I have a linkedlist where each element has key and value(ArrayList<dataStructure>). I want to merge the elements having same key.
Iterator<CElem> oItr = linkedList.iterator();
{
while (oItr.hasNext())
{
CElem outer = oItr.next();
Iterator<CElem> iItr = linkedList.iterator();
{
while (iItr.hasNext())
{
CElem inner = iItr.next();
if (outer.equals(inner))
continue;
if (outer.getKey().equals(inner.getKey()))
{
outer.getValues().addAll(inner.getValues());
iItr.remove();
}
}
}
}
}
Though I am using the iterators remove methog getting a java.util.ConcurrentModificationException. What should be changed to get rid of this.
You remove element with one of your iterators, thus the second of them does not know about this removal and ConcurrentModificationException is thrown
BTW:
you should consider using some multimap in place of list that is having key-values pairs
Add the elements you want to remove in another List and then loop in that list at the end to remove these elements.
Alternatively, use a Map/Set.
Both your iterators are traversing the linked list
Iterator<CElem> oItr = linkedList.iterator();
....
Iterator<CElem> iItr = linkedList.iterator();
probably iItr should be for the inner array list?
UPDATE Scratch above answer I misread the question. The challenge though is that you have two iterators traversing the list, so while you use one iterator's remove() method, the other still detects the concurrent modification.
Normally, to remove duplicates from a list, you can just run them through a Set (e.g. a HashSet) but that won't work for you as it's only key duplication, not the entire member of the list.
I'd take an approach where I try to find and capture the duplicated keys and their values in a separate list and then merge in and remove the duplicates as a separate step.
The problem is that when you use the iItr.remove() it modifies the list, which iItr is happy with because it knows what changed, but oItr isn't. There are three possible solutions to this that I can see:
Switch to a concurrent list (e.g. ConcurrentLinkedQueue - but see then answers at Lock-Free Concurrent Linked List in Java for warnings about this)
Switch to a set structure, e.g. TreeSet, which will keep your items unique automatically (but won't preserve their order)
Make sure you don't use the other iterator after removing from one of them -- you could do this by switching which element you are removing, i.e. change iItr.remove() to:
oItr.remove();
break;
This would cause the first instance of each key to be removed, rather than subsequent ones, which might not be the behaviour you want -- in that case you could try iterating over the lists backwards.
Will this work?
Iterator<CElem> oItr = linkedList.iterator();
{
while (oItr.hasNext())
{
CElem outer = oItr.next();
Iterator<CElem> iItr = linkedList.iterator();
{
while (iItr.hasNext())
{
CElem inner = iItr.next();
if (outer.equals(inner))
continue;
if (outer.getKey().equals(inner.getKey()))
{
inner.getValues().addAll(outer.getValues());
outer.remove();
break;
}
}
}
}
}
Related
I have defined a ConcurrentLinkedQueue and looping over it like :
ConcurrentLinkedQueue clq
.forEach((carTask -> {
Now if I delete carTask from the queue. which is an element
in the queue while we are doing a forEach will that lead to any issue ?
One alternative I think of is put the elements to be deleted
in a separate List and once the forEach is over simply delete all
the elements in the Queue from the List.
I am not sure if this alternative approach will work perfectly
Any suggestions or better way to approach this problem ?
This specific example won't result in an error, because this implementation of Queue allows concurrent modifications.
But removing elements from Java collections via the remove method, while you're looping over the elements, will often result in a ConcurrentModificationException. Instead, it's a good habit to use an Iterator and call its remove method:
Collection<String> c = new LinkedList<>();
c.add("foo");
c.add("bar");
for(Iterator<String> it = c.iterator(); it.hasNext(); ) {
String s = it.next();
if(s.equals("foo")) {
it.remove();
}
}
// collection now contains only "bar"
In addition to preventing ConcurrentModificationException, this is preferable because the time complexity of remove for many collections is linear. Since this queue is implemeneted as a linked list, the list must be traversed until the element to be removed is located. With an Iterator, you've already located the element, and can remove it "in place".
What if you locate the element you want to remove, and it's the last element in the queue? Calling remove on that item requires a traversal all the way to the end of the queue again. In addition, it requires a call to equals for every preceding element. You definitely don't want to put the items to be removed into a list, because you'd have the same negative performance characteristics (along with the allocation of another List).
This question already has answers here:
Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop
(31 answers)
Closed 8 years ago.
In Java, is it legal to call remove on a collection when iterating through the collection using a foreach loop? For instance:
List<String> names = ....
for (String name : names) {
// Do something
names.remove(name).
}
As an addendum, is it legal to remove items that have not been iterated over yet? For instance,
//Assume that the names list as duplicate entries
List<String> names = ....
for (String name : names) {
// Do something
while (names.remove(name));
}
To safely remove from a collection while iterating over it you should use an Iterator.
For example:
List<String> names = ....
Iterator<String> i = names.iterator();
while (i.hasNext()) {
String s = i.next(); // must be called before you can call i.remove()
// Do something
i.remove();
}
From the Java Documentation :
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. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than
risking arbitrary, non-deterministic behavior at an undetermined time
in the future.
Perhaps what is unclear to many novices is the fact that iterating over a list using the for/foreach constructs implicitly creates an iterator which is necessarily inaccessible. This info can be found here
You don't want to do that. It can cause undefined behavior depending on the collection. You want to use an Iterator directly. Although the for each construct is syntactic sugar and is really using an iterator, it hides it from your code so you can't access it to call Iterator.remove.
The behavior of an iterator is
unspecified if the underlying
collection is modified while the
iteration is in progress in any way
other than by calling this method.
Instead write your code:
List<String> names = ....
Iterator<String> it = names.iterator();
while (it.hasNext()) {
String name = it.next();
// Do something
it.remove();
}
Note that the code calls Iterator.remove, not List.remove.
Addendum:
Even if you are removing an element that has not been iterated over yet, you still don't want to modify the collection and then use the Iterator. It might modify the collection in a way that is surprising and affects future operations on the Iterator.
for (String name : new ArrayList<String>(names)) {
// Do something
names.remove(nameToRemove);
}
You clone the list names and iterate through the clone while you remove from the original list. A bit cleaner than the top answer.
The java design of the "enhanced for loop" was to not expose the iterator to code, but the only way to safely remove an item is to access the iterator. So in this case you have to do it old school:
for(Iterator<String> i = names.iterator(); i.hasNext();) {
String name = i.next();
//Do Something
i.remove();
}
If in the real code the enhanced for loop is really worth it, then you could add the items to a temporary collection and call removeAll on the list after the loop.
EDIT (re addendum): No, changing the list in any way outside the iterator.remove() method while iterating will cause problems. The only way around this is to use a CopyOnWriteArrayList, but that is really intended for concurrency issues.
The cheapest (in terms of lines of code) way to remove duplicates is to dump the list into a LinkedHashSet (and then back into a List if you need). This preserves insertion order while removing duplicates.
I didn't know about iterators, however here's what I was doing until today to remove elements from a list inside a loop:
List<String> names = ....
for (i=names.size()-1;i>=0;i--) {
// Do something
names.remove(i);
}
This is always working, and could be used in other languages or structs not supporting iterators.
Yes you can use the for-each loop,
To do that you have to maintain a separate list to hold removing items and then remove that list from names list using removeAll() method,
List<String> names = ....
// introduce a separate list to hold removing items
List<String> toRemove= new ArrayList<String>();
for (String name : names) {
// Do something: perform conditional checks
toRemove.add(name);
}
names.removeAll(toRemove);
// now names list holds expected values
Make sure this is not code smell. Is it possible to reverse the logic and be 'inclusive' rather than 'exclusive'?
List<String> names = ....
List<String> reducedNames = ....
for (String name : names) {
// Do something
if (conditionToIncludeMet)
reducedNames.add(name);
}
return reducedNames;
The situation that led me to this page involved old code that looped through a List using indecies to remove elements from the List. I wanted to refactor it to use the foreach style.
It looped through an entire list of elements to verify which ones the user had permission to access, and removed the ones that didn't have permission from the list.
List<Service> services = ...
for (int i=0; i<services.size(); i++) {
if (!isServicePermitted(user, services.get(i)))
services.remove(i);
}
To reverse this and not use the remove:
List<Service> services = ...
List<Service> permittedServices = ...
for (Service service:services) {
if (isServicePermitted(user, service))
permittedServices.add(service);
}
return permittedServices;
When would "remove" be preferred? One consideration is if gien a large list or expensive "add", combined with only a few removed compared to the list size. It might be more efficient to only do a few removes rather than a great many adds. But in my case the situation did not merit such an optimization.
Those saying that you can't safely remove an item from a collection except through the Iterator aren't quite correct, you can do it safely using one of the concurrent collections such as ConcurrentHashMap.
Try this 2. and change the condition to "WINTER" and you will wonder:
public static void main(String[] args) {
Season.add("Frühling");
Season.add("Sommer");
Season.add("Herbst");
Season.add("WINTER");
for (String s : Season) {
if(!s.equals("Sommer")) {
System.out.println(s);
continue;
}
Season.remove("Frühling");
}
}
It's better to use an Iterator when you want to remove element from a list
because the source code of remove is
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null;
so ,if you remove an element from the list, the list will be restructure ,the other element's index will be changed, this can result something that you want to happened.
Use
.remove() of Interator or
Use
CopyOnWriteArrayList
Is there a quick way of removing the
null elements from a linked list?
The only way i know is to iterate on the
elements and remove the null ones.
I don't see how multithreading would help-- linked list doesn't have direct access to its members.
////////////////////////////////
EDIT: One I could think of is putting the elements into a set then back to the list again. This wouldn't retain the order, However would work every otherwise(?)
Unless you have an iterator pointing to the middle of the list, multithreading is not going to help* . A simple iteration with a ListIterator<T> should do the trick:
ListIterator<String> iter = list.listIterator();
while (iter.hasNext()) {
if (iter.next() == null) {
iter.remove();
}
}
* That is before we take into consideration that the linked list is not thread-safe without external synchronization.
What do you mean by fastest? The least number of operations is probably just going through one-by-one. If you use multithreading you would likely have to either chop up the list and put it back together or get the indexes and then go through the list in reverse and remove each one manually.
while(list.remove(null)); would work but might be slow on large list
Edit: In java 8 you can do list.parallelStream().filter((e)->e!=null) if you need the result as a linked list again you can do new LinkedList(Arrays.asList(stream.toArray));
This is pretty fast way. Worst case though it will be slower than just traversing once. In most cases it will be faster though
ArrayList a = new ArrayList<Integer>();
Collections.sort(a);
for(int i=a.size()-1; i>-1; i--)
{
if(a.get(i)==null)
a.remove(i);
else
break;
}
I am getting an exception when I try to remove elements from CopyOnWriteArrayList using an iterator.
I have noticed that it is documented
Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw UnsupportedOperationException.
(from http://download.oracle.com/javase/6/docs/api/java/util/concurrent/CopyOnWriteArrayList.html)
Now, surprisingly i can iterate it with foreach and use the remove() function . But then I get the famous bug - when trying to remove an item from a list using a for loop - you skip the element next to the removed element.
any suggestions then?
Iterate over the collection choosing all the elements you want to delete and putting those in a temporary collection. After you finish iteration remove all found elements from the original collection using method removeAll.
Would that work out for you? I mean, not sure if deletion logic is more complicated than that in your algorithm.
EDIT: I'm an idiot. I missed the fact that this is a copy-on-write list so every removal means a new copy. So my suggestions below are likely to be suboptimal if there's more than one removal.
Same as for any other list whose iterator doesn't support remove, or anything where you're not using an iterator. There are three basic techniques that come to mind to avoid this bug:
Decrement the index after removing something (being careful not to do anything with the index until the next iteration). For this you'll obviously have to use a for(int i=0; i < ... style of for loop, so that you can manipulate the index.
Somehow repeat what the inside of the loop is doing, without literally going back to the top of the loop. Bit of a hack - I would avoid this technique.
Iterate over the list in reverse (from end to start, instead of from start to end). I prefer this approach as it's the simplest.
Since this is a CopyOnWriteArrayList it is totally safe to remove elements while iterating with forEach. No need for fancy algorithms.
list.forEach(e -> {
if (shouldRemove(e))
list.remove(e);
});
EDIT: Well of course that works if you want to delete elements by reference, not by position.
Ususlly you would iterate first gathering elemenet to be deleted in a separate list then delete them outside the for each loop (which is disguised iterator based loop anyway)
Something like this:
int pos = 0;
while(pos < lst.size() ) {
Foo foo = lst.get(pos);
if( hasToBeRemoved(foo) ) {
lst.remove(pos);
// do not move position
} else {
pos++;
}
}
You could use Queue instead of List.
private Queue<Something> queue = new ConcurrentLinkedQueue<Something>();
It's thread safe and supports iterator.remove(). Be aware of the thread-safe behavior of Queue iterators, though (check the javadoc).
If you want to delete all use just clear(). If you want to keep elements put them in a temporary ArrayList and get them back from there.
List<Object> tKeepThese= new ArrayList<>();
for(ListIterator<Object> tIter = theCopyOnWriteArrayList; tIter.hasNext();)
{
tObject = tIter.next();
if(condition to keep element)
tKeepThese.add(tObject);
}
theCopyOnWriteArrayList.clear();
theCopyOnWriteArrayList.addAll(tKeepThese);
the shortest and most efficient way:
List<String> list = new CopyOnWriteArrayList<>();
list.removeIf(s -> s.length() < 1);
internally it creates an temporary array with the same length and copies all elements where the predicate returns true.
keep in mind that if you use this method to actually iterate over the elements to perform some action, these actions cannot be performed in paralell anymore since the removeIf-call is atomic and will lock the traversal for other threads
Below works fine with CopyOnWriteArrayList
for(String key : list) {
if (<some condition>) {
list.remove(key);
}
}
could not find anything on this, wondering if anyone knew about this or a possible workaround. I am using JDOM and working with an xml schema.
I have created a List of which are just xml tags. The algorithm's aim is to iterate through the List of elements and remove the element if a condition is met (in this case if it starts with a certain string). See below:
for (Element appinfo : appinfos) {
if (appinfo.getText().startsWith(
PARAMETER_DESCRIPTION_APPINFO)) {
removeAppInfoElement(appinfo, name, appinfo.getText());
}
}
However, the loop appears to be attempting to iterate to the element it just removed.
Does anyone see anything wrong with this? Do I need to abandon the enhanced for loop or dig deeper for cause of problem?
I suppose you're talking about ConcurrentModificationException. Try to use iterator instead.
Yes that wont work.
Add all the items you want to remove to a new collection and then do a removeAll with those elements on the original collection.
You cannot remove elements from a Collection directly as you iterate over it - this causes issues because the Iterator has no idea that the element has been removed.
Instead of the enhanced for-loop, use the Iterator directly and call the remove() function, for example:
for (Iterator it = appinfos.iterator(); it.hasNext();) {
Element appinfo : it.next();
if (someCondition) {
it.remove();
}
}
willcodejavaforfood's answer is one way of doing this.
An alternative, which may be better or worse depending on style and what else you want to do in the loop, is to get the Iterator explicitly and use its remove method:
final Iterator<Element> iter = appinfos.iterator();
while (iter.hasNext()) {
if (iter.next().getText().startsWith(
PARAMETER_DESCRIPTION_APPINFO)) {
iter.remove();
}
}
This of course only works if a simple removal from the collection is what you want to do. When invoking potentially complex methods that will directly remove from the underlying collection, the best approach is to take a copy of the collection initially, then iterate over this copy.
In all cases, modifying a collection while you are iterating over it will generally cause Bad Things to happen.