What is the difference between HashMap and ConcurrentHashMap?

HashMap and ConcurrentHashMap are implementations of hash tables in Java, but with different characteristics in terms of thread safety and performance.

HashMap:

• Not thread-safe.
• Concurrent access from multiple threads without external synchronization can lead to unpredictable behavior, including data loss or infinite loops.
• Used in single-threaded or synchronized contexts.

ConcurrentHashMap:

• Thread-safe and optimized for concurrent access.
• Allows multiple threads to read and write without locking the entire structure.
• Uses internal segmentation mechanisms or other techniques (depending on Java version) to minimize locking and improve performance.

Example of using ConcurrentHashMap:

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

In summary, the main difference is that ConcurrentHashMap supports concurrent access without external synchronization, whereas HashMap is not designed for multi-threaded use without additional measures.