From my understanding the ListIterator.add() adds an element before the current element in the list, not after it. how to modify a list while iterating (intermediate) anthony explains Spying on a smartphone remotely by the authorities: feasibility and operation. This example shows how to replace all elements after a modification: The new set could be just a reference to the original set and the transformation function -- technical in place. We have seen that moving forward in the list using a for-loop and removing elements from it might cause us to skip a few elements. An iterator for lists that allows the programmer Book or a story about a group of people who had become immortal, and traced it back to a wagon train they had all been on. 1 @TheNewIdiot: The code in the related question already does. In Java, can you modify a List while iterating through it? Book set in a near-future climate dystopia in which adults have been banished to deserts. There are 3 problems in your approach to solve the problem. Do I have the right to limit a background check? CopyOnWriteArrayList returns an iterator that does not support the remove() method. Would it be possible for a civilization to create machines before wheels? Peek (as in stack operations) is mainly for seeing the current status but not changing it as the meaning applies (if we are not quantum computing :) ). rev2023.7.7.43526. (Ep. We'll cover a few examples using a while loop, Java 8, and a few common libraries. A sci-fi prison break movie where multiple people die while trying to break out. how to modify a list while iterating (intermediate) anthony explains #402 - YouTube *normally* you can't modify a list while iterating over it but I show a little trick that makes. If it is, then the exception is expected. Find centralized, trusted content and collaborate around the technologies you use most. Is religious confession legally privileged? How to print and connect to printer using flutter desktop via usb? To learn more, see our tips on writing great answers. unsynchronized concurrent modification. Making statements based on opinion; back them up with references or personal experience. Is it not safe to insert into a list while iterating through it? If it is not modified the third item should be printed. total useless. Other than Will Riker and Deanna Troi, have we seen on-screen any commanding officers on starships who are married? Obviously you need to make sure there is an end condition (like with any recursive code or queue processing). ", @merlin2011: and that's why I've got the paragraph about "if you're very lucky", Fair enough. Nevertheless, I expect that when the next() is called there are two possibilities: either the list is not modified or it is. I was just trying to indicate that the list was changing during iteration. I think you are looking for something like. @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, Why on earth are people paying for digital real estate? I found, "Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress." The notable exception to this are streams whose sources are concurrent collections, which are specifically designed to handle concurrent modification. I suspect the answer is simply that you've got a race condition between the different threads. It would really help if you could make this question self-contained. Beware, as the peek() documentation says This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline, so it shouldnt be used like this. Pretty new to Java here. @Pshemo, one solution to that is to create a new instance of a Collection like ArrayList with the items inside your primary list; iterate over the new list, and do the operation on the primary list: new ArrayList<>(users).stream.forEach(u -> users.remove(u)); @Blauhirn Solve what? Connect and share knowledge within a single location that is structured and easy to search. Characters with only one possible next character. So this approach isn't any better, but it's a different way to do it. Would a different way of doing it, maybe using the set(E e) method of a ListIterator, be better? But your statement is the one I was looking for. java - Modify a list while it is being iterating I suspect the answer is simply that you've got a race condition between the different threads. API Note: This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline: reference: I don't understand why this operation is "mainly" for debugging. Why did the Apple III have more heating problems than the Altair. In the movie Looper, why do assassins in the future use inaccurate weapons such as blunderbuss? In the movie Looper, why do assassins in the future use inaccurate weapons such as blunderbuss? speaking, impossible to make any hard guarantees in the presence of 587), The Overflow #185: The hardest part of software is requirements, Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Temporary policy: Generative AI (e.g., ChatGPT) is banned, Testing native, sponsored banner ads on Stack Overflow (starting July 6). https://docs.oracle.com/javase/7/docs/api/java/lang/Iterable.html, https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html, https://docs.oracle.com/javase/7/docs/api/java/util/List.html, https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html, https://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html, https://docs.oracle.com/javase/7/docs/api/java/util/ListIterator.html, https://docs.oracle.com/javase/8/docs/technotes/guides/language/foreach.html, https://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html. Is there a deep meaning to the fact that the particle, in a literary context, can be used in place of , Cannot assign Ctrl+Alt+Up/Down to apps, Ubuntu holds these shortcuts to itself, Identifying large-ish wires in junction box. remove (int index) and remove (Object element), you cannot use them to remove items while iterating over ArrayList in Java because they will throw ConcurrentModificationException if called during iteration. When are complicated trig functions used? What could cause the Nikon D7500 display to look like a cartoon/colour blocking? What is the reasoning behind the USA criticizing countries and then paying them diplomatic visits? I could not easily find documentation for "structural modification" which is what I was looking for! Not the answer you're looking for? Learn more about Stack Overflow the company, and our products. Let's look at the alternatives: This is a simple solution for the underlying problem of your first code: A ConcurrentModificationException is thrown because you iterate through the list and removing from it at the same time. Java 8 Iterating List from JDK 1.0 to Java 1.8 version in 7 ways. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Not the answer you're looking for? Inserts the specified element into the list (optional operation). How to merge two lists of objects by comparing field value in the given objects in Java, Selection of First L Items of ArrayList of size N > L and Insertion to Another ArrayList in Java, No signature of method: java.util.ArrayList.getAt() is applicable for argument types: (HashMap) values: [[:]], Is using an arraylist of Tuple(double,int,int) slower than two arraylists. Otherwise I cannot count the exception in such situation. 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. There are several ways to iterate over List in Java. ArrayList remove () method Example Now, let's see an example of removing elements from ArrayList while looping using for () loop and ArrayList.remove () method, which is wrong, and the program will throw ConcurrentModificationExcetpion upon execution. Sorry I wasn't clear. Let's begin with the approach traditionally used before Java 8. The ArrayList iterator implementation appears to only detect invalid modicifications on the call to next(), not on the call to hasNext(). Would a room-sized coil used for inductive coupling and wireless energy transfer be feasible? ANTLR What is simpliest way to realize python like indent-depending grammar? For the remaining task, you dont need a stream: @SergeyLagutin and @Apostolos thanks for your reply. You can do it using streams map function like below, get result in new stream for further processing. It's no magic bullet, anymore. Invitation to help writing and submitting papers -- how does this scam work? Why did Indiana Jones contradict himself? When are complicated trig functions used? Thanks for contributing an answer to Stack Overflow! In Java, can you modify a List while iterating through it. Connect and share knowledge within a single location that is structured and easy to search. Find centralized, trusted content and collaborate around the technologies you use most. Remove elements from a list while iterating over it in Java @Augustas It all depends on how you want to modify this list. Also see the documentation redistribution policy. Why QGIS does not load Luxembourg TIF/TFW file? For instance, ["apple", "orange"] to ["iapple", "iorange"]. Here's the link to the documentation quoted by @ZouZou in the comments, it states that: 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. This would be easy to do in a linkedlist or any data structure which supports indexing, but with a set it can be tricky. Concurrent stream sources are those whose Spliterator reports the CONCURRENT characteristic. Brute force open problems in graph theory. Every functional programmer will see map and understand what it does. There are similar questions on SO talking about remove or add element while iterating a Java Set. This is the right way. Asking for help, clarification, or responding to other answers. Why Is PNG file with Drop Shadow in Flutter Web App Grainy? Making statements based on opinion; back them up with references or personal experience. This class is designed for observer lists, which are rarely modified and often traversed. In general, if a function returns a mutated version of the object, it should copy the original one, and mutate and return the copy, leaving the original intact. The . "When to use LinkedList over ArrayList?" ListIterator (Java Platform SE 8 ) java.util Interface ListIterator<E> All Superinterfaces: Iterator <E> public interface ListIterator<E> extends Iterator <E> An iterator for lists that allows the programmer to traverse the list in either direction, modify the list during iteration, and obtain the iterator's current position in the list. I really just meant to point out reassigning references does only that -- it changes the reference but not the object the reference referred to. Iterate Over Unmodifiable Collection in Java - GeeksforGeeks Basic for Loop 2.1 boolean hasNext() This method tells us whether the collection has the next element to fetch. Java - How to modify all elements of a List? Extract data which is inside square brackets and seperated by comma, A sci-fi prison break movie where multiple people die while trying to break out. (Ep. EDIT: got it. 2. Of course, the developer has to understand how it works and use it properly. Why do complex numbers lend themselves to rotation? I was just trying to indicate that the list was changing during iteration. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. I needed a way to remove elements on a List while iterating through it. ConcurrentModificationException on a best-effort basis. Simply replacing one element by another doesn't count as a structural modification. Guide to the Java 8 forEach Connect and share knowledge within a single location that is structured and easy to search. (Ep. For example, what if we have. Are there ethnically non-Chinese members of the CCP right now? when you construct a string, say new String("hello"), you can't further modify it's inner value. Is the part of the v-brake noodle which sticks out of the noodle holder a standard fixed length on all noodles? Here is simple example how to modify string element. 15amp 120v adaptor plug for old 6-20 250v receptacle? Instead of creating strange things, you can just filter() and then map() your result. 587), The Overflow #185: The hardest part of software is requirements, Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Temporary policy: Generative AI (e.g., ChatGPT) is banned, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Adding elements to a collection during iteration, Exception in thread "main" java.util.ConcurrentModificationException.