Hashtable vs HashMap: Thread Safety, Null Handling, and Performance Explained

Thread Safety

Hashtable : uses synchronized methods, ensuring thread safety but causing significant performance overhead due to lock contention.

HashMap : not synchronized, thus not thread‑safe; to achieve safety you can wrap it with Collections.synchronizedMap(new HashMap<>()) or use ConcurrentHashMap for better concurrency.

Null Value Handling

Hashtable : does not allow null keys or values; inserting null throws NullPointerException.

HashMap : permits one null key and multiple null values.

Inheritance and Interface Implementation

Hashtable : extends the legacy Dictionary class, which is deprecated in modern Java.

HashMap : extends AbstractMap and implements the Map interface, offering a more modern design.

Capacity and Resizing Mechanism

Hashtable : default initial capacity 11, load factor 0.75; expands linearly, leading to frequent reallocations and lower efficiency.

HashMap : default initial capacity 16, load factor 0.75; doubles capacity when size reaches 75% of current capacity, providing better space utilization and query performance.

Iterator

Hashtable : iterator is thread‑safe because it acquires the same lock used for modifications.

HashMap : iterator is not thread‑safe; concurrent modifications cause ConcurrentModificationException. Use external synchronization or ConcurrentHashMap in multithreaded contexts.