How do HashTable and ConcurrentHashMap data structures differ in characteristics and operation principles?

HashTable and ConcurrentHashMap are data structures for storing key-value pairs, but they differ in characteristics and ways of handling multithreading.

• HashTable:

  • Synchronized (thread-safe) collection, all methods are synchronized.
  • Uses locking on the entire object, which can lead to bottlenecks under high contention.
  • Deprecated, modern applications more often use ConcurrentHashMap.

• ConcurrentHashMap:

  • More modern and efficient implementation for concurrent access.
  • Uses segmented locking or other mechanisms (in newer Java versions — CAS operations and internal structures), allowing multiple threads to read and write simultaneously without full locking.
  • Provides high performance under concurrent access.

Summary: If you need a thread-safe dictionary with high performance, it is better to use ConcurrentHashMap, while HashTable is used for compatibility with legacy code.