Concurrent modification exception

I have an ArrayList that I want to iterate over. While iterating over it I have to remove elements at the same time. Obviously this throws a java.util.ConcurrentModificationException. What is the best practice to handle this problem? Should I clone the list first? I remove the elements not in the loop itself but another part of the code. My code looks like this: public class Test() a.doSomething might call Test.removeA(); Two options: • Create a list of values you wish to remove, adding to that list within the loop, then call originalList.removeAll(valuesToRemove) at the end • Use the remove() method on the iterator itself. Note that this means you can't use the enhanced for loop. As an example of the second option, removing any strings with a length greater than 5 from a list: List list = new ArrayList(); ... for (Iterator iterator = list.iterator(); iterator.hasNext(); ) From the JavaDocs of the ArrayList The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. You are trying to remove value from list in advanced "for loop", which is not possible, even if you apply any trick (which you did in your code). Better way is to code iterator level as other advised here. I wonder how people have not suggested traditional for loop approach. for...

• Instead of iterating through collections classes , we can iterate through the arrays. This works perfectly fine for smaller lists what about the bigger list? It’s very basic we know that if the array size is huge then it affects the performance. Hence, this method is only effective for smaller size arrays. • The next method is the Synchronized block method, Here we actually lock the list in a synchronized block to avoid the exception. Isn’t that cool? but guess what this is also not an effective method to avoid Exception Why? Because the purpose of multithreading is not being used. • The better way is we have Methods: Here two ways are proposed of which starting with the naive one and ending up with the optimal approach to reach the goal. • Using Loops: We used the Iterator remove() method instead of that we can use a for loop to avoid ConcurrentModificationException in a Single-threaded environment. If we add any extra objects then we can ensure that our code takes care of them. • Using the remove() Method: We can use the remove method to remove the object from the collection. Here there is a problem that is you can only remove the same object and not any other from the list Example 1:

Prerequisites: The ConcurrentHashMap class is introduced in JDK 1.5 belongs to java.util.concurrent package, which implements ConcurrentMap as well as to Serializable interface also. ConcurrentHashMap is an enhancement of HashMap as we know that while dealing with Threads in our application HashMap is not a good choice because performance-wise HashMap is not up to the mark. ConcurrentHashMap is a thread-safe implementation of the Map interface in Java, which means multiple threads can access it simultaneously without any synchronization issues. It’s part of the java.util.concurrent package and was introduced in Java 5 as a scalable alternative to the traditional HashMap class. One of the key features of the ConcurrentHashMap is that it provides fine-grained locking, meaning that it locks only the portion of the map being modified, rather than the entire map. This makes it highly scalable and efficient for concurrent operations. Additionally, the ConcurrentHashMap provides various methods for atomic operations such as putIfAbsent(), replace(), and remove(). Output Map size: 3 Value of A: 1 Map size: 2 Key points of ConcurrentHashMap: • The underlined data structure for ConcurrentHashMap is • ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications. • At a time any number of threads are applicable for a read operation without locking the ConcurrentHashMap object which is not there in HashMap. • In ConcurrentHashMap...

ConcurrentModificationException has thrown by methods that have detected concurrent modification of an object when such modification is not permissible. If a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this ConcurrentModificationException. Here we will be understanding this exception with an example of why it occurs and how changes are made simultaneously which is the root cause for this exception. In the later part, we will understand how to fix it up. Example 1: ConcurrentModificationException Output Explanation: ConcurrentModificationException is thrown when the next() method is called as the iterator is iterating the List, and we are making modifications in it simultaneously. Now in order to avoid this exception so let us do discuss a way out of using iterator directly which is as follows: Example 2: Resolving ConcurrentModificationException

Apart from the ConcurrentModificationException is another nightmare for Java developers. What makes this error tricky is the word concurrent, which always mislead Java programmers that this exception is coming because multiple threads are trying to modify the collection at the same time. Then begins the hunting and debugging, they spent countless hours to find the code which has the probability of concurrent modification. While in reality, ConcurrentModficationException can also come in a single-threaded environment. To give you an example, just you broke the rule of not modifying a Collection during iteration. How does Java know to throw ConcurrentModificationExeption? It uses a modCount, which keeps track of how many times a list is modified structurally. Structural modifications are those that change the size of the list, which may affect the progress of iteration and may yield incorrect results. Both Iterator and ListIterator use this field to detect unexpected change. Other methods of List which structurally modify List also use this method like add(), remove(). Problem: loop over an ArrayList and remove selected elements, but remove() is throwing " Exception in thread "main" java.util.ConcurrentModificationException". Cause: The real cause of ConcurrentModfiicationException is inconsistent modCount. When you are next() method keep track of modCount. If you modify the collection by adding or removing elements then modCount will change and it will not match with the ex...

Closed 8 years ago. I have tried to reinstall Minecraft with Forge multiple times, but Minecraft still won't launch. When I check "Launcher Visibility" and change it to "Hide Launcher" and re-open, I get this error log in the console: [13:20:11] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker [13:20:11] [main/INFO] [LaunchWrapper]: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker [13:20:11] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker [13:20:11] [main/INFO] [FML]: Forge Mod Loader version 7.10.18.1180 for Minecraft 1.7.10 loading [13:20:11] [main/INFO] [FML]: Java is Java HotSpot(TM) 64-Bit Server VM, version 1.8.0_20, running on Windows 7:amd64:6.1, installed at C:\Program Files\Java\jre1.8.0_20 [13:20:11] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker [13:20:11] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker [13:20:11] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker [13:20:11] [main/ERROR] [LaunchWrapper]: Unable to launch java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(Unknown Source) ~[?:1.8.0_20] at java.util.ArrayList$Itr.remove(Unknown Source) ~[?:1.8.0_20] at net.minecraft.launchwrapper.Launch.launch(Launch.java:117) [launchwrapper-1.9.j...

The ConcurrentModificationException comes under RuntimeException, and may be thrown by the methods that have detected concurrent modification of an object when such modification is not allowed. When a thread is iterating over a Collection, generally it is not allowed for another thread to modify a Collection. The To understand the ConcurrentModificationException consider the following scenario:- TestCase 1:- import java.util.ArrayList; import java.util.List; import java.util.Iterator; public class TestCase1 Output:- size before modification :5 size after modification :6 So using the concurrent Collection classes or Synchronized Collection Method we can avoid the ConcurrentModificationException in the multi-threaded environment. Use Synchronized Collection Methods if you need to ensure data consistency and each thread needs to have an up-to-date view of the map. Use concurrent Collection classes if performance is critical, and each thread only inserts data, with reads happening less frequently. • ← JavaScript: an Overview • →