hash map

[sweety sinha]

it SCJP book by K Siera & Berts Bates it is written that "hash map can have one null key & multiple null values" null values is alright but how can it has null key. then can values be retrieved from it.

please explain

[Sagar Rohankar]

sweety sinha wrote: "hash map can have one null key & multiple null values" null values is alright but how can it has null key. then can values be retrieved from it.

Had you tried to put a "null" value as a key in HashMap instance, lets try it and tell us whether you're able to fetch the respective "value" from it ?

HashMap map = new HashMap();
map.put(null, "My Value");
sysout(map.get(null));

[Ninad Kulkarni]

HashMap object hold key and it's corresponding value.

See the following non-generic code example

import java.util.*;
public class Test{
	public static void main(String[] args){
		HashMap hm = new HashMap();
		hm.put(null,"Maths");
		hm.put(null,"English");
		System.out.println(hm.get(null));
	}
}

Ouput is English not Maths.
The second put method call changes value to English. Initially null key point to Maths but now point to English.
In such a way HashMap have unique keys and multiple values. Multiple values may be same or different but keys must have unique values.

I hope this clears

[sweety sinha]

thanks guys for quick response

[David Marco]

And watch out for TreeMap and SortedMap, which NOT ALLOWS for null keys (NullPointerException will arise at runtime).

Greetings.

[Ruben Soto]

Just to point that a TreeMap can have a null key, as long as the TreeMap is constructed using a Comparator which handles keys with the null value.