Hashtable and Hashmap Doubt

[Abhi vijay]

I read in K&B Book that,

Hashmap allows one null key and multiple null values in a collection. wheras Hashtable dosent let you add anything that's null.

Why is this distinction as both are unsorted, unordered?

[Punit Singh]

Hashtable is thread-safe Abhi, and it was there in java since 1.0, you can see its naming convention "t" in Hashtable is written in small. It was designed such that it will not allow any null key or null value.

 Hashtable ht=new Hashtable();
	ht.put(new Object(),null);//NullPointerException
	ht.put(null, new Object());//NullPointerException   

But HashMap is added in java 1.2 version, so designer thought null should be allowed. Reason I do not know, any sheriff or knowledgeable guy could say the reason.

   HashMap hm=new HashMap();
     hm.put(null, "value");
     hm.put(null, "value");
     hm.put(null, "value");
     for (Object o :hm.entrySet()) {
		System.out.println(o);
	}  

[Duran Harris]

Punit,

You just inserted MULTIPLE null keys into the hashMap...
why was there no exception?

[Punit Singh]

HashMap does not store duplicate values. So although I have inserted multiple key, it will store only one null key and set the latest value to it.

  HashMap hm=new HashMap();  
    hm.put(null, "1");  
    hm.put(null, "2");  
    hm.put(null, "3");  
    for (Object o :hm.entrySet()) {  
    System.out.println(o);  
}      

Run this code, and tell me what you learned ?

[Vijitha Kumara]

Duran Harris wrote:

You just inserted MULTIPLE null keys into the hashMap...
why was there no exception?

If the map previously contained the same key the old value is replaced.

[Duran Harris]

Oh ok...Got it