Concatenating elements of ArrayList while iterating

Question

I am interested in concatenating a string to elements of an arraylist while iterating.When I run the below code,I am getting a ConcurrentModificationException.My code is:

ArrayList<String> list=new ArrayList<String>();
list.add("000);
list.add("00");
list.add("0");
for(String s:list)
{
    if(s.length()==1)
    {
        String s2="000" + s;
        list.add(s2);

    }


    if(s.length()==2)
    {
        String s2="00" + s;

    }


    if(s.length()==3)
    {
        String s2="0" + s;
    }
}

My question is how to add the concatenated strings back to list without using StringBuilder as when I use a StringBuilder,it is causing other parts of my program to malfunction? So just need some guidance.

The code you've given will *not* cause an exception. Please provide a short but complete program *that demonstrates the problem*, as well as telling us what you're trying to achieve. — Jon Skeet, Nov 05 '14 at 18:02
What I want to do is if my arraylist contains a binary string "00",I want to represent 4 bits for the number.So "00" will become "0000".Similarly,if it contains "0" it should have 3 zeroes appended to the front.In short, all my strings should come out having a length of 4.Hope this helps — Java_stack1, Nov 05 '14 at 18:09
Your question is confusingly worded. You don't want to add elements to the list, you want to prepend characters to the Strings contained within your list. — Sean Bright, Nov 05 '14 at 18:21
In future, you should put the details in the *question* - not just in comments. — Jon Skeet, Nov 05 '14 at 19:00

Sean Bright · Answer 1 · 2014-11-05T18:19:53.897

1

I would just use List.set:

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

list.add("000");
list.add("00");
list.add("0");

for (int i = 0; i < list.size(); i++) {
    String value = list.get(i);
    list.set(i, ("0000" + value).substring(value.length()));
}

edited Nov 05 '14 at 18:19

answered Nov 05 '14 at 18:14

Sean Bright

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score 0 · Answer 2 · answered Nov 05 '14 at 18:08

The exception will occur if you change the contents of the list (i.e. add or remove an element) anywhere in your for loop.

ArrayList<String> list=new ArrayList<String>();
list.add("000);
list.add("00");
list.add("0");
ArrayList<String> itemsToAdd = new ArrayList<String>();  // <======
for(String s:list) {
    if(s.length()==1) {
        String s2="000" + s;
        //list.add(s2);   DONT DO THIS
        itemsToAdd.add(s2);  // DO THIS INSTEAD
    }

    if(s.length()==2) {
        String s2="00" + s;
    }


    if(s.length()==3) {
        String s2="0" + s;
    }
}

// NOW APPEND THE LIST
// This is safe as the iteration is complete.
list.addAll(itemsToAdd.add);

score 0 · Answer 3 · answered Nov 05 '14 at 18:09

0

make a second list
add the new elements to that second list
add all the elements in the second list back into the first one after you're done iterating

answered Nov 05 '14 at 18:09

Sam I am says Reinstate Monica

30,851
12
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Thanks.But will this practice retain the order of elements in new list similar to that in original? – Java_stack1 Nov 05 '14 at 18:18
@RabiaMairaj it depends on how you implement it, but if you implement it in the most straightforward way, the elements will be added in the same order that they would have been added in your method had it worked – Sam I am says Reinstate Monica Nov 05 '14 at 18:57

score 0 · Answer 4 · edited May 23 '17 at 11:56

Have a look at this one: Modifying Java ArrayList while iterating over it

Short story: Java won't let you change a list that you're iterating over. Instead, make a copy by putting the list to be cloned into the new arraylist's constructor arguments and then change the new list:

ArrayList<String> list=new ArrayList<String>();
list.add("000);
list.add("00");
list.add("0");
ArrayList<String> listSecond =new ArrayList<String>(list);

for(String s:list)
{
    if(s.length()==1)
    {
        String s2="000" + s;
        listSecond.add(s2);

    }...

score 0 · Answer 5 · answered Nov 05 '14 at 18:12

Your list.add(s2) is a problem. You just can't play with lists while iterating (well with an iterator you can delete items). Here's a possible solution:

    ArrayList<String> list=new ArrayList<String>();
    list.add("000");
    list.add("00");
    list.add("0");
    List<String> newOnes = new ArrayList<>();
    for(String s:list)
    {
        if(s.length()==1)
        {
            String s2="000" + s;
            newOnes.add(s2);

        }


        if(s.length()==2)
        {
            String s2="00" + s;

        }


        if(s.length()==3)
        {
            String s2="0" + s;
        }
    }
    list.addAll(newOnes);

score 0 · Answer 6 · answered Nov 05 '14 at 18:15

create a tempList and add the element to tempList and at last assign tempList to list

public static void main (String[] args) throws Exception
    {
            ArrayList<String> list=new ArrayList<String>();
            ArrayList<String> tempList=new ArrayList<String>();
            list.add("000");
            list.add("00");
            list.add("0");

            for(String s:list)
            {
                if(s.length()==1)
                {
                    String s2="000" + s;
                    tempList.add(s2);

                }


                if(s.length()==2)
                {
                    String s2="00" + s;
                    tempList.add(s2);

                }


                if(s.length()==3)
                {
                    String s2="0" + s;
                    tempList.add(s2);
                }
            }
            list=tempList;
            System.out.println(list);

        }

SkyMaster · Answer 7 · 2014-11-05T21:07:35.567

Modifying your list while iterating: (more generic and short)

        ArrayList<String> list= new ArrayList<>();
        String zeros = "00000000";
        list.add("000");
        list.add("00");
        list.add("0");
        for(int i=0;i<list.size();i++) {
            String s = list.get(i) + zeros.substring(0,4-list.get(i).length());
            list.set(i,s);
        }           
        for (String s : list)
            System.out.println(s);

Concatenating elements of ArrayList while iterating

7 Answers7