for-each | BestHub

Java Captain

Nov 12, 2023 · Fundamentals

Why HashMap.keySet() Traversal Involves Two Passes and How entrySet()/Map.forEach() Is More Efficient

This article explains why iterating a Java HashMap via keySet() incurs two passes—once to obtain an iterator and again to fetch values—while entrySet() or Map.forEach() performs a single pass, detailing the underlying iterator implementations and their bytecode behavior.

HashMapIteratorJava

0 likes · 9 min read

Why HashMap.keySet() Traversal Involves Two Passes and How entrySet()/Map.forEach() Is More Efficient

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