How do ConcurrentHashMap and HashMap containers differ in behavior and characteristics?

HashMap and ConcurrentHashMap are implementations of the Map interface in Java, but they differ in behavior in a multithreaded environment:

• HashMap is not thread-safe. If multiple threads modify a HashMap simultaneously without external synchronization, it can lead to unpredictable behavior, including data loss or infinite loops.

• ConcurrentHashMap is a thread-safe implementation optimized for concurrent access. It uses internal mechanisms like segmentation or CAS operations to ensure high performance under concurrent access.

Main differences:

• Operations on HashMap are not synchronized, whereas on ConcurrentHashMap they are synchronized at the segment or individual element level.
• ConcurrentHashMap does not allow null keys or values, but HashMap does.
• During iteration, ConcurrentHashMap provides weak consistency (may see changes made after the start of iteration), whereas HashMap does not.

Example of using ConcurrentHashMap:

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

Use ConcurrentHashMap when safe concurrent access to the map from multiple threads is required without external synchronization.