It also allows to replace the current element via set () method. My impression is that Java is not particularly suitable for FP. Different maturities but same tenor to obtain the yield. the list during iteration, and obtain the iterator's Why did Indiana Jones contradict himself? How to Use Iterator in Java? Java 8 Iterator Examples on ArrayList Science fiction short story, possibly titled "Hop for Pop," about life ending at age 30, My manager warned me about absences on short notice. It is a bit faster as it moves no elements to be removed later. Is the part of the v-brake noodle which sticks out of the noodle holder a standard fixed length on all noodles? Because of ConcurrentModificationException. Java Collections Framework. Thanks. The ArrayList iterator implementation appears to only detect invalid modicifications on the call to next(), not on the call to hasNext(). Why free-market capitalism has became more associated to the right than to the left, to which it originally belonged? Iterate through List in Java Java: adding elements to a collection during iteration. Thanks for contributing an answer to Stack Overflow! Can we use work equation to derive Ohm's law? The functional approach would be to create a new set by applying a transformation function to the original set. 2nd - In your code you are modifying a reference to a string, not the string itself. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Were Patton's and/or other generals' vehicles prominently flagged with stars (and if so, why)? See. In Java, can you modify a List while iterating through it? I'm interested to see what other answers pop up as well. Copyright 1993, 2023, Oracle and/or its affiliates. 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). rev2023.7.7.43526. Adding to @Simon's answer, you could use a reversed for loop to go through your array to remove items you don't want. Modifying Objects within stream in Java8 while iterating This to get the count of the number of files. @JonSkeet He is talking about the code which he posted as an answer I suppose. 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 :) ). (Ep. Typo in cover letter of the journal name where my manuscript is currently under review. You get the exception if and only if the two threads happen to overlap in time when modifying the list. 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. 1 As I understand it, the variable in the enhanced for loop is akin to a parameter passed to a method or constructor: changes to the state of the referenced object are permitted, but assigning a new object will not cause the new object to be assigned to the collection element. List users: Yes, you can modify state of objects inside your stream, but most often you should avoid modifying state of source of stream. Do you mind providing code example? First option is the simpler one, create a new set. How does the theory of evolution make it less likely that the world is designed? its correctness: ConcurrentModificationException should be used only 15amp 120v adaptor plug for old 6-20 250v receptacle? Learn more about Stack Overflow the company, and our products. Save my name, email, and website in this browser for the next time I comment. Book set in a near-future climate dystopia in which adults have been banished to deserts. Avoiding the ConcurrentModificationException in Java Instead of creating strange things, you can just filter() and then map() your result. This is much more readable and sure. Any way to make this stream more efficient? Modifying an Array list while I am iterating over it, In need of iterating and modifying arraylist (or similar) at the same time. We'll begin by defining a list of countries for our examples: List<String> countries = Arrays.asList ( "Germany", "Panama", "Australia" ); 2.1. Java 8 - How to merge/concatenate/join two lists into single list ? We can do that by using hasNext (), next (), previous () and hasPrevious () methods. Replace fragment with another fragment inside ViewPager, display:table-cell not working on an input element, inlining failed in call to always_inline _mm_mullo_epi32: target specific option mismatch, Adding Convenience Initializers in Swift Subclass, Working example for JavaScriptResult in asp.net mvc, Proper session hijacking prevention in PHP. Modifying original List throws CMEx : While iterating List / ArrayList, if we try to modify original List like adding / removing elements then program throws ConcurrentModificationException. Has a bill ever failed a house of Congress unanimously? 1st - You can't modify the contents of a Set while you are iterating it. Pretty new to Java here. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. A. In Java, can you modify a List while iterating through it? I understand I wasn't using the iteration variable. One workaround is to iterate backward in the list, which does not skip anything. Submit a bug or feature For further API reference and developer documentation, see Java SE Documentation. 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. From the javadoc for add(): The new element is inserted before the implicit cursor: a subsequent call to previous() would return the new element. Why free-market capitalism has became more associated to the right than to the left, to which it originally belonged? on Stack Overflow, Just had to do something very similar (hence why I'm here), ended up using Java8's Collection.removeIf(PredicateIterate Over Unmodifiable Collection in Java - GeeksforGeeks 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. Invitation to help writing and submitting papers -- how does this scam work? Do you need an "Any" type when implementing a statically typed programming language? total useless. Adding, removing and printing from ArrayList with Iterator. Brute force open problems in graph theory. What are the advantages and disadvantages of the callee versus caller clearing the stack after a call? 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, Sci-Fi Science: Ramifications of Photon-to-Axion Conversion, Characters with only one possible next character, Different maturities but same tenor to obtain the yield. How JVM stack, heap and threads are mapped to physical memory or operation system. Java - How to modify all elements of a List? Find centralized, trusted content and collaborate around the technologies you use most. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Therefore, our printConsumer is simplified: name -> System.out.println (name) And we can pass it to forEach: names.forEach (name -> System.out.println (name)); Since the introduction of Lambda expressions in Java 8, this is probably the most common way to use the forEach method. What would stop a large spaceship from looking like a flying brick? Therefore, it In the movie Looper, why do assassins in the future use inaccurate weapons such as blunderbuss? rev2023.7.7.43526. Why did Indiana Jones contradict himself? Thanks for contributing an answer to Stack Overflow! Do I have the right to limit a background check? I didn't get what your conversation is about ;) Anyway the code is added. When practicing scales, is it fine to learn by reading off a scale book instead of concentrating on my keyboard? "But I'm not modifying the set, I am modifying the objects within it", which leads to problem two. Why modify a collection/array/list while iterating over it? Can a lambda be used to change a List's values in-place ( without creating a new list)? Science fiction short story, possibly titled "Hop for Pop," about life ending at age 30. How can I learn wizard spells as a warlock without multiclassing? I understand that in Java a Collection should not be modified while iterating through it, such as removing or adding elements. Bonus: if you iterate backwards, you can remove elements while iterating. For instance, ["apple", "orange"] to ["iapple", "iorange"]. A sci-fi prison break movie where multiple people die while trying to break out. Can Visa, Mastercard credit/debit cards be used to receive online payments? Modify property value of the objects in list using Java 8 streams Were Patton's and/or other generals' vehicles prominently flagged with stars (and if so, why)? By mkyong | Last updated: May 12, 2021 Viewed: 49,452 (+470 pv/w) Tags: iterator | java 8 | java collections | list | loop list | predicate Characters with only one possible next character. (Ep. :) Just curious, why does it got downvoted? speaking, impossible to make any hard guarantees in the presence of Simply replacing one element by another doesnt count as a structural modification. Given that, this code should work to set the new element as the next in the iteration: This will work except when the list starts iteration empty, in which case there will be no previous element. Creates various stream-related objects which might not be the most effective option. Find centralized, trusted content and collaborate around the technologies you use most. (Or not, it depends - but premature optimization is the root of all evil), Add elements to a List while iterating over it. You can do it using streams map function like below, get result in new stream for further processing. 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. Is there a distinction between the diminutive suffices -l and -chen? To learn more, see our tips on writing great answers. 5 Answers Sorted by: 80 There are several ways to do this. EDITED the code to produce the exception, please note the list content: The behavior you are trying to reproduce is highly timing-dependent. to traverse the list in either direction, modify (Java) [duplicate], Java: adding elements to a collection during iteration, Why on earth are people paying for digital real estate? How I can add objects into an ArrayList when I'm using this ArrayList in a for loop? I don't think data-race matters here, am I right? (Ep. 15amp 120v adaptor plug for old 6-20 250v receptacle? subsequent call to. To learn more, see our tips on writing great answers. It only takes a minute to sign up. I was just trying to indicate that the list was changing during iteration. It will modify your original list. But that is often not the case. Ok, I searched, what's this part on the inner part of the wing on a Cessna 152 - opposite of the thermometer, A sci-fi prison break movie where multiple people die while trying to break out, Cultural identity in an Multi-cultural empire. When are complicated trig functions used? Book set in a near-future climate dystopia in which adults have been banished to deserts. 7 Answers Sorted by: 195 If you wanna create new list, use Stream.map method: List<Fruit> newList = fruits.stream () .map (f -> new Fruit (f.getId (), f.getName () + "s", f.getCountry ())) .collect (Collectors.toList ()) If you wanna modify current list, use Collection.forEach: fruits.forEach (f -> f.setName (f.getName () + "s")) Share But generally you should avoid. Java 8 introduced the default method removeIf on the Collection interface. Java 8 How to check whether a number exists in an Arrays or List or Stream ? and may throw ConcurrentModificationException or even other unexpected exceptions like NPE: In this solution the original list is not modified, but should contain your expected result in a new list that is accessible under the same variable as the old one. I have a list of Fruit objects in ArrayList and I want to modify fruitName to its plural name. Maybe I'm too greedy ;) Could you please update the answer so I can accept it. What is the reasoning behind the USA criticizing countries and then paying them diplomatic visits? docs.oracle.com/javase/7/docs/api/java/util/concurrent/, Why on earth are people paying for digital real estate? Do I have the right to limit a background check? I looked in javadoc for List, but it's not there. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. If this method returns true that indicates there few elements in the collection. Is religious confession legally privileged? @emory Yap. The operation is there and can be used for whatever the developer thinks it fits. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Notice that the above snippet is not modifying the lists structure meaning: no elements are added or removed and the lists size remains constant. hmm. Would a room-sized coil used for inductive coupling and wireless energy transfer be feasible? The only down-side of this approach is that you need to switch your for-each to a while. In fact, the Javadoc for the Exception addresses this point very specifically. For the remaining task, you dont need a stream: @SergeyLagutin and @Apostolos thanks for your reply. 15amp 120v adaptor plug for old 6-20 250v receptacle? 587), The Overflow #185: The hardest part of software is requirements, Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Deleting an item from a Set while iterating, Comparing list elements in an effective way, A Range object for Java that partially implements `List`, Replacing consecutive equal elements in a list with a new element, Comparing each element of a list to all other elements of the same list, Given a list of words, remove the shorter word of every pair, Remove elements compared with other elements in a Set based on a condition, Characters with only one possible next character, Travelling from Frankfurt airport to Mainz with lot of luggage. Syntax: Collections.unmodifiableCollection (collection) Parameters: This method takes the collection as a parameter for which an unmodifiable view is to be returned. My manager warned me about absences on short notice. I'm not sure if this applies to your case (if deleting from the list will be frequent or not), but I thought I'd mention this just in case. Java 8 stream doesn't allow to modify the pointer itself and hence if you declare just count as a variable and try to increment within the stream it will never work and throw a compiler exception in the first place. Create a pointer to memory (a new obj in this case) and have the property of the object modified. This example shows how to replace all elements after a modification: Modifying Java ArrayList while iterating over it, Java - adding elements to list while iterating over it. this does not work when adding to a list (if 'c' is a List<>) I get java.util.ConcurrentModificationException. Within the for loop you are changing the value of str, but not doing anything else with it. Is there a distinction between the diminutive suffixes -l and -chen? Does being overturned on appeal have consequences for the careers of trial judges? have you tried it before asking this question? (Ep. You would need to create a new Set with the new values. As the API documentation for peek shows, it is there to show intermediate level results for debugging. 2. In order to do that, you would have to look at the implementation of the iterator for ArrayList. To learn more, see our tips on writing great answers. Removes from the list the last element that was returned by, Returns the next element in the list and advances the cursor position. Would a different way of doing it, maybe using the set(E e) method of a ListIterator, be better? How to print and connect to printer using flutter desktop via usb? You can use ListLterator for modifying list during iterating,it provides more functionality than iterator. That list is identical to the original list. What does that mean? Let's begin with the approach traditionally used before Java 8. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. CopyOnWriteArrayList returns an iterator that does not support the remove() method. This might be a little late. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. What are solutions if you want to modify users? Decrementing index. As it was mentioned before - you can't modify original list, but you can stream, modify and collect items into new list. In the movie Looper, why do assassins in the future use inaccurate weapons such as blunderbuss? Ok, I searched, what's this part on the inner part of the wing on a Cessna 152 - opposite of the thermometer, Non-definability of graph 3-colorability in first-order logic. rev2023.7.7.43526. java - Removing elements on a List while iterating through it - Code Easy solution is to create a copy of the list and iterate through that. Thanks for contributing an answer to Stack Overflow! :). 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). I think this will throw a ConcurrentModificationException because java does not like it when you add/remove items from a collection it is iterating over. Eg:- If you want to remove all even numbers from a list, you can do it as follows. Java 8 How to store multiple values for single key in HashMap . How to modify a Collection while iterating using for-each loop without ConcurrentModificationException? This was a simple example based on different code where the iteration variable is used. Using Thread.sleep() cannot reliably force an overlap between two threads because the kernel can always decide to schedule threads arbitrarily after they awaken. To safely update items in the list, use map(): To safely remove items in place, use filter(): Thanks for contributing an answer to Stack Overflow! If I got it correctly, the output should be either three items printed or some of them printed and an exception, no matter what. Notice that the above snippet is not modifying the list's structure - meaning: no elements are added or removed and the lists' size remains constant. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site. (*) Unless it's a LinkedList, which excels here. Note: Instead of using a while-loop it can also be written as: If you want to mutate the existing list, removeIf is the solution I would go with. @Rad, Your conclusion is logically sound given the assumption that the iterator's. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. I suspect the answer is simply that you've got a race condition between the different threads. is it possible to modify all elements of a list in java? yes but not with one liner. How can I use a map to modify existing elements in a stream? 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. Here is simple example how to modify string element. But what about changing the elements in a List? Notice that the above snippet is not modifying the list's structure - meaning: no elements are added or removed and the lists' size remains constant. A+B and AB are nilpotent matrices, are A and B nilpotent? Find centralized, trusted content and collaborate around the technologies you use most. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. EDIT: got it. Scripting on this page tracks web page traffic, but does not change the content in any way. There is nothing wrong with the idea of modifying an element inside a list while traversing it (dont modify the list itself, thats not recommended), but it can be better expressed like this: At the end the whole list will have the letter "D" as its content. 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), java stream mutate data with terminal operation, Split a list into sublists based on a condition with Stream api, How to modify an element of the Stream based on the value in a HashMap, Add an element to the list if it doesn't find it with lambda. Connect and share knowledge within a single location that is structured and easy to search. Can the Secret Service arrest someone who uses an illegal drug inside of the White House? I understand I wasn't using the iteration variable. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. - Jon Skeet Jul 25, 2014 at 7:54 It would really help if you could make this question self-contained. Is there a legal way for a country to gain territory from another through a referendum? What is the significance of Headband of Intellect et al setting the stat to 19? Let's look at the alternatives: Iterating over a copy, removing from original 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. Accidentally put regular gas in Infiniti G37. Thou Shalt Not Modify A List During Iteration - Unspecified Behaviour Supposedly something like this (illegal code): But we all know that the above code is not allowed. For example, what if we have 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. But what about changing the elements in a List?
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