Differences Between Java HashMap and Hashtable

HashMap and Hashtable comparisons are common Java interview questions that test a programmer's understanding of collection classes and their appropriate usage.

This article examines both the differences and similarities between HashMap and Hashtable.

Differences Between HashMap and Hashtable

Both classes implement the Map interface, but they differ mainly in thread safety, synchronization, and speed.

HashMap is almost equivalent to Hashtable except that HashMap is non‑synchronized and permits null keys and values, while Hashtable does not allow nulls.

HashMap is non‑synchronized; Hashtable is synchronized, making it thread‑safe for multiple threads to share. Java 5 introduced ConcurrentHashMap as a more scalable alternative to Hashtable.

HashMap’s iterator is fail‑fast, throwing ConcurrentModificationException if another thread modifies the map structurally, whereas Hashtable’s enumerator is not fail‑fast. This reflects the broader difference between Enumeration and Iterator.

Because Hashtable is synchronized, it is slower than HashMap in single‑threaded environments; when synchronization is unnecessary, HashMap offers better performance.

HashMap does not guarantee that the order of its elements remains constant over time.

Important Terminology

1) synchronized means only one thread can modify a Hashtable at a time; a thread must acquire the lock before updating, and other threads must wait for the lock to be released.

2) Fail‑safe relates to iterators. If an Iterator or ListIterator is created and another thread structurally modifies the collection, a ConcurrentModificationException is thrown. Non‑structural changes via set() are allowed, but further structural changes after a set() may cause an IllegalArgumentException .

3) A structural change refers to inserting or removing an element, which alters the map’s internal structure.

Can We Synchronize a HashMap?

HashMap can be synchronized using the following statement:

Map m = Collections.synchronizeMap(hashMap);

Conclusion

Hashtable and HashMap differ primarily in thread safety and speed. Use Hashtable only when full thread safety is required; otherwise, for Java 5 or later, prefer ConcurrentHashMap for concurrent scenarios.