mayhem

    public static void main(String[] args)
    {
        List<Integer> list = new ArrayList<>();
        List<Integer> subList = list.subList(0, 0);
        list.addAll(subList);
        list.addAll(subList);
    }

    public static void main(String[] args)
    {
        List<Integer> list = new ArrayList<>();
        List<Integer> subList = list.subList(0, 0);
        list.addAll(subList);
        list.addAll(subList);
    }

Throws a ConcurrentModificationException in the last line, and I have no clue why.

8 Replies

JavaBot•3y ago

Hey, @Pleezon! Please remember to /close this post once your question has been answered!

PleezonOP•3y ago

    public static void main(String[] args)
    {
        List<Integer> list = new ArrayList<>();
        List<Integer> subList = new ArrayList<>(list.subList(0, 0));
        list.addAll(subList);
        list.addAll(subList);
    }

    public static void main(String[] args)
    {
        List<Integer> list = new ArrayList<>();
        List<Integer> subList = new ArrayList<>(list.subList(0, 0));
        list.addAll(subList);
        list.addAll(subList);
    }

Does not throw an exception

dan1st•3y ago

sublist doesn't return a copy

PleezonOP•3y ago

    public static void main(String[] args)
    {
        List<Integer> list = new ArrayList<>();
        List<Integer> subList = list.subList(0, 0);
        list.addAll(new ArrayList<>(subList));
        list.addAll(new ArrayList<>(subList));
    }

    public static void main(String[] args)
    {
        List<Integer> list = new ArrayList<>();
        List<Integer> subList = list.subList(0, 0);
        list.addAll(new ArrayList<>(subList));
        list.addAll(new ArrayList<>(subList));
    }

throws an exception

dan1st•3y ago

sublist returns a view on the list you cannot iterate and write on the same list at the same time and iterating over the sublist iterates on the original list

PleezonOP•3y ago

but like why does List<Integer> subList = list.subList(0, 0); still iterate whilst list.addAll(new ArrayList<>(subList)); runs

dan1st•3y ago

the problem is iteratimg over the sublist don't use the sublist after the original list changed

PleezonOP•3y ago

like.. this copies the sublist right? this does so too one of them works why don't both work :loldog:

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4 lines of mayhem