Planned lessonMID-005-L02

ConcurrentHashMap

Lesson 2 of 5
Mid-level

On This Lesson 4 topics

In this lesson
Planned topics
Draft outline details
Practice

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

use atomic map operations correctly.

Planned topics

Fine-grained internal locking and lock-free reads allow high concurrency.
putIfAbsent, computeIfAbsent, compute, merge are atomic per key.
Keep the function passed to compute/merge short, side-effect free, and non-blocking; do not update the same map inside it.
No null keys or values.
size() and iteration are weakly consistent; they give a snapshot-like view, not a frozen one. Do not use them for exact decisions.

Draft outline details

Demonstration

java

Map<String, LongAdder> views = new ConcurrentHashMap<>();
views.computeIfAbsent(sku, k -> new LongAdder()).increment();
Map<String, Integer> stock = new ConcurrentHashMap<>();
stock.merge(sku, 1, Integer::sum);

Check your understanding

Which call replaces get-then-put counting?

Show answer

merge(key, 1, Integer::sum).

Practice

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