ConcurrentModificationException

Architecture Digest

Mar 18, 2025 · Fundamentals

Understanding Java foreach Loop, HashMap Iteration, and ConcurrentModificationException

This article explains how Java's foreach loop is implemented using iterators, demonstrates the bytecode differences between array and collection traversal, analyzes why modifying a HashMap during foreach can trigger ConcurrentModificationException, and shows the correct way to safely modify collections with an iterator.

CollectionsConcurrentModificationExceptionHashMap

0 likes · 11 min read

Understanding Java foreach Loop, HashMap Iteration, and ConcurrentModificationException

Cognitive Technology Team

Jun 23, 2024 · Fundamentals

Understanding Fail-Fast Iterators and ConcurrentModificationException in Java Collections

The article explains why fail‑fast iterators throw ConcurrentModificationException when a collection is modified during iteration, outlines two common scenarios that trigger the exception, and presents both incorrect and correct Java code examples along with strategies such as avoiding modifications, using synchronization, iterator.remove(), or switching to fail‑safe concurrent collections.

CollectionsConcurrencyConcurrentModificationException

0 likes · 5 min read

Understanding Fail-Fast Iterators and ConcurrentModificationException in Java Collections

Architect's Guide

Jun 2, 2024 · Fundamentals

Pitfalls and Best Practices of Arrays.asList and ArrayList.subList in Java

This article explains how Java's Arrays.asList creates a fixed‑size list that throws UnsupportedOperationException on add/remove, and how ArrayList.subList returns a mutable view whose non‑structural changes affect both the original list and the sublist while structural changes can trigger ConcurrentModificationException, providing code examples and practical guidelines.

ArrayList.subListArrays.asListCollections

0 likes · 9 min read

Pitfalls and Best Practices of Arrays.asList and ArrayList.subList in Java

IT Services Circle

Jan 10, 2024 · Backend Development

Why Using forEach to Remove Elements from an ArrayList Throws ConcurrentModificationException and How to Delete Safely

The article explains why iterating an ArrayList with forEach and removing elements causes a ConcurrentModificationException, analyzes the underlying fail‑fast iterator behavior, and presents three correct ways—using Iterator.remove, removeIf, and collecting then removing—to delete elements safely.

ArrayListConcurrentModificationExceptionIterator

0 likes · 4 min read

Why Using forEach to Remove Elements from an ArrayList Throws ConcurrentModificationException and How to Delete Safely

Java Architect Essentials

Mar 15, 2023 · Backend Development

Why Deleting Elements from a Java List Inside a For Loop Is Unsafe and How to Do It Correctly

This article explains why deleting elements from a Java List while iterating with a for loop is unsafe, demonstrates the correct approach using an Iterator or removeIf, and analyzes common pitfalls of both classic for and enhanced foreach loops.

Best PracticesConcurrentModificationExceptionIterator

0 likes · 7 min read

Why Deleting Elements from a Java List Inside a For Loop Is Unsafe and How to Do It Correctly

macrozheng

Feb 20, 2023 · Fundamentals

8 Reliable Ways to Remove Elements from a Java List (And Why Some Fail)

This article examines eight common techniques for deleting elements from a Java List, explains why certain loop‑based approaches cause errors or unexpected results, and highlights the three reliable methods—including reverse for‑loops, iterator.remove, and Stream filtering—that work correctly.

CollectionConcurrentModificationExceptionIterator

0 likes · 9 min read

8 Reliable Ways to Remove Elements from a Java List (And Why Some Fail)

Cognitive Technology Team

Apr 4, 2022 · Fundamentals

Why Removing Elements from a HashMap During a For‑Each Loop Causes ConcurrentModificationException and How to Fix It

Iterating a HashMap with a for‑each loop while calling its remove method triggers a ConcurrentModificationException because the loop uses an internal iterator that becomes inconsistent with the map’s modCount, and the article explains the cause, shows decompiled code, and provides correct removal approaches using the iterator or removeIf.

CollectionsConcurrentModificationExceptionHashMap

0 likes · 6 min read

Why Removing Elements from a HashMap During a For‑Each Loop Causes ConcurrentModificationException and How to Fix It

Selected Java Interview Questions

Sep 30, 2021 · Fundamentals

Understanding the Role of modCount in Java HashMap's Fail‑Fast Mechanism

The article clarifies that the modCount field in java.util.HashMap is used to implement the fail‑fast iterator behavior, explains why the field was volatile in early JDK versions but not in later ones, and shows that ConcurrentModificationException can be thrown even in single‑threaded scenarios when the iterator detects structural changes.

ConcurrentModificationExceptionHashMapJDK

0 likes · 5 min read

Understanding the Role of modCount in Java HashMap's Fail‑Fast Mechanism

Selected Java Interview Questions

Jun 4, 2021 · Fundamentals

Why Removing Elements During forEach on an ArrayList Triggers ConcurrentModificationException and How to Fix It

The article explains that deleting elements from an ArrayList inside a forEach loop can cause a ConcurrentModificationException because the iterator's expected modification count diverges from the actual count, and it shows correct ways to remove items safely using an explicit Iterator or a CopyOnWriteArrayList.

ArrayListConcurrentModificationExceptionIterator

0 likes · 5 min read

Why Removing Elements During forEach on an ArrayList Triggers ConcurrentModificationException and How to Fix It

Top Architect

Mar 29, 2021 · Fundamentals

Pitfalls of Using Arrays.asList and ArrayList.subList in Java

This article explains common traps when using Java's Arrays.asList and ArrayList.subList methods, including why add operations throw UnsupportedOperationException, how subList creates a view that shares modifications with the original list, and the circumstances that lead to ConcurrentModificationException.

ArrayList.subListArrays.asListCollections

0 likes · 8 min read

Pitfalls of Using Arrays.asList and ArrayList.subList in Java

Java Captain

Mar 27, 2021 · Fundamentals

Pitfalls of Using Arrays.asList and ArrayList.subList in Java

This article explains common pitfalls when using Java's Arrays.asList and ArrayList.subList methods, illustrating why add operations cause UnsupportedOperationException, how subList creates a view that shares modifications with the original list, and provides best‑practice recommendations to avoid runtime errors.

ArrayList.subListArrays.asListCollections

0 likes · 8 min read

macrozheng

Jun 15, 2020 · Backend Development

Avoid ConcurrentModificationException: Safe Ways to Remove Elements While Iterating in Java

This article explains why removing items from a Java List inside a foreach loop triggers java.util.ConcurrentModificationException and demonstrates three reliable alternatives—using Iterator.remove(), a forward for‑loop with index adjustment, and a reverse for‑loop—to safely modify collections during iteration.

ConcurrentModificationExceptionIteratorJava

0 likes · 7 min read

Avoid ConcurrentModificationException: Safe Ways to Remove Elements While Iterating in Java

Java Captain

Jun 10, 2020 · Fundamentals

Understanding java.util.ConcurrentModificationException and Safe Ways to Remove Elements from a List

This article explains why using a foreach loop to remove elements from a Java List triggers java.util.ConcurrentModificationException, analyzes the underlying iterator mechanism, and presents three safe alternatives: iterator.remove(), forward-index loop with index adjustment, and reverse-index loop.

ArrayListCollectionConcurrentModificationException

0 likes · 6 min read

Understanding java.util.ConcurrentModificationException and Safe Ways to Remove Elements from a List

Architect's Tech Stack

Jun 7, 2020 · Backend Development

How to Avoid java.util.ConcurrentModificationException When Removing Elements from a List

This article explains why a foreach loop over an ArrayList can throw java.util.ConcurrentModificationException when removing elements, analyzes the underlying iterator mechanism, and presents three safe alternatives—using Iterator.remove(), a forward-index for loop with index correction, and a reverse for loop—to modify collections without errors.

ArrayListConcurrentModificationExceptionIterator

0 likes · 7 min read

How to Avoid java.util.ConcurrentModificationException When Removing Elements from a List

Selected Java Interview Questions

Apr 27, 2020 · Fundamentals

How to Remove Elements from a List Without Causing java.util.ConcurrentModificationException

This article explains why removing items from a Java List inside a foreach loop triggers a ConcurrentModificationException and demonstrates three correct approaches—using Iterator.remove(), a forward indexed for loop with index correction, and a reverse for loop—each with full code examples and output.

ArrayListConcurrentModificationExceptionIterator

0 likes · 6 min read

How to Remove Elements from a List Without Causing java.util.ConcurrentModificationException

FunTester

Aug 29, 2019 · Fundamentals

Avoid Removing Elements While Iterating: Java ConcurrentModificationException Example and Fix

During a product bug investigation, the author demonstrates how removing entries from a Java HashMap while iterating triggers a ConcurrentModificationException, provides the faulty code, shows the exception stack trace, and presents a safe fix that collects keys first and removes them after iteration.

BugFixConcurrentModificationExceptionHashMap

0 likes · 3 min read

Avoid Removing Elements While Iterating: Java ConcurrentModificationException Example and Fix

Qunar Tech Salon

Jun 3, 2015 · Fundamentals

Understanding HashMap ConcurrentModificationException and Using ConcurrentHashMap in Java

This article explains why iterating and removing entries from a HashMap throws ConcurrentModificationException, describes the internal iterator mechanism, and provides correct solutions using an Iterator or replacing HashMap with ConcurrentHashMap, including detailed explanations of ConcurrentHashMap's structure and its get, put, remove, size, and iteration implementations.

CollectionsConcurrentHashMapConcurrentModificationException

0 likes · 11 min read

Understanding HashMap ConcurrentModificationException and Using ConcurrentHashMap in Java