Would it be possible for a civilization to create machines before wheels? References, when reassigned, don't change the actual object it refers to. speaking, impossible to make any hard guarantees in the presence of To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Create a pointer to memory (a new obj in this case) and have the property of the object modified. I know it's not really long, but is there a better way to do this? When are complicated trig functions used? To get rid from ConcurrentModificationException Use CopyOnWriteArrayList. What is the significance of Headband of Intellect et al setting the stat to 19? Critical issues have been reported with the following SDK versions: com.google.android.gms:play-services-safetynet:17.0.0, Flutter Dart - get localized country name from country code, navigatorState is null when using pushNamed Navigation onGenerateRoutes of GetMaterialPage, Android Sdk manager not found- Flutter doctor error, Flutter Laravel Push Notification without using any third party like(firebase,onesignal..etc), How to change the color of ElevatedButton when entering text in TextField. Scripting on this page tracks web page traffic, but does not change the content in any way. 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. @JonSkeet He is talking about the code which he posted as an answer I suppose. Does being overturned on appeal have consequences for the careers of trial judges? An iterator for lists that allows the programmer I made the comment only because you opened by trying to explain why the exception occurs at all. Whats the point of adding the elements to a new list? Thank you and ZouZou for your answers. How can I learn wizard spells as a warlock without multiclassing? Use CopyOnWriteArrayList So performance considerations listed in other posts still hold. 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. "But I'm not modifying the set, I am modifying the objects within it", which leads to problem two. 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. Learn several ways of iterating over Java 8 Streams using indices Read more 2. for Loop First, let's review some for loop options. How to troubleshoot crashes detected by Google Play Store for Flutter app, Cupertino DateTime picker interfering with scroll behaviour. There are several ways to iterate over List in Java. Otherwise I cannot count the exception in such situation. Were Patton's and/or other generals' vehicles prominently flagged with stars (and if so, why)? The notable exception to this are streams whose sources are concurrent collections, which are specifically designed to handle concurrent modification. As it was mentioned before - you can't modify original list, but you can stream, modify and collect items into new list. In Java, can you modify a List while iterating through it? However, this approach is the most efficient one, especially for LinkedList where it is \$O(n)\$ (it's \$O(n^2)\$ for ArrayList because it has to copy array data on each remove(index) call). Submit a bug or feature For further API reference and developer documentation, see Java SE Documentation. This not only solves the problem unambiguously, but avoids the gotchas of not knowing how an iterator is going to act. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. The best answers are voted up and rise to the top, Not the answer you're looking for? Would a room-sized coil used for inductive coupling and wireless energy transfer be feasible? 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. 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. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. 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. Iterating using Iterator/ListIterator allows to add/remove element and these modification (add/remove) is reflected in the original List. There is nothing wrong with the idea of modifying an element inside a list while traversing it (don't modify the list itself, that's not recommended), but it can be better expressed like this: for (int i = 0; i < letters.size (); i++) { letters.set (i, "D"); } At the end the whole list will have the letter "D" as its content. Thank you and ZouZou for your answers. However, internally (looking at the JDK code) it seems to still use an iterator and call iterator.remove(). All rights reserved. @Augustas It all depends on how you want to modify this list. 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. (Ep. if you add a element into list here, you get infinity loop: myList.add(current + "one more"); @yaroslavTir yes? Java - How to modify all elements of a List? 15amp 120v adaptor plug for old 6-20 250v receptacle? 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. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Obviously you need to make sure there is an end condition (like with any recursive code or queue processing). There is nothing wrong with the idea of modifying an element inside a list while traversing it (don't modify the list itself, that's not recommended), but it can be better expressed like this: At the end the whole list will have the letter "D" as its content. Java 8 How to check whether a number exists in an Arrays or List or Stream ? How can I use a map to modify existing elements in a stream? Asking for help, clarification, or responding to other answers. I didn't get what your conversation is about ;) Anyway the code is added. rev2023.7.7.43526. What is the verb expressing the action of moving some farm animals in a field to let them eat grass or plants? when you construct a string, say new String("hello"), you can't further modify it's inner value. Java: adding elements to a collection during iteration. 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. I like it! Do I have the right to limit a background check? There might be some trick with ListIterator, but the easiest solution is probably an old style index loop. Iterator Methods Iterator interface has a total of 4 methods but initially, it was introduced with 3 methods but java 8 has come up with a new method. If it is, then the exception is expected. 2.1 boolean hasNext() This method tells us whether the collection has the next element to fetch. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Do I remove the screw keeper on a self-grounding outlet? Countering the Forcecage spell with reactions? See. shouldn't ".map(f -> new Fruit(f.getId(), f.getName() + "s", f.getCountry())" line have one more closing ")" at the end. But here is one of the usage. When to Use a Parallel Stream in Java | Baeldung Commercial operation certificate requirement outside air transportation. Modifying Objects within stream in Java8 while iterating Use CopyOnWriteArrayList @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? Would a different way of doing it, maybe using the set(E e) method of a ListIterator, be better? 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." Java 8 - How to merge/concatenate/join two lists into single list ? My impression is that Java is not particularly suitable for FP. When are complicated trig functions used? Could I modify an element using enhanced for loop - Coderanch and if you want to remove it, do the following: Java 8's stream() interface provides a great way to update a list in place. 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. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. (Ep. To safely update items in the list, use map (): List<String> letters = new ArrayList <> (); // add stuff to list letters = letters.stream ().map (x -> "D" ).collect (Collectors.toList ()); To safely remove items in place, use filter (): But its performance in all other scenarios is so terrible that it should be practically never used. You would have to consider every possible interleaving of the other thread code with the main thread's code to draw any conclusion about that. In the movie Looper, why do assassins in the future use inaccurate weapons such as blunderbuss? So, I'm not talking about modifying the object stored at an element; I'm talking about changing what the object is. For example, what if we have Return Value: This method returns an unmodifiable view of the specified collection. It would really help if you could make this question self-contained. Java 8 introduced the removeIf() method to the Collection interface. Asking for help, clarification, or responding to other answers. Removes from the list the last element that was returned by, Returns the next element in the list and advances the cursor position. It doesn't matter if we use map, foreach or other stream methods. (Ep. Why free-market capitalism has became more associated to the right than to the left, to which it originally belonged? Is there any potential negative effect of adding something to the PATH variable that is not yet installed on the system? No stream at all: just for modifying certain property from object collection you could directly use forEach with a collection as follows, We can change the property via map without creating new objects. So for example, if you change your in-loop sleep to Thread.sleep(2500) then you'll get the exception at the start of the second iteration, because the remove() call will occur before it. Invitation to help writing and submitting papers -- how does this scam work? Do I have the right to limit a background check? to detect bugs. rev2023.7.7.43526. 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. But what about changing the elements in a List? EDITED the code to produce the exception, please note the list content: The behavior you are trying to reproduce is highly timing-dependent. 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. @emory Yap. How can I learn wizard spells as a warlock without multiclassing? Why QGIS does not load Luxembourg TIF/TFW file? Connect and share knowledge within a single location that is structured and easy to search. Why did Indiana Jones contradict himself? What is the reasoning behind the USA criticizing countries and then paying them diplomatic visits? What I would like to know is to modify the elements containing in the Set. "When to use LinkedList over ArrayList?" Which may not be acceptable in most cases. Java 8 introduced the default method removeIf on the Collection interface. When to use LinkedList over ArrayList in Java? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Convert ArrayList With Quotes Read from an external txt file to ArrayList Without the Quotes Using Java 8, Modify property value of the objects in list using Java 8 streams, How to modify a field in each element of list in java 8 using stream, Java 8 - Stream - value update while collecting, Java 8 streams - modifying all elements in a group, Java stream: Difficulty in object property update, Modify objects while iterating 2 Lists using java stream, Using streams Java 8 to modify in List Java. Does not actually modify the existing list, so if references to the list are spread around various variables, you still have some old elements that just shouldn't be in that list. the list during iteration, and obtain the iterator's It also allows to replace the current element via set () method. Maybe I'm too greedy ;) Could you please update the answer so I can accept it. Countering the Forcecage spell with reactions? Different maturities but same tenor to obtain the yield. Fail-fast operations throw 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. Streams will make it in only one loop. What is the number of ways to spell French word chrysanthme ? 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. would be returned by. I am coming from Python. 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. The only down-side of this approach is that you need to switch your for-each to a while. There is nothing wrong with the idea of modifying an element inside a list while traversing it (don't modify the list itself, that's not recommended), but it can be better expressed like this: At the end the whole list will have the letter "D" as its content. I think you can use nums.removeAll(toRemove), @maaartinus True, but it does depend a little bit on how, It used to be a native call when Java was terribly slow. There are several ways to do this. But when you only know that you have a. @BhaveshDangi sort action will be done in plural list. We'll cover a few examples using a while loop, Java 8, and a few common libraries. Peek method should only be used for debugging. How does the theory of evolution make it less likely that the world is designed? My boss claims this code is fine (and it does appear to work), but I still don't think it is correct. 10 Answers Sorted by: 116 Yes, you can modify state of objects inside your stream, but most often you should avoid modifying state of source of stream. So, I'm not talking about modifying the object stored at an element; I'm talking about changing what the object is. Thanks. yes but not with one liner. This means that if we are working with it, we can use ideas of functional programming to achieve the same results again: List<Integer> integers = newArrayList(1, 2, 3); integers.removeIf(i -> i == 2); assertThat(integers).containsExactly(1, 3); to traverse the list in either direction, modify Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. A simple way to do this is to convert to a list and back: This is clearly a bit more work than you would expect, but if mass-replacing is behavior you anticipate then probably don't use a set. is it possible to modify all elements of a list in java? Supposedly something like this (illegal code): But we all know that the above code is not allowed. For example, in Java to modify a collection when another thread is iterating over it. If you want to use a list in multiple threads and at least one of them is modifying it, you should use an implementation which supports that, such as CopyOnWriteArrayList. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Connect and share knowledge within a single location that is structured and easy to search. Connect and share knowledge within a single location that is structured and easy to search. I was just trying to indicate that the list was changing during iteration. Java - How to remove items from a List while iterating? This method may be called repeatedly to Has a bill ever failed a house of Congress unanimously? Why free-market capitalism has became more associated to the right than to the left, to which it originally belonged? @Jordan Silva: I would use map() rather than abuse peek(): map(f -> { f.setName(f.getName() + "s"); return f;}). To learn more, see our tips on writing great answers. You can't modify a Collection while iterating over it using an Iterator, except for Iterator.remove (). In fact, the Javadoc for the Exception addresses this point very specifically. Is it possible to achieve this in some other way? In Java, can you modify a List while iterating through it? For the remaining task, you dont need a stream: @SergeyLagutin and @Apostolos thanks for your reply. Verify performance isn't an issue (no linked lists - but ArrayList is fine). Languages which give you access to the AST to modify during compilation? Characters with only one possible next character. Therefore, it Of course, the developer has to understand how it works and use it properly. position backwards. But generally you should avoid. Code Review Stack Exchange is a question and answer site for peer programmer code reviews. Do we have any way to achieve same behavior using Java 8 streams ? Does being overturned on appeal have consequences for the careers of trial judges? You cannot modify a String in java, they are immutable. If it is not modified the third item should be printed. To learn more, see our tips on writing great answers. I looked in javadoc for List, but it's not there. Using Thread.sleep() cannot reliably force an overlap between two threads because the kernel can always decide to schedule threads arbitrarily after they awaken. rev2023.7.7.43526. Here is simple example how to modify string element. This method may be called repeatedly to iterate through the list, I was just trying to indicate that the list was changing during iteration. More generally, looking for a ConcurrentModificationException is not a reliable way of detecting multiple threads modifying and reading an object simultaneously. 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). Concepts of modifying the element in collection while iterating? One workaround is to iterate backward in the list, which does not skip anything. In this article, we will discuss and learn with different illustrations on how to add/remove an element to List/ArrayList while iterating, First, we will understand what happens when we are iterating and modifying the List/ArrayList at the same time. 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), 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. Java 8 Find First and Last entries in a Map or HashMap ? It's completing the iteration between the second and third items being added. When are complicated trig functions used? Connect and share knowledge within a single location that is structured and easy to search. So this approach isn't any better, but it's a different way to do it. would be wrong to write a program that depended on this exception for If that's a problem, you'll have to maintain a flag of some sort to indicate this edge case. The element is inserted immediately before the element that Book set in a near-future climate dystopia in which adults have been banished to deserts. A. The second option is to use string builders instead of strings, which are mutable string creators/placeholders. Basic for Loop There are several workarounds to deal with this problem. For instance, ["apple", "orange"] to ["iapple", "iorange"]. or intermixed with calls to, Returns the previous element in the list and moves the cursor 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. And if you wanted to collect the removes: Out of the other presented solutions, all but the one using Java 8 seem to be O(n**2), i.e., too slow when the list(*) gets a bit bigger. How does the theory of evolution make it less likely that the world is designed? A+B and AB are nilpotent matrices, are A and B nilpotent? This is the approach I would recommend in most cases. ArrayList provides the remove() methods, e.g. Modifying an Array list while I am iterating over it, In need of iterating and modifying arraylist (or similar) at the same time. Do you mind providing code example? Iterator<Integer> iterator = Arrays.asList ( 1, 2, 3 ).iterator (); 2. The for-each loop sets the value of str to each element. From my understanding the ListIterator.add() adds an element before the current element in the list, not after it. Java 8 provides a safer method to conditionally remove items from stream using a filter Remove elements from collection using Java 8 Lambda expression List<String> names = new ArrayList (asList ( "munish", "ravneesh", "rajneesh" )); names.removeIf (name -> "munish" .equalsIgnoreCase (name)); System.out.println ( "names = " + names); total useless. Any performance difference between using Streams and forEach in this case? Eg:- If you want to remove all even numbers from a list, you can do it as follows. Yep. To do structural modification on the source of the stream, as Pshemo mentioned in his answer, one solution 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 operations on the primary list. 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! 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). 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. (Ep. Is there a distinction between the diminutive suffixes -l and -chen? If you wanna create new list, use Stream.map method: If you wanna modify current list, use Collection.forEach: You can use just forEach. Instead of modifying the collection I would suggest using, That is actually correct, but: this depends on the specific class that is used here. 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 :) ). 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. How to Use Iterator in Java? Java 8 Iterator Examples on ArrayList EDIT: got it. This class is designed for observer lists, which are rarely modified and often traversed. this does not work when adding to a list (if 'c' is a List<>) I get java.util.ConcurrentModificationException. Simply replacing one element by another doesnt count as a structural modification. That list is identical to the original list. In Java, can you modify a List while iterating through it? List users: Yes, you can modify state of objects inside your stream, but most often you should avoid modifying state of source of stream. How to modify a Collection while iterating using for-each loop without ConcurrentModificationException? Can we use work equation to derive Ohm's law? 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. @TheNewIdiot: The code in the related question already does. Why did Indiana Jones contradict himself? My boss claims this code is fine (and it does appear to work), but I still don't think it is correct. Method for modifying an element in an Array list? you need to use. Find centralized, trusted content and collaborate around the technologies you use most. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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. The size of the List is not being changed, but the object at the index is changing, so technically the List is being modified. 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. 1. How can I remove a mystery pipe in basement wall and floor? It's no magic bullet, anymore. It might be the right question: Iterator implementation. Simply replacing one element by another doesn't count as a structural modification. Examples: Input: HashMap: {1=Geeks, 2=ForGeeks, 3=GeeksForGeeks}, value = "ForGeeks" Output: {1=Geeks, 3=GeeksForGeeks} Input: HashMap: {1=G, 2=e, 3=e, 4=k, 5=s}, value = k Output: {1=G, 2=e, 3=e, 5=s} Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing.
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