Hash Map and Hash Table in Java

Mohan Rathour

15y3kJava

Hi,
What is the difference between Hash Map and Hash Table in Java ?

3 Replies

Know the answer? Post it — somebody with the same question will find it here.

Prabhu Raja15y ago
Accepted answer
Hi,

Hope it will useful for you.

Hashtable

Hashtable is basically a datastructure to retain values of key-value pair.

It didn't allow null for both key and value. You will get NullPointerException if you add null value.
It is synchronized. So it comes with its cost. Only one thread can access in one time

Hashtable; cityTable = new Hashtable();
cityTable.put(1, "Lahore");
cityTable.put(2, "Karachi");
cityTable.put(3, null); /* NullPointerEcxeption at runtime*/

System.out.println(cityTable.get(1));
System.out.println(cityTable.get(2));
System.out.println(cityTable.get(3));

HashMap

Like Hashtable it also accepts key value pair.

It allows null for both key and value
It is unsynchronized. So come up with better performance

HashMap productMap = new HashMap();
productMap.put(1, "Keys");
productMap.put(2, null);
0
Satyapriya Nayak15y ago
Accepted answer
Hi Mohan,

1.Hashtable is synchronized while access to the HashMap is not synchronized.
2.HashMap allows null values as Key, while Hashtable doesn't.
3.Iterator in a HashMap is fail-safe(Fail-safe is relevant from the context of iterators.If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally",a concurrent modification exception will be thrown) while the enumerator of HashTable is not.

Thanks
0
Javeed M Shaikh15y ago
Hi Mohan,

1. Hashtable is synchronized and whereas HashMap is not.
2. Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.

Please do not forget to mark "Accepted Answer".
0