Iterators were introduced mainly because enumerators couldn't handle deleting while enumerating. When should I use the different types of why and because in German? To fix this, you need to. Update: The OP want to know whether exactly one of the following two cases must occur: Jon Skeet's answer points out a case where less than three elements are printed, without an exception, which implies that the answer is no. Find centralized, trusted content and collaborate around the technologies you use most. TikZ / foreach: read out sequence of Unicode symbols. To learn more, see our tips on writing great answers. Did anybody use PCBs as macro-scale mask-ROMS? The initialization is executed. There might be some trick with ListIterator, but the easiest solution is probably an old style index loop. ListIterator example 4. Why do secured bonds have less default risk than unsecured bonds? Are "pro-gun" states lax about enforcing "felon in possession" laws. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. remove both elements when a duplicate element is found) this is the way to go. So you save memory and time. There are several ways to iterate over List in Java. First, every time you remove an element, the indexes are reorganized. You are using fast enumeration, which protects the list that you are iterating through. Not the answer you're looking for? What is the best way to set up multiple operating systems on a retro PC? Why is there current if there isn't any potential difference? Why might a civilisation of robots invent organic organisms like humans or cows? If the only modification is to remove the current element, you can make the second approach work by using itr.remove() (that is, use the iterator's remove() method, not the container's). I could not easily find documentation for "structural modification" which is what I was looking for! Basic List operations 4. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. The third alternative, is to create a new Collection, iterate over the original, and add all the members of the first Collection to the second Collection that are not up for deletion. How do I continue work if I love my research but hate my peers? interface provides. Calling external applications/bat files using QGIS Graphical Modeller. Remove two objects at the same time from an ArrayList without causing a ConcurrentModificationException? Why does Ash say "I choose you" instead of "I chose you" or "I'll choose you"? You can call iterator.remove() to savely remove the current item from list. preferring the first approach for the simple reason of readability)? I would choose the second as you don't have to do a copy of the memory and the Iterator works faster. Making statements based on opinion; back them up with references or personal experience. Stuff that was removed from set in the meantime will be ignored. Does this change of data work or is it temporary (only to be lost when the activation record is deleted)? Does the policy change for AI-generated content affect users who (want to) How to create a function to retains only the elements in the list that are contained in the specified list without using any third 3rd party. Live Demo. This is much more readable and sure. It would really help if you could make this question self-contained. Is it true that the Chief Justice granted royal assent to the Online Streaming Act? What method you use might depend on what you are intending to do, In Java 8, there is another approach. But generally you should avoid. (Specifically for when trying to categorize an adult), Reductive instead of oxidative based metabolism. Never considered this as I typically just used it to remove a single item I was looking for. Just curious, why do you create a copy of foolist rather than just looping through foolist in the first example? Using Thread.sleep() cannot reliably force an overlap between two threads because the kernel can always decide to schedule threads arbitrarily after they awaken. However if you truly want to iterate through all of your elements and remove a specific one during the loop, you may want to shift the index backwards afterwards. Good to know. But couldn't you do item[i].getKey().equals(key))? Does a Wildfire Druid actually enter the unconscious condition when using Blazing Revival? How do I remove filament from the hotend of a non-bowden printer? rev2023.6.8.43485. You dont need to create an another list emp1 as you are not modifying the structure of the list you are modifying the existing elements which is an object in this case. Second, not all List implementations offer direct access to the elements (as ArrayList does). Modifying Java ArrayList while iterating over it. I read somewhere that you cannot replace values in a list while iterating using a for loop but have no idea how to fix that. This way you'll safely remove all elements that generate a removal set from your setOfElementsToRemove(). I have productListArray and I would like to add product into it but each product has unique ID and unique name and I am getting some errors, Space Invaders generate initial Asteroids Java Failure, arrayList only updates 2 elements when using forEach, Adding object to arraylist unless it's already there. A method is provided to obtain a list iterator that starts at a specified position in the list. Instead of creating strange things, you can just filter() and then map() your result. Then just call remove on each element you want to remove. Is a house without a service ground wire to the panel safe? TikZ / foreach: read out sequence of Unicode symbols, Luzern: Walking from Pilatus Kulm to Frakigaudi Toboggan, Reductive instead of oxidative based metabolism. If the list is sorted, and you want to remove consecutive elements you can create a sublist and then clear it: Since the sublist is backed by the original list this would be an efficient way of removing this subcollection of elements. int size = al.size (); // Iterate list of objects. Why does Ash say "I choose you" instead of "I chose you" or "I'll choose you"? What is the best way to set up multiple operating systems on a retro PC? Following example uses replaceAll() method to replace all the occurance of an element with a different element in a list. Making statements based on opinion; back them up with references or personal experience. Then you can use a second list to store the data you want to add. Connect and share knowledge within a single location that is structured and easy to search. Your fix should be e.setOrders (e.orders += 1). Suppose we have the following collection of books. Because there is so little going on in each iteration: Is there risk? Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Is it possible to achieve this in some other way? This includes ArrayList. How to Carry My Large Step Through Bike Down Stairs? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. 1: ConcurrentModificationException 2: , 1: List Index Index 2: , .exe with Digital Signature, showing SHA1 but the Certificate is SHA384, is it secure? In your paragraph about JDK8 Streams you mention to. We have to consider that only the structural space of the collection is the one being created, the objects inside the collections are not being copied. Yes, you can modify or update the values of objects in the list in your case likewise: However, the above statement will make updates on the source objects. This answer is deleting the item from the list which isn't the answer to the question. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. I'll proof read my code better next time. Possible plot hole in D&D: Honor Among Thieves. ClamAV detected Kaiji malware on Ubuntu instance. Find centralized, trusted content and collaborate around the technologies you use most. Asking for help, clarification, or responding to other answers. So, we iterate only once over the collection and that would be efficient. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. @JonSkeet He is talking about the code which he posted as an answer I suppose. How to add element in List while iterating in java? tmux: why is my pane name forcibly suffixed with a "Z" char? Can existence be justified as better than non-existence? Are interstellar penal colonies a feasible idea? @TheNewIdiot: No, I've just spotted the last line of the post, which refers to code using an enhanced for loop. The question is how to replace/update/swap an item in the list while iterating. It is possible to implement a Set that allows its elements to be removed whilst iterating over it. You will always get a java.util.ConcurrentModificationException. But what about changing the elements in a List? Is it better to not connect a refrigerator to water supply to prevent mold and water leaks, ClamAV detected Kaiji malware on Ubuntu instance, Short story about flowers that look like seductive women, Possible plot hole in D&D: Honor Among Thieves. Eg:- If you want to remove all even numbers from a list, you can do it as follows. Making statements based on opinion; back them up with references or personal experience. Can I remove from a list while iterating it? Yep your second answer will work, other than possibly running into memory problems +1 though. Short story about flowers that look like seductive women. I know that removing an element from a list while iterating it is not recommended. Paper with potentially inappropriately-ordered authors, should a journal act? See. I thought I would point out that the ListIterator which is a special kind of Iterator is built for replacement. List users: Yes, you can modify state of objects inside your stream, but most often you should avoid modifying state of source of stream. Using. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Its cursor always lies between the previous and next elements. From a performance standpoint, these methods should be used with caution. First off all you need to use an iterator, to remove the item. My boss claims this code is fine (and it does appear to work), but I still don't think it is correct. Can you aid and abet a crime against yourself? EDIT: oops, I see Peter Nix had already suggested the toRemove idea (although with an unnecessarily hand-rolled removeAll). Get sets of keys by calling the Map.keySet () method 2. Connect and share knowledge within a single location that is structured and easy to search. Your question is a bit confusing so I'll answer what I think I understand; your question comes to this: how to remove an item from a list while the list is iterated concurrently and items are being removed OR how to avoid ConcurrentModificationException. Oops, sorryit is implied that I would use the iterator's remove method, not the container's. Asking for help, clarification, or responding to other answers. Is this photo of the Red Baron authentic? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. While iterating through a list, an item can possibly be removed. Even though it works under the condition people have pointed out, I would only use it temporarily and change it to: There is no need to check if i==3, simply remove it before the for loop. Yep. how to get curved reflections on flat surfaces? And a synchronized class? I am trying to iterate (or use a for each loop) on a Linked list class and be able to change the item (when found) to a passed in parameter. It's not a good idea to use an enhanced for loop in this case, you're not using the iteration variable for anything, and besides you can't modify the list's contents using the iteration variable. You need to think about how list.remove(index) works. One can learn lot of things from this question and the answers below. This might be a little late. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Therefore, for a list of n length, there are n+1 possible cursors. You can make use of the removeIf to remove data from a list conditionally. This is stated in the doc : "A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification." (Java) [duplicate], Java: adding elements to a collection during iteration, Self-healing code is the future of software development, How to keep your new tool from gathering dust, We are graduating the updated button styling for vote arrows, Statement from SO: June 5, 2023 Moderator Action. Thinking about rejoining the workforce how do I refernece a company that no longer exists on a resume? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Problem Description. , . For loop doesn't loop through all the index, ConcurrentModificationException when removing item from a list. But your statement is the one I was looking for. How to Carry My Large Step Through Bike Down Stairs? Why does voltage increase in a series circuit? Does the policy change for AI-generated content affect users who (want to) How can I iterate over an object while modifying it in Java? How to avoid java.util.ConcurrentModificationException when iterating through and removing elements from an ArrayList, LisIterator has add() but doesn't iterate over new added elements, How to remove element from list while iterating the same list in golang, Looping area calculations for multiple rasters in R. What are the Star Trek episodes where the Captain lowers their shields as sign of trust? Method for modifying an element in an Array list? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. In the first approach the disadvantage is that we have to iterate twice. The first technique consists in collecting all the objects that we want to delete (e.g. Can anyone explain why it behaves like this? .exe with Digital Signature, showing SHA1 but the Certificate is SHA384, is it secure? This would support the claim that, at least in this case, iterator approach should be faster. But the continue makes sure all the following elements are printed as well, which a break couldn't do. Setup Let us define our Item object first. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. Connect and share knowledge within a single location that is structured and easy to search. I want to iterate over list and increment orders by one. Java itself provides several ways of finding an item in a list: The contains method The indexOf method An ad-hoc for loop The Stream API 3.1. contains () List exposes a method called contains: boolean contains(Object element) If we encounter what appears to be an advanced extraterrestrial technological device, would the claim that it was designed be falsifiable? How many numbers can I generate and be 90% sure that there are no duplicates? Peek method should only be used for debugging, Update element in the list while iterating Java8, Modify property value of the objects in list using java 8 streams, docs.oracle.com/javase/8/docs/api/java/util/stream/, Self-healing code is the future of software development, How to keep your new tool from gathering dust, We are graduating the updated button styling for vote arrows, Statement from SO: June 5, 2023 Moderator Action. Also, I thought I would post my solution because there seems to be a need and I wanted to contribute the the excellent resource that is Stack Overflow. speaking, impossible to make any hard guarantees in the presence of While notifying the listeners of an event, other listeners may no longer be needed. I think his question is more of "why do I sometimes get lucky" rather than "why did the Exception occur at all? It might be the right question: Iterator implementation. The listIterator () method of java.util.ArrayList class is used to return a list iterator over the elements in this list (in proper sequence). Not the answer you're looking for? rev2023.6.8.43485. Not the answer you're looking for? Any suggestions to avoid concurrent modification issue in this JAVA for loop please? It's just that its often a, Last edit is exactly right. To learn more, see our tips on writing great answers. This prevents that each removal, changes the index of future potential removals. Remove elements from collection while iterating Ask Question Asked 11 years, 1 month ago Modified 3 months ago Viewed 399k times 318 AFAIK, there are two approaches: Iterate over a copy of the collection Use the iterator of the actual collection For instance, I am trying to identify this bone I found on the beach at the Delaware Bay in Delaware. 1. You can read more in the docs for Iterator. Is it possible to determine a maximum L/D possible, ClamAV detected Kaiji malware on Ubuntu instance, Is it better to not connect a refrigerator to water supply to prevent mold and water leaks, Skeleton for a command-line program that takes files in C, Find Roman numerals up to 100 that do not contain I". docs.oracle.com/javase/7/docs/api/java/util/concurrent/, Self-healing code is the future of software development, How to keep your new tool from gathering dust, We are graduating the updated button styling for vote arrows, Statement from SO: June 5, 2023 Moderator Action. In fact, the Javadoc for the Exception addresses this point very specifically. Not the answer you're looking for? So if you get into the last iteration of the loop before the remove() call, then you won't get an exception - hasNext() will just return false. Asking for help, clarification, or responding to other answers. Guava MultiMap and ConcurrentModificationException, Most Efficient Way to Remove Objects With Null Attributes From a List, Need an api to remove objects which are started with a specific charecter without using another collection inside the method, ConcurrentModificationException(list) or another datastructur for "searching/comparing and filtering", How to safely remove other elements from a Collection while iterating through the Collection, Element removing while collection iterating, Java analog of iterator to remove objects from collection, Java: removing elements from a collection, Remove elements from collection while iterating, Remove from a collection during iteration, iterating through collection - removing other element, Using Iterators to remove elements from a Java Collection. If it is, then the exception is expected. When remove(idx) gets called, the element at idx index gets deleted and all the next elements gets shifted to left. Does the policy change for AI-generated content affect users who (want to) Iteration over a list (ConcurrentModificationException), Deal with concurrent modification on List without having ConcurrentModificationException, Running into java.util.ConcurrentModificationException while iterating list, Java How to add to an array list while looping, Concurrent modification excpetion with iterator adding to arraylist, ConcurrentModificationException when iterate through List, Java modifying list concurrently at different places, ConcurrentModificationException while iterating through List, altough not modifying it. In this tutorial, we'll review the different ways to do this in Java. Java: adding elements to a collection during iteration. @merlin2011: Yes - it's not clear that the OP fully understood that part either @Rad: By the time it's trying to print the 4th item, there, Modify a list while it is being iterating, Self-healing code is the future of software development, How to keep your new tool from gathering dust, We are graduating the updated button styling for vote arrows, Statement from SO: June 5, 2023 Moderator Action. And how much overhead does copying the list create? Overview There are several options to iterate over a collection in Java. You can't modify a Collection while iterating over it using an Iterator, except for Iterator.remove(). ConcurrentModificationException on a best-effort basis. Except possibly one: you could use a Collections.newSetFromMap(new ConcurrentHashMap(sizing params)). setOfElementsToRemoveIncludingThePassedValue() will generate a set of elements to remove that includes the value passed to it. Fundamentally, you're modifying a list in one thread while iterating over it in another, and the list implementation you're using does not support that. More generally, looking for a ConcurrentModificationException is not a reliable way of detecting multiple threads modifying and reading an object simultaneously. Edit: Also if you are using a constant value in loop break condition like in above example i<10; you might get ArrayIndexOutOfBounds exception. I tried with streams but it is only mapping the orders. Two ways of replacing the elements using ListIterator shown below are: Replacing First element Is 'infodumping' the important parts of a story via an in-universe lesson in school/documentary/the news/other educational medium bad storytelling? I think the standard implementations (HashSet, TreeSet etc.) 2 You don't need to create an another list emp1 as you are not modifying the structure of the list you are modifying the existing elements which is an object in this case. Isn't it O(n^2)? From my understanding the ListIterator.add() adds an element before the current element in the list, not after it. If you have enough memory for one copy of the set, I'll assume you also have enough memory for two copies. How many numbers can I generate and be 90% sure that there are no duplicates? Thought I would choose the second as you do item [ I ].getKey ( ) 2!, looking for a ConcurrentModificationException remove that includes the value passed to it D. How to add element in an Array list, see our tips on writing great answers a collection iteration! To obtain a list Iterator that starts at a specified position in the docs for.. Java for loop please share knowledge within a single location that is and... Claim that, at least in this tutorial, we & # ;. N'T modify a collection in Java, I see Peter Nix had already suggested the toRemove (... The objects that we want to iterate over list in Java feed, copy and this! Way you 'll safely remove all even numbers from a list intending to this. Thought I would point out that the Chief Justice granted royal assent to the Online Act. Would choose the second as you do n't have to do a copy of foolist rather than just looping foolist. That generate a set that allows its elements to remove a single item I was looking for help if want... Over list and increment orders by one read more in the docs for Iterator JonSkeet He is about., Iterator approach should be used with caution in D & D: Honor Among Thieves RSS reader possibly... That there are n+1 possible cursors technologies you use most ) will generate a set that allows elements. You remove an element in list while iterating through a list to delete e.g! Following example uses replaceAll ( ) exactly right wire to the elements in a list, after... My peers developers & technologists share private knowledge with coworkers, Reach developers & technologists share knowledge. Set that allows its elements to be removed approach for the simple reason of readability?! Collection and that would be efficient time you remove an element in list while iterating in Java simple of! Set that allows its elements to be lost when the activation record is deleted ) crime against yourself look seductive... An unnecessarily hand-rolled removeAll ) this way you 'll safely remove all even from... Can just filter ( ) will generate a set of elements to be lost when the activation record is )... My code better next time the docs for Iterator of n length, there no... N'T have to iterate over a collection while iterating in Java bonds less! Can call iterator.remove ( ) will generate a set that allows its elements to remove from... Possible to implement a set of elements to be removed when remove ( idx gets. On what you are iterating through unsecured bonds really help if you could use a second to!: you could make this question and the Iterator works faster ListIterator is. Second as you do n't have to iterate over a collection during iteration I generate and 90... Of Iterator is built for replacement is structured and easy to search from my the! Specifically for when trying to categorize an adult ), Reductive instead of `` chose... Abet a crime against yourself what I was looking for unsecured bonds Ash say `` I assume... Could n't you do item [ I ].getKey ( ) over in. Or responding to other answers that the Chief Justice granted royal assent to the elements ( ArrayList. Going on in each iteration: is there current if there is another approach, showing SHA1 but Certificate... 'Ll safely remove all elements that generate a set of elements to that! Right question: Iterator implementation cursor always lies between the previous and next elements gets shifted left... Statement is the way to set up multiple operating systems on a retro?... An unnecessarily hand-rolled removeAll ) copying the list which is what I was looking for that was removed set... Just looping through foolist in the list while iterating index ) works help if you have enough memory for copy... Of the removeIf to remove data from a list of n length, are... Work or is it true that the Chief Justice granted royal assent to the question understanding the (. This way you 'll safely remove all even numbers from a list question and answers! With potentially inappropriately-ordered authors, should a journal Act the activation record is deleted?! I think the standard implementations ( HashSet, TreeSet etc. remove a location. Than unsecured bonds D & D: Honor Among Thieves list implementations offer direct access the... N'T loop through all the index of future potential removals docs for Iterator `` pro-gun states! Would choose the second as you do n't have to iterate over list increment! Possibly running into memory problems +1 though remove the current element in while! Reading an object simultaneously elements are printed as well, which a break could n't do meantime be... Sure all the java replace item in list while iterating of future potential removals with a `` Z '' char be 90 % sure there... Collection during iteration for `` structural modification '' which is a special kind of is! Of detecting multiple threads modifying and reading an object simultaneously cursor always lies between the previous and elements. You do item [ I ].getKey ( ) ; // iterate of. One copy of the removeIf to remove data from a list, is it possible to achieve this in other... // iterate list of objects not after it if you could make question... Posted as an answer I suppose to set up multiple operating systems on a resume but is! Are several options to iterate over a collection during iteration of keys calling. You could use a Collections.newSetFromMap ( new ConcurrentHashMap < SomeClass, Boolean > ( sizing )! '' char choose the second as you do item [ I ].getKey ( ) method 2 our tips writing. Question self-contained stuff that was removed from set in the list that you are iterating through a list iterating. The Chief Justice granted royal assent to the question is how to add element in an Array list in iteration. Ca n't modify a collection in Java Among Thieves even numbers from a Iterator. The indexes are reorganized > ( sizing params ) ) yep your second answer will,... Index gets deleted and all the index of future potential removals to achieve this in.! All the objects that we have to do this in some other way removed from set in the list not! Javadoc for the simple reason of java replace item in list while iterating ) why is there current if there so! Create a copy of the memory and the answers below which a break n't! Answer is deleting the item ) adds an element before the current in. Your statement is the best way to set up multiple operating systems on a resume a way... 90 % sure that there are no duplicates with Digital Signature, showing SHA1 but the continue makes all. The technologies you use might depend on what you are intending to do in! Like humans or cows current element in an Array list exactly right might be some trick ListIterator! Memory for two copies Specifically for when trying to categorize an adult ), instead... Actually enter the unconscious condition when using Blazing Revival documentation for `` structural modification '' which is what was! Causing a ConcurrentModificationException is not recommended right question: Iterator implementation on writing great answers, for ConcurrentModificationException! Of Unicode symbols a method is provided to obtain a list, you can just filter ( ) (., sorryit is implied that I would point out that the ListIterator which is any! N length, there is another approach the answers below learn more, see tips... Gets deleted and all the occurance of an element from a list conditionally technique consists in collecting the. To subscribe to this RSS feed, copy and paste this URL into your RSS reader read. Is only mapping the orders is not a reliable way of detecting multiple threads modifying and reading an simultaneously. Collection in Java edit: oops, I see Peter Nix had already suggested the toRemove (. Would use the Iterator works faster the first approach the disadvantage is that we want to delete ( e.g suppose! From list in list while iterating might a civilisation of robots invent organic organisms like humans or?. Than just looping through foolist in the list, not all list implementations direct... Could not easily find documentation for `` structural modification '' which is what I was looking.! Carry my Large Step through Bike Down Stairs condition when using Blazing Revival savely remove current... For help, clarification, or responding to other answers then the Exception addresses this point very Specifically about! ) adds an element from a list conditionally data you want to delete ( e.g Certificate SHA384! As follows set that allows its elements to be removed references or experience... Are iterating through a list while iterating in Java the value passed to it same time from an ArrayList causing! Once over the collection and that would be efficient how much overhead does copying the list while iterating it. Non-Bowden printer future potential removals delete ( e.g systems on a resume just call remove on each you! In the list while iterating over it a crime against yourself deleted and all the occurance of an element the... Answer will work, other than possibly running into memory problems +1 though oops, I 'll read... That starts at a specified position in the first approach the disadvantage is that we to. Coworkers, Reach developers & technologists worldwide my understanding the ListIterator.add ( ) an. Al.Size ( ) your result already suggested the toRemove idea ( although with unnecessarily!