How I can add objects into an ArrayList when I'm using this ArrayList in a for loop?

Question

I need to add some objects to an ArrayList that I am using in a for loop. This is the code:

        List<Bulto> bultosHijos = bultoDAO.findChilds(idBulto);
        List<Bulto> bultosMasProfundos = new ArrayList();

        for (Bulto bulto : bultosHijos) {
            List<Bulto> bultosNietos = bultoDAO.findChilds(bulto.getIdBulto());

            if (!bultosNietos.isEmpty()) {
                bultosHijos.addAll(bultosHijos.size(), bultosNietos);
            } else {
                bultosMasProfundos.add(bulto);
            }
        }

This is throwing me an "Current Modification Exception". I try changing the type that return my DAO, but I can not. How can I avoid this error and do this?

Thank's a lot

EDIT 1:

Thanks for all your replies! I am using now ListIterator and with the code below, works well! But if I do not use listIterator.previous() the while loop exits immediatly and I do not want this. Is ok?

        List<Bulto> bultosHijos = bultoDAO.findChilds(idBulto);
        List<Bulto> bultosMasProfundos = new ArrayList();
        ListIterator<Bulto> listIterator = bultosHijos.listIterator();

        while (listIterator.hasNext()) {
            Bulto bulto = listIterator.next();
            List<Bulto> bultosNietos = bultoDAO.findChilds(bulto.getIdBulto());

            if (!bultosNietos.isEmpty()) {
                for (Bulto bultoNieto : bultosNietos) {
                    listIterator.add(bultoNieto);
                    listIterator.previous();
                }
            } else {
                bultosMasProfundos.add(bulto);
            }
        }

You can use ListIterator to iterate. stackoverflow.com/questions/7409740/… — Niraj Chapla
– Niraj Chapla, Commented Dec 24, 2014 at 10:10

Braj · Accepted Answer · 2014-12-24 10:04:16Z

3

Use ListIterator to add or remove concurrently while iterating arrayList. When you try to add or remove a object from a list by using forloop it will surely throws the Current Modification Exception. So you can't modify the list. By using ListIterator you can add remove object from the list.

Here is sample code:

List<String> list1=new ArrayList<String>();
list1.add("a");
list1.add("b");
// c is missing in the list
list1.add("d");
list1.add("e");
ListIterator<String> it=list1.listIterator();
while(it.hasNext()){
    if(it.next().equals("b")){
        // add c in the list after b
        it.add("c");
    }
}
System.out.println(Arrays.toString(list1.toArray(new String[]{})));

output:

[a, b, c, d, e]

edited Dec 24, 2014 at 10:04

Braj

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answered Dec 24, 2014 at 9:58

newuser

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1 Comment

Braj Over a year ago

I have added sample code as well to make it more clear.

Community · Accepted Answer · 2017-05-23 10:33:39Z

1

When looping through an ArrayList if you want to make any modifications to it, you must then loop via iterator, as the regular for loop won't help you then, look here.

Iterator<String> iter = myArrayList.iterator();

while (iter.hasNext()) {
    String str = iter.next();

    if (someCondition)
        iter.remove();
}

edited May 23, 2017 at 10:33

CommunityBot

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answered Dec 24, 2014 at 9:59

Muhammed Refaat

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Comments

chiastic-security · Accepted Answer · 2014-12-24 10:01:44Z

1

The only way you're allowed to add to an ArrayList while you're looping over it is by using an iterator:

ListIterator<String> iterator = bultosHijos.iterator();
while (iterator.hasNext()) {
    String str = iterator.next();
    // now if we want to...
    iterator.add("some other string");
}

The iterator also has a .remove(), and a .set() for editing an element.

There is no .addAll() on a ListIterator, so if you want to add lots of elements, you have to create your own loop and add them one by one:

for (String s: bultosNietos)
    iterator.add(s);

Any other way of modifying the list will give a ConcurrentModificationException.

answered Dec 24, 2014 at 10:01

chiastic-security

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Comments

Chowdappa · Accepted Answer · 2014-12-24 10:05:06Z

0

You should use Concurent package. Use http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CopyOnWriteArrayList.html

answered Dec 24, 2014 at 10:05

Chowdappa

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