query regarding null key in hashmap

[shreyans ricky]

Hi,

I am aware that we can have 1 null key in a hashmap.
If I try to put second null key in the hashmap, previous one is overridden.
Hashmap performs all the operation by evaluating the keys for methods hashcode and equals to check the existance of key.
My question here is: Is it possible to take hashcode of null value ?
If yes, how ?
If no, how does duplicate null key is avoided in the hashmap ??

Sample code :

Map<String, Integer> map = new HashMap<String, Integer>();

// put null key
map.put(null , 6);
// put duplicate key
map.put(null , 9);

Iterator<String> itr = map.keySet().iterator();
while (itr.hasNext()) {
String str = itr.next();
System.out.println(map.get(str));
}

The output will be 9




Regards,
Shreyans

[William Brogden]

Browsing the source code for HashMap.java I find special handling of the null key - for example in the get method:

   public V get(Object key) {
        if (key == null)
            return getForNullKey();
        int hash = hash(key.hashCode());
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }
    /**
     * Offloaded version of get() to look up null keys.  Null keys map
     * to index 0.  This null case is split out into separate methods
     * for the sake of performance in the two most commonly used
     * operations (get and put), but incorporated with conditionals in
     * others.
     */
    private V getForNullKey() {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }

Bill

[shreyans ricky]

Thanks Bill !!