Question

Is there anyway I can print out the Keys with the values that had collisions ? If a key has two values I want to be able to print out those values how can I do that I am inside the hash map class and making modification to it.

Answer 1

If a key has two values…

In a HashMap, a key cannot have two values. If you call map.put(key,value) with an existing key, the old value is removed from the map, and is returned by put().

One way to have multiple values per key is by using HashMap<K,Collection<V>>. This automatically provides the functionality you want, since you can simply examine the contents of the value collection after you’ve added the new element to it.

There are also third-party classes that provide this functionality, such as MultiValueMap.

edit:

If you’re talking about multiple keys ending up in the same bucket, then you need to modify HashMap‘s put() method:

public V put(K key, V value) {
    if (key == null)
        return putForNullKey(value);
    int hash = hash(key.hashCode());
    int i = indexFor(hash, table.length);
    if (table[i] != null) {
        // TODO: there's already something in this bucket
    }
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {
        ...

(Add your code where the TODO line is.)

You’ll need to make similar changes to putForNullKey() and other related methods, such as putForCreate().