Monday, 9 November 2015

How null key is handled in HashMap?

How null key is handled in HashMap?

Since equals() and hashCode() are used to store and retrieve values, how does it work in case of null key?

Null key is handled specially in HashMap, there are two separate method for that putForNullKey(V value) and getForNullKey(). Later is offloaded version of get() to look up null keys.

Null keys always 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.

In short, equals() and hashcode() method are not used in case of null keys in HashMap, here is how nulls are retrieved from HashMap.

    private V getForNullKey() {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }

    /**
     * Offloaded version of put for null keys
     */
    private V putForNullKey(V value) {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        modCount++;
        addEntry(0, null, value, 0);
        return null;
    }


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