How do HashMap and ConcurrentHashMap differ in Java in terms of functionality and usage in multithreaded segments?

HashMap and ConcurrentHashMap differ in how they operate in a multithreaded environment:

• HashMap is not thread-safe. Concurrent access from multiple threads without external synchronization can lead to data integrity issues and even infinite loops.

• ConcurrentHashMap is a thread-safe implementation that allows multiple threads to read and write simultaneously without locking the entire collection. It uses segmented locking or other mechanisms (depending on the Java version) to improve performance in a multithreaded environment.

Use HashMap in single-threaded scenarios or with external synchronization, and ConcurrentHashMap when efficient concurrent access without locking the entire collection is required.

Example of using ConcurrentHashMap:

import java.util.concurrent.ConcurrentHashMap;

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("key1", 1);
map.putIfAbsent("key2", 2);

// Safe for multithreaded access
int value = map.get("key1");