How to add values to a list while iterating it

Question

I Have a scenario such like this

List<String> xxx = new ArrayList
for(String yyy : xxx){
   for(String zzz:xyxy){
     if(!zzz.equals(yyy)){
       xxx.add(zzz);
     }
   }
}

But i get java.util.ConcurrentModificationException: null exception.Can anyone help me solve this issue.? Can anyone give me alternate method to perform this ?

As an aside, you need to work on your variable naming skills. Your sample code is hard to follow solely due to the non-descriptive names. — MattLBeck, May 15 '14 at 09:51

awksp · Accepted Answer · 2014-05-15T09:54:09.243

10

Looking at the ArrayList API:

The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.

So you're going to need to explicitly get a ListIterator and use that to properly alter your ArrayList. Also, I don't believe you can use a for-each loop because the iterators in those are separate from your explicitly retrieved iterator. I think you'll have to use a while loop:

List<String> xxx = new ArrayList<>();
ListIterator<String> iterator = xxx.listIterator();
while (iterator.hasNext()) {
    String s = iterator.next();
    for (String zzz : xyxy) {
        if (!zzz.equals(s)) {
            iterator.add(zzz); //<-- Adding is done through the iterator
        }
    }
}

edited May 15 '14 at 09:54

answered May 15 '14 at 09:47

awksp

11,764
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Yo thanks bro .. It worked like a gem ...! – Deepak Ramakrishnan Kalidass May 15 '14 at 13:12
What is xyxy in above for each loop? – rogue lad Dec 05 '14 at 11:43
@Rajper I presume it's an `Iterable` of some kind. What specific kind it is, I don't know. It was in the original question, so I just left it that way. – awksp Dec 05 '14 at 17:23
Yeah definitely it is some kind of String collection, but what I was trying to ask; It must be declared or initialized somewhere in the snippet. – rogue lad Dec 08 '14 at 07:20
@Rajper I assume the missing declaration was just forgotten or something. It isn't a significant detail for the question. – awksp Dec 08 '14 at 07:49

score 1 · Answer 2 · answered May 15 '14 at 09:47

Add them in another list and then add that list to xxx;

List<String> xxx = new ArrayList<>();
List<String> additional = new ArrayList<>();
for (String yyy : xxx) {
    for (String zzz : xyxy) {
        if (!zzz.equals(yyy)) {
            additional.add(zzz);
        }
    }
}
xxx.addAll(additional);

score 1 · Answer 3 · answered May 15 '14 at 09:58

1

To modify a list while iterating you can declare the arraylist as "java.util.concurrent.CopyOnWriteArrayList"

public static void main(String[] args) {
    List<String> xxx = new CopyOnWriteArrayList();
    xxx.add("3");
    xxx.add("4");

    List<String> xyxy = new ArrayList();
    xyxy.add("1");
    xyxy.add("2");
    xyxy.add("3");

    for (String yyy : xxx) {
        for (String zzz : xyxy) {
            if (!zzz.equals(yyy)) {
                xxx.add(zzz);
            }
        }
    }

    System.out.println(xxx);
}

answered May 15 '14 at 09:58

user3636135

21
1

However, according to the online API, "The iterator will not reflect additions, removals, or changes to the list since the iterator was created". This could be problematic, depending on what OP wants. Also, `CopyOnWriteArrayList`s are horrible for performance if you don't have a ton more reads than writes – awksp May 15 '14 at 10:00
Agreed. CopyOnWriteArrayList will be effective when number of modifications are minimal compared to number of traversal operations. – user3636135 May 15 '14 at 10:14
Which doesn't seem to be the case here... – awksp May 15 '14 at 10:16

score 0 · Answer 4 · answered May 15 '14 at 09:48

0

Don't use a foreach loop, but use a normal loop like:

int originalSize=xxx.size();
for (int i=0;i<originalSize;i++) {
    xxx.add("New string appended at the end");
}

This allows you to iterate only on the items present in the list before you start to iterate, and it works if your only operation on the list is to add some new item at the end of it.

answered May 15 '14 at 09:48

WoDoSc

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1

Don't do this! `xxx.size()` is chaging while `originalSize` stays the same. – Dmitry Ginzburg May 15 '14 at 09:49
@ginz this is done on purpose! To iterate only on the items that are originally contained in the list (and not in the new items added during the loop) – WoDoSc May 15 '14 at 09:51
1

It's not the same behavior as in author's code. – Dmitry Ginzburg May 15 '14 at 09:51
The intent of the OP is not really specified. But if you set the condition for ending the loop by checking xxx.size() you have the risk to have an infinite loop (if you continue to add new item). – WoDoSc May 15 '14 at 09:54

Mustafa sabir · Answer 5 · 2014-05-15T10:02:29.433

0

It means you cannot access and modify your collection (List) at the same time. To do so you must use an iterator. I dont understand what xyxy is in your code. but a possible iteration and adittion of element might look like this.

        List<String> xxx = new ArrayList<String>();

        xxx.add("hello");
        xxx.add("hi");
        xxx.add("hh");

        ListIterator<String> it = xxx.listIterator();
        while (it.hasNext()) {

            if (!it.next().equals("hi")) {
                it.remove();
                it.add("hi");

            }
        }

        System.out.println(xxx);
          }
      }

edited May 15 '14 at 10:02

answered May 15 '14 at 09:49

Mustafa sabir

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I believe this would still throw a `ConcurrentModificationException` because you're still modifying the `ArrayList` in a way that isn't through the iterator itself – awksp May 15 '14 at 09:51

How to add values to a list while iterating it

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