What is the difference between ConcurrentHashMap and Collections.synchronizedMap?

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**ConcurrentHashMap** — fine-grained locking, better concurrency. **synchronizedMap** — single lock for all operations. ```java // ConcurrentHashMap — concurrent reads and writes ConcurrentHashMap map = new ConcurrentHashMap<>(); map.put("key", 1); // Lock on bucket only map.compute("key", (k, v) -> v + 1); // Atomic operation // Collections.synchronizedMap — blocks entire map Map syncMap = Collections.synchronizedMap(new HashMap<>()); syncMap.put("key", 1); // Lock on entire map ``` **Key differences:** - CHM: no null keys/values, better throughput. - synchronizedMap: wraps any map, blocks all reads during writes. - CHM: iterators are weakly consistent (no ConcurrentModificationException). - synchronizedMap: must manually synchronize iteration.

Java: What is the difference between ConcurrentHashMap and Collections.synchronizedMap?