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ConcurrentModificationException When removing element using list iterator java

I have an issue removing the 1st and 2nd element of my list even by using the iterator.
I have read the following threads but can't fix my issue (those were the most relevant but I checked other material as well):
ConcurrentModificationException when trying remove element from list
Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop
So my code looks like this:
List<List<String>> list = cnf.read();
List<List<String>> nlist = new ArrayList<>();
for (List<String> l : list) {
if (l.size() <= 3) {
nlist.add(l);
} else {
int size = l.size();
while (size > 3) {
List<String> three = l.subList(0, 2);
three.add("Y" + (count++));
//Iterator itr = l.iterator();
ListIterator itr = l.listIterator();
int v = 0;
while (itr.hasNext()) {
itr.next();
if (v == 0 || v == 1) {
itr.remove();
v++;
}
}
l.add(0, "Y" + (count++));
size--;
nlist.add(three);
}
nlist.add(l);
}
}
for (List<String> l : nlist) {
System.out.println(l.toString());
System.out.println(l.size());
}
I get a ConcurrentModificationException at the print statement here :
System.out.println(l.toString());
I tried using iterators for my 2 for loops as well but It doesn't seem to make a difference!
I am new to posting questions so let me know If I am doing it right!
Thank you.

After A long debugging, here is the solution.
The sublist function passes by reference and not by value, a sublist created by ArrayList.subList call keeps a reference to the original list and accesses its elementData array directly.
For this reason, when adding an element to the "three" list, we alter the state of the original list. this happens here:
three.add("Y" + (count++));
A way of fixing it for this specific case is to create and initialize the "three" list the following way:
String one = l.get(0);
String two = l.get(1);
List<String> three = new ArrayList<>();
three.add(one);
three.add(two);
three.add("Y" + (count));
This allows us to manipulate our lists without getting Concurrency Exceptions (ConcurrentModificationException). However, if you are manipulating big lists, I would suggest you use another less hardcoded method for list creation.
I will mark this thread as answered and hope it helps people.

Deleting specific object from ArrayList using for-loop

I am trying to delete one object from an ArrayList, but after iterating through the list with the for loop i'm stuck at what to do next. nameInput is a lowercase string from the user.
If i run this it prints the object from arr list equal to the input from nameInput. But I cannot understand how to go from printing that object to deleting it?
I'm sure this is a stupid question but the 50+ answers i have read and tried all seem to fail me (or more likely I fail to understand them). I have tried the list.remove and removeIf.
private ArrayList<Arr> arr = new ArrayList<>();
private void removeItem() {
for (Object arr : arr) {
if (((Arr) arr).getName().equals(nameInput())) {
System.out.println(arr);
break;
} else {
System.out.println("Error");
}
}
}

Using for loop
List<Arr> arr = new ArrayList<>();
for (Arr item : arr) {
if (item.getName().equals(nameInput())) {
arr.remove(item);
break;
}
}
If not call break after remove element, you get ConcurrentElementException
Note from #Aomine: you have to implement correct Arr.equals() method.
Using Iterator
List<Arr> arr = new ArrayList<>();
Iterator<Arr> it = arr.iterator();
while (it.hasNext()) {
Arr items = it.next();
if (item.getName().equals(nameInput())) {
it.remove();
break; // you can continue iterating and remove another item
}
}
Using Streams
List<Arr> arr = new ArrayList<>();
arr.removeIf(item -> item.getName().equals(nameInput()));
Remove all items that match given condition
This is not good to remove element from ArrayList. In case you know that you have to remove element from the middle of the List, do use LinkedList.

You are trying to remove an item while you are traversing/iterating the list in the for loop. You cannot remove an item from the list iterating it in a for loop. Use an Iterator instead and invoke arr.remove().

If you use Java 8 you could do
private void removeItem() {
arr.removeIf(t -> t.getName().equals(nameInput));
}
Note that this will remove all objects with name equal to nameInput
Also you should change your declaration of arr to
List<Arr> arr = new ArrayList<>();

A couple of things here...
The loop variable receiver type should ideally be Arr instead of Object as the list contains Arr objects. This also means you no longer need the cast you're performing.
You could remove the item via remove(Object o) but this requires overriding equals and hashcode based on name only. Another option is via an iterator but this would mean changing your code completely. Thus, to keep it as close to your code as possible you can use a for loop; get the index which the object is located and then remove.
Thus, you can do:
for(int i = 0; i < arr.size(); i++){
if (arr.get(i).getName().equals(nameInput)) {
Arr obj = arr.remove(i); // remove the item by index
System.out.println(obj); // print the object
break; // terminate the loop iteration
}
}

How to obtain first 5 values from a LinkedHashSet?

I have a LinkedHashSet which contains multiple number of values.
LinkedHashSet<String> lhs = new LinkedHashSet<String>();
I want to iterate through the set of values and display the first five values from the number of items stored in the set. I have used a for loop to iterate through values and display data, see below:
for (String sent : lhs) {
text.append(sent);
}
This outputs all the values stored in the LinkedHashSet. What alterations should I make to my code in order to only get the first 5 values from the set.

You can get your sets Iterator
Iterator<String> it = yourSet.iterator();
and move to next() element N times (assuming that it still hasNext() element)
int counter = 0;
while(it.hasNext() && counter++ < N){
String element = it.next();
...
}

int i = 0;
for (String sentences : lhs) {
if (i > 4) break;
text.append(sentences);
i++;
}

If you had java8, then I would suggest something like this:
yourLinkedHashSet
.stream()
.limit(5)
.forEachOrdered(text::append);
Explanation:
stream() will take one String after another from the collection
limit(5) will stop the calculations after five elements are processed
forEachOrdered(...) takes an action for each item, one after another
text::append is the action to be done per item

You can use subList without counter declaration
Iterator<String> iter = new ArrayList<>(lhs).subList(5, lhs.size())
.iterator();
while (iter.hasNext()) {
text.append(iter.next());
}

Java - Collection.remove() behaves differently in different conditions

This is a follow up to my previous question :
Collection - Iterator.remove() vs Collection.remove()
The below two pieces of code , which apparently differs only by a single line , but one throws exception and other don't . Can you please explain the difference ?
List<String> list = new ArrayList<String>
(Arrays.asList("noob1","noob2","noob3"));
System.out.println(list);
for (String str : list) {
if (str.equals("noob2")) {
list.remove(str);
}
}
runs fine , but if i change the condition to
if (!str.equals("noob2"))
the code throws exception !

What happens in this situation is you are removing the second list element.
List<String> list = new ArrayList<String>
(Arrays.asList("noob1", "noob2", "noob3", "noob4"));
System.out.println(list);
for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {
String str = iterator.next();
if (str.equals("noob3")) {
System.out.println("Checking "+str);
list.remove(str);
}
}
System.out.println(list);
prints
[noob1, noob2, noob3, noob4]
Checking noob1
Checking noob2
Checking noob3
[noob1, noob2, noob4]
By removing the second last element you have reduced the size to the number of elements which you have iterated over.
// from ArrayList.Itr
public boolean hasNext() {
return cursor != size;
}
This causes the loop to exit early before the concurrent modifcation check is performed in next(). If you remove any other element next() is called and you get a CME.
BTW Something which also bypasses the check is
for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {
String str = iterator.next();
System.out.println("Checking "+str);
if (str.equals("noob2")) {
list.remove("noob1");
list.remove("noob3");
}
}
as long as the size of the collection is the same as the index it is up to, the check is not performed.

The for loop is just a simplified syntax for an iterator scan of the list. The iterator may throw an exception if the list is modified under it, but it is not guaranteed. Because of hasNext, iterators are often working one element ahead, making the first case less likely to be affected by list modification. By the time "noob2" is removed, the iterator already knows about "noob3".

Actually you should never remove collections' elements during "casual" iterating. When you have to modify your collection in some loop you have to use iterator to make these operations.
public class Test {
public static void main(String... args) {
List<String> list = new ArrayList<String>(Arrays.asList("noob1", "noob2", "noob3"));
System.out.println(list);
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
String str = iterator.next();
if (!str.equals("noob2")) {
iterator.remove();
}
}
System.out.println(list);
}
}

I suppose the exception is thown because you are trying to change a collection you are looping on... and not because the if condition.
I suggest you to create a new list only containing the items that verify the condition. Add them to the new list and avoid to change the original collection.

It's because you are trying to remove from a Collection you are currently iterating through. Making a minor alteration you can do what you want to do:
String[] strValues = {"noob1","noob2","noob3"}; // <<< Array
List<String> list = new ArrayList<String>(Arrays.asList(strValues));
System.out.println(list);
for (String str : strValues) { // << List is duplicate of array so can iterate through array
if (!str.equals("noob2")) {
list.remove(str);
}
}
That should work. Hopefully

Well, your first case doesn't throw the Exception because, the iterator returns false for Iterator.hasNext() at index 2 as you remove the element at index 1.
Iterator<String> itr = list.iterator();
while(itr.hasNext()){
String s= itr.next();
if(s.equals("noob2")){
list.remove(s); // size of the list is 2 here
System.out.println(itr.hasNext());// this returns false as it doesn't have anything at index 2 now.(on 2nd iteration )
}
}
You can test it clearly using a simple for-loop:
for (int i=0; i<list.size(); i++) {
if (list.get(i).equals("noob2")) {
System.out.println(list.get(i));
System.out.println(list.size());
list.remove(list.get(i));
System.out.println(list.size());
}
}
Output:
[noob1, noob2, noob3]
noob2
3
2
Notice the size of the list after you remove the element, which fails after incrementing. 2<2 which is false

How to avoid java.util.ConcurrentModificationException when iterating through and removing elements from an ArrayList

I have an ArrayList that I want to iterate over. While iterating over it I have to remove elements at the same time. Obviously this throws a java.util.ConcurrentModificationException.
What is the best practice to handle this problem? Should I clone the list first?
I remove the elements not in the loop itself but another part of the code.
My code looks like this:
public class Test() {
private ArrayList<A> abc = new ArrayList<A>();
public void doStuff() {
for (A a : abc)
a.doSomething();
}
public void removeA(A a) {
abc.remove(a);
}
}
a.doSomething might call Test.removeA();

Two options:
Create a list of values you wish to remove, adding to that list within the loop, then call originalList.removeAll(valuesToRemove) at the end
Use the remove() method on the iterator itself. Note that this means you can't use the enhanced for loop.
As an example of the second option, removing any strings with a length greater than 5 from a list:
List<String> list = new ArrayList<String>();
...
for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {
String value = iterator.next();
if (value.length() > 5) {
iterator.remove();
}
}

From the JavaDocs of the ArrayList
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.

You are trying to remove value from list in advanced "for loop", which is not possible, even if you apply any trick (which you did in your code).
Better way is to code iterator level as other advised here.
I wonder how people have not suggested traditional for loop approach.
for( int i = 0; i < lStringList.size(); i++ )
{
String lValue = lStringList.get( i );
if(lValue.equals("_Not_Required"))
{
lStringList.remove(lValue);
i--;
}
}
This works as well.

In Java 8 you can use the Collection Interface and do this by calling the removeIf method:
yourList.removeIf((A a) -> a.value == 2);
More information can be found here

You should really just iterate back the array in the traditional way
Every time you remove an element from the list, the elements after will be push forward. As long as you don't change elements other than the iterating one, the following code should work.
public class Test(){
private ArrayList<A> abc = new ArrayList<A>();
public void doStuff(){
for(int i = (abc.size() - 1); i >= 0; i--)
abc.get(i).doSomething();
}
public void removeA(A a){
abc.remove(a);
}
}

While iterating the list, if you want to remove the element is possible. Let see below my examples,
ArrayList<String> names = new ArrayList<String>();
names.add("abc");
names.add("def");
names.add("ghi");
names.add("xyz");
I have the above names of Array list. And i want to remove the "def" name from the above list,
for(String name : names){
if(name.equals("def")){
names.remove("def");
}
}
The above code throws the ConcurrentModificationException exception because you are modifying the list while iterating.
So, to remove the "def" name from Arraylist by doing this way,
Iterator<String> itr = names.iterator();
while(itr.hasNext()){
String name = itr.next();
if(name.equals("def")){
itr.remove();
}
}
The above code, through iterator we can remove the "def" name from the Arraylist and try to print the array, you would be see the below output.
Output : [abc, ghi, xyz]

Do the loop in the normal way, the java.util.ConcurrentModificationException is an error related to the elements that are accessed.
So try:
for(int i = 0; i < list.size(); i++){
lista.get(i).action();
}

Here is an example where I use a different list to add the objects for removal, then afterwards I use stream.foreach to remove elements from original list :
private ObservableList<CustomerTableEntry> customersTableViewItems = FXCollections.observableArrayList();
...
private void removeOutdatedRowsElementsFromCustomerView()
{
ObjectProperty<TimeStamp> currentTimestamp = new SimpleObjectProperty<>(TimeStamp.getCurrentTime());
long diff;
long diffSeconds;
List<Object> objectsToRemove = new ArrayList<>();
for(CustomerTableEntry item: customersTableViewItems) {
diff = currentTimestamp.getValue().getTime() - item.timestamp.getValue().getTime();
diffSeconds = diff / 1000 % 60;
if(diffSeconds > 10) {
// Element has been idle for too long, meaning no communication, hence remove it
System.out.printf("- Idle element [%s] - will be removed\n", item.getUserName());
objectsToRemove.add(item);
}
}
objectsToRemove.stream().forEach(o -> customersTableViewItems.remove(o));
}

One option is to modify the removeA method to this -
public void removeA(A a,Iterator<A> iterator) {
iterator.remove(a);
}
But this would mean your doSomething() should be able to pass the iterator to the remove method. Not a very good idea.
Can you do this in two step approach :
In the first loop when you iterate over the list , instead of removing the selected elements , mark them as to be deleted. For this , you may simply copy these elements ( shallow copy ) into another List.
Then , once your iteration is done , simply do a removeAll from the first list all elements in the second list.

In my case, the accepted answer is not working, It stops Exception but it causes some inconsistency in my List. The following solution is perfectly working for me.
List<String> list = new ArrayList<>();
List<String> itemsToRemove = new ArrayList<>();
for (String value: list) {
if (value.length() > 5) { // your condition
itemsToRemove.add(value);
}
}
list.removeAll(itemsToRemove);
In this code, I have added the items to remove, in another list and then used list.removeAll method to remove all required items.

Instead of using For each loop, use normal for loop. for example,the below code removes all the element in the array list without giving java.util.ConcurrentModificationException. You can modify the condition in the loop according to your use case.
for(int i=0; i<abc.size(); i++) {
e.remove(i);
}

Sometimes old school is best. Just go for a simple for loop but make sure you start at the end of the list otherwise as you remove items you will get out of sync with your index.
List<String> list = new ArrayList<>();
for (int i = list.size() - 1; i >= 0; i--) {
if ("removeMe".equals(list.get(i))) {
list.remove(i);
}
}

You can also use CopyOnWriteArrayList instead of an ArrayList. This is the latest recommended approach by from JDK 1.5 onwards.

Do somehting simple like this:
for (Object object: (ArrayList<String>) list.clone()) {
list.remove(object);
}

An alternative Java 8 solution using stream:
theList = theList.stream()
.filter(element -> !shouldBeRemoved(element))
.collect(Collectors.toList());
In Java 7 you can use Guava instead:
theList = FluentIterable.from(theList)
.filter(new Predicate<String>() {
#Override
public boolean apply(String element) {
return !shouldBeRemoved(element);
}
})
.toImmutableList();
Note, that the Guava example results in an immutable list which may or may not be what you want.

for (A a : new ArrayList<>(abc)) {
a.doSomething();
abc.remove(a);
}

"Should I clone the list first?"
That will be the easiest solution, remove from the clone, and copy the clone back after removal.
An example from my rummikub game:
SuppressWarnings("unchecked")
public void removeStones() {
ArrayList<Stone> clone = (ArrayList<Stone>) stones.clone();
// remove the stones moved to the table
for (Stone stone : stones) {
if (stone.isOnTable()) {
clone.remove(stone);
}
}
stones = (ArrayList<Stone>) clone.clone();
sortStones();
}

I arrive late I know but I answer this because I think this solution is simple and elegant:
List<String> listFixed = new ArrayList<String>();
List<String> dynamicList = new ArrayList<String>();
public void fillingList() {
listFixed.add("Andrea");
listFixed.add("Susana");
listFixed.add("Oscar");
listFixed.add("Valeria");
listFixed.add("Kathy");
listFixed.add("Laura");
listFixed.add("Ana");
listFixed.add("Becker");
listFixed.add("Abraham");
dynamicList.addAll(listFixed);
}
public void updatingListFixed() {
for (String newList : dynamicList) {
if (!listFixed.contains(newList)) {
listFixed.add(newList);
}
}
//this is for add elements if you want eraser also
String removeRegister="";
for (String fixedList : listFixed) {
if (!dynamicList.contains(fixedList)) {
removeResgister = fixedList;
}
}
fixedList.remove(removeRegister);
}
All this is for updating from one list to other and you can make all from just one list
and in method updating you check both list and can eraser or add elements betwen list.
This means both list always it same size

Use Iterator instead of Array List
Have a set be converted to iterator with type match
And move to the next element and remove
Iterator<Insured> itr = insuredSet.iterator();
while (itr.hasNext()) {
itr.next();
itr.remove();
}
Moving to the next is important here as it should take the index to remove element.

List<String> list1 = new ArrayList<>();
list1.addAll(OriginalList);
List<String> list2 = new ArrayList<>();
list2.addAll(OriginalList);
This is also an option.

If your goal is to remove all elements from the list, you can iterate over each item, and then call:
list.clear()

What about of
import java.util.Collections;
List<A> abc = Collections.synchronizedList(new ArrayList<>());

ERROR
There was a mistake when I added to the same list from where I took elements:
fun <T> MutableList<T>.mathList(_fun: (T) -> T): MutableList<T> {
for (i in this) {
this.add(_fun(i)) <--- ERROR
}
return this <--- ERROR
}
DECISION
Works great when adding to a new list:
fun <T> MutableList<T>.mathList(_fun: (T) -> T): MutableList<T> {
val newList = mutableListOf<T>() <--- DECISION
for (i in this) {
newList.add(_fun(i)) <--- DECISION
}
return newList <--- DECISION
}

Just add a break after your ArrayList.remove(A) statement