# Concurrent Collections — Advanced Java

Source: https://www.geekswithgeeks.com/en/advanced-java/adv-concurrent-collections

> Share data safely across threads with ConcurrentHashMap, BlockingQueue for producer/consumer handoff, and CopyOnWriteArrayList for read-heavy lists.

## Pick the right one

`ConcurrentHashMap` for shared maps (atomic `compute`, `merge`). `BlockingQueue` (`ArrayBlockingQueue`, `LinkedBlockingQueue`) for producer/consumer handoff. `CopyOnWriteArrayList` for read-heavy, rarely-written lists. Avoid `Collections.synchronizedMap` — it locks the whole map.

## Producer/consumer with BlockingQueue

`put()` blocks the producer when the queue is full, `take()` blocks the consumer when it's empty — no manual `wait`/`notify` needed.

```java
BlockingQueue<Job> queue = new LinkedBlockingQueue<>(100);

// producer
queue.put(job);

// consumer
Job job = queue.take();
```