When should you use ConcurrentHashMap or another concurrent collection?

Chooses concurrent collections by required atomic operations and iteration semantics rather than treating them as synchronized replacements.

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TL;DR

ConcurrentHashMap supports concurrent reads and updates without one global lock around every operation. Methods such as putIfAbsent, compute, and merge provide atomic operations for a key and are safer than a separate check followed by a write. Its iterators are weakly consistent: they do not throw ConcurrentModificationException and may reflect some updates during traversal. Null keys and values are not allowed.

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Concurrent collection contracts, compound operations, weakly consistent traversal, key behavior, and bounded-state design.

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Overview
A concurrent collection makes documented operations safe during concurrent access, but callers still need an atomic application-level protocol.
ConcurrentHashMap supports concurrent reads and updates without one global lock around every operation. Methods such as putIfAbsent, compute, and merge provide atomic operations for a key and are safer than a separate check followed by a write. Its iterators are weakly consistent: they do not throw ConcurrentModificationException and may reflect some updates during traversal. Null keys and values are not allowed.
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Using contains-then-put instead of an atomic map method.
Assuming iteration is a fixed snapshot.
Treating concurrent as bounded or transactionally consistent.

When should you use ConcurrentHashMap or another concurrent collection?

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