Beyond the collections overview
You already know ConcurrentHashMap exists from S06. Here is what senior loops ask:
- No global lock on
get— reads are usually lock-free. - Updates lock only a bin (bucket), not the entire map.
size()is an estimate under concurrency — do not use it as an exact audit in a hot loop.
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Still to come · 2 sections
- computeIfAbsent — cache pattern
- merge for counters
Test yourself
Answer these before moving on — recall is what makes it stick.