HashMap vs Hashtable in Java

HashMap vs. Hashtable in Java: Key Differences

For Java & Spring Boot developers, understanding the subtle differences between HashMap and Hashtable is crucial. Both implement the Map interface, storing key-value pairs, but differ significantly in synchronization and null handling.

Synchronization: Hashtable is synchronized, meaning it's thread-safe. Multiple threads can access it concurrently without data corruption. HashMap is not synchronized; concurrent access requires external synchronization (e.g., Collections.synchronizedMap()).

Null Values: HashMap allows one null key and multiple null values. Hashtable permits neither null keys nor null values.

Performance: HashMap's lack of synchronization makes it generally faster than Hashtable in single-threaded environments.

Example:

// HashMap example
HashMap<String, Integer> hashMap = new HashMap<>();
hashMap.put("one", 1);
hashMap.put("two", 2);
hashMap.put(null, 3); // Allowed


// Hashtable example
Hashtable<String, Integer> hashtable = new Hashtable<>();
hashtable.put("one", 1);
hashtable.put("two", 2);
//hashtable.put(null, 3); // Throws NullPointerException

In Spring Boot: Prefer HashMap for most cases unless thread safety is explicitly required. If concurrency is a concern, use ConcurrentHashMap which offers better performance than synchronized HashMap or Hashtable. Avoid Hashtable due to its performance overhead in single-threaded applications. Choose the data structure that best suits your specific needs – performance vs. thread safety.