Hash Map and Hash Table in Java

Question

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

Prabhu Raja · 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);

Satyapriya Nayak · 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

Javeed M Shaikh · Answer

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".