Map object

[priyaguptan]

I learnt that Map is an interface that maps unique keys to values. A key is an object that we use to retrieve a value at a later date. Give a key and a value, we can store the value in a Map object..Now my confusion is if Map is an Interface how come it can be instantiated i.e.,how can we have Map as an object. Kindly clarify me.

[javacodez]

We are not instantiating the interfaces. We are actually instantiating classes that implement these interfaces. For eg: in our case to store a key value pair we actually use a Hashmap or Hashtable which implements the Map interface.

[albinjoseph]

We cannot instantiate an Interface. But we can instantiate a class that implement an interface. For eg: We have Hashtable or HashMap which implements the Map interface. So we can assign a Hashtable or HashMap to a Map object. For eg:

[java]
Map map = new HashMap();
[/java]

[priyaguptan]

Thanks Albin and javacodez .But I want to know what is the difference in the below two instantiations?

HashMap hm=new HashMap();

and

Map m=new HashMap();

When do we use the former and when do we use the latter?

[albinjoseph]

HashMap hm = new HashMap(); This code is very specific and hm can contain only a HashMap. But when we say Map m = new HashMap(), m becomes more generic. At later time we can assign a Hashtable to m if we want. But the same thing is not possible with hm.

[priyaguptan]

Ok thanks a lot Albin.