ConcurrentModificationException at for each android

Question

I am passing Arraylist of ParseObject, and then i am putting one one foreach loop to extract the items with a condition when user object is not equals to null. There are two problems which i am facing. 1. If i am doing the following lines of code by passing different data to another list and then pass that list in my adapter, i am getting random data with numbers for example: If on item # 1 the name is "MAC" then it is showing in item 3.

ArrayList<ParseObject> checkRequestedNetArrayList = new ArrayList<ParseObject>();
requestedNetArrayList = (ArrayList<ParseObject>) objects;
MyResponsibilitesActivity.requestedNetArrayList = requestedNetArrayList;
adapterRequest = new GenericAdapter<ParseObject>(
    getApplicationContext(),
    requestedNetArrayList,
    R.layout.requested_trust_net_list_item,
    requestedDataBinder);

requestListView.setAdapter(adapterRequest);

requestedNetArrayList = (ArrayList<ParseObject>) objects;

for(ParseObject object: objects){
    System.out.println(object);
    object.getParseObject("user");
    if(object.has("user")){

        checkRequestedNetArrayList.add(object);

    }else{
        checkRequestedNetArrayList.remove(object);
    }

}

adapterRequest = new GenericAdapter<ParseObject>(
    getApplicationContext(),
    checkRequestedNetArrayList,
    R.layout.requested_trust_net_list_item,
requestedDataBinder);

requestListView.setAdapter(adapterRequest);

If i am doing the following line of code to just direct giving the items in the same list, i am getting the java.util.ConcurrentModificationException

for(ParseObject object: objects){

    if(object.has("user")){

        requestedNetArrayList.add(object);

    }
}
else{
    requestedNetArrayList.remove(object);
}

adapterRequest = new GenericAdapter<ParseObject>(
    getApplicationContext(),
    requestedNetArrayList,
    R.layout.requested_trust_net_list_item,
    requestedDataBinder);

requestListView.setAdapter(adapterRequest);

}

Please help me out here.

After reformatting the second block of code, I realized that it's not even correct. Your braces are mismatched, and you have an else construct hanging in the middle of nowhere. Please fix your code so that it at least compiles. — Sam Estep
– Sam Estep, Commented Jul 4, 2015 at 13:55

Community · Accepted Answer · 2020-06-20 09:12:55Z

6

You can not remove an element from list while accessing it. You have to use Iterator.

Where ever you are removing the object, use it.remove();

Iterator<ParseObject> it = objects.iterator();
while(it.hasNext()){
    Object object = it.next();
    //your stuff
    it.remove();
}

I think you might want to check this article about deep copy also.

UPDATE

Since you want to add elements to the list it is not directly possible with iterator. Now you are facing problem because you are directly assigning objects to requestedNetArrayList instead of that do it in the following way :

    ArrayList<ParseObject> requestedNetArrayList = new ArrayList<>(objects);

Then iterate over objects as you are doing now, and remove from or add to requestedNetArrayList (which you are pretty much already doing).

edited Jun 20, 2020 at 9:12

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answered Jul 4, 2015 at 13:53

Karthik

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2 Comments

android_explorer Over a year ago

thanks for your answer here. I just need to know one thing that, can you please tell me how to add items with that Iterator, like i understand it.remove part but didn't understand that how to add items in it, and how to pass that list further.

Karthik Over a year ago

I have updated the answer. With the first version of answer I was just trying to explain why you were getting that exception. Just changing the initialisation of requestedNetArrayList would solve your problem.

AterLux · Accepted Answer · 2015-07-04 19:12:33Z

1

When you make iteration using for-each construction for Collection

for (Object x : collection) ...

you have implicit creation of Iterator object for that Collection. This iterator performs a check: is collection was changed since iterator was created? If so, throwing an exception. So, you should avoid to any modify to your collection, until iterator done. That means, you should not use add and remove.

In either way, it is better to access ArrayList by index, because it will prevent creation of Iterator object. Like this:

for (int i = objects.size() - 1; i >= 0; i--) {
    ParseObject object = objects.get(i);
    // when iterating from tail to head, you can safely add or remove objects to/from this array list
}

edited Jul 4, 2015 at 19:12

answered Jul 4, 2015 at 18:58

AterLux

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5 Comments

android_explorer Over a year ago

thanks for your answer. It is giving this error at count() The method count() is undefined for the type List<ParseObject>. Can you please help here

AterLux Over a year ago

Sorry. I misprinted. use size() instead of count()

android_explorer Over a year ago

i just change it. Actually i have to pass data to array list for passing it in adapter. Can you please make a condition according to my condition, in which i need to send pass certain data with my condition for example object.has("user") and in that condition data will be added to array list so i can pass it to my adapter

AterLux Over a year ago

I cannot understand logic of your application. I think you only need to remove uneccesary items, but no add (because it will result in duplicates). So, just remove add branch

android_explorer Over a year ago

thanks for your reply. You are right, i need to remove un necessary items from the list. But i also have to pass array list to my adapter. Thats why i was saying that before sending it to the adapter i need to remove the items.

Rohan Kamat · Accepted Answer · 2015-07-04 14:10:07Z

0

Instead of assigning the reference of objects to requestedNetArrayList, create a new ArrayList with the same contents

requestedNetArrayList=new ArrayList<ParseObject>(objects);

Then you can iterate on objects and modify requestedNetArrayList.

answered Jul 4, 2015 at 14:10

Rohan Kamat

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