multiple null as key to hashmap

can we have multiple null  as key to hashmap. i was not clear on this concept. please advise.thanks  in advance
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gudii9Asked:
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for_yanConnect With a Mentor Commented:
So you cannot have multiple nulls as a key, as well as any other key - it should be unique
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for_yanCommented:
null can be a key,
but second time as any other key with HashMap it will overwrite thae Entry
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for_yanCommented:
As you see "ab" is overwritten by "cd" :

        HashMap<String,String> m = new HashMap<String,String>();
        m.put(null, "ab");

         m.put(null, "cd");
        
          m.put("a","nm");

        String sss = m.get(null);

        System.out.println(sss);
        

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Output:

cd

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for_yanCommented:



see here:
http://download.oracle.com/javase/1.4.2/docs/api/java/util/HashMap.html

This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.)
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for_yanCommented:

As opposed to HashMap,
Hashtable does not allow null as the key  and throws NullPointer exception

       Hashtable<String,String> t = new Hashtable<String,String>();

         t.put(null, "ef");

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Exception in thread "main" java.lang.NullPointerException
	at java.util.Hashtable.put(Hashtable.java:399)

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reijnemansConnect With a Mentor Commented:
Hi

As described by for_jan it's not possible to add multiple null key-values into a hashmap. But why do you want to put multiple null values as key in a hashmap? Maybe we could figure out a solution for that?

Regards,

reijnemans


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