| Synchronization | Not synchronized (not thread safe) | Internally synchronized (thread safe) |
| Null Elements | Allows one null key and any number of null values | Doesn’t allow null keys and null values |
| Iterators | Returns fail-fast iterators | Returns fail-safe Enumerations |
| Class Extension | Extends AbstractMap class | Extends Dictionary class |
| Traversal | Returns only iterators | Returns both Iterator and Enumeration |
| Speed | Fast | Slow |
| Legacy Class | Not a legacy class | Is a legacy class |
| Preferred Use | Preferred in single-threaded applications; for multi-threaded, wrap using Collections.synchronizedMap() | Not preferred nowadays for multi-threaded applications; ConcurrentHashMap is a better option |