Explain what a ConcurrentHashMap is and how it ensures safety during parallel data access.

ConcurrentHashMap is a thread-safe implementation of the Map interface in Java, designed for efficient operation in a multithreaded environment. Unlike a regular HashMap, which is not synchronized, ConcurrentHashMap allows multiple threads to read and modify the map simultaneously without locking the entire structure.

Safety during concurrent access is achieved through internal segmentation of the map or by using modern synchronization mechanisms (in newer Java versions, CAS — compare-and-swap, and other optimizations are used). This allows locking only specific parts of the map during write operations, while read operations occur without locks, increasing performance.

Example usage:

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("key1", 1);
int value = map.get("key1");

In this example, multiple threads can safely call put and get simultaneously without risking inconsistent data or exceptions.