Collision resolution in Java HashMap

Question

Java HashMap uses put method to insert the K/V pair in HashMap. Lets say I have used put method and now HashMap<

Answer 1

It could have formed a linked list, indeed. It's just that Map contract requires it to replace the entry:

V put(K key, V value)

Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)

http://docs.oracle.com/javase/6/docs/api/java/util/Map.html

For a map to store lists of values, it'd need to be a Multimap. Here's Google's: http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Multimap.html

A collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values.

Edit: Collision resolution

That's a bit different. A collision happens when two different keys happen to have the same hash code, or two keys with different hash codes happen to map into the same bucket in the underlying array.

Consider HashMap's source (bits and pieces removed):

public V put(K key, V value) {
    int hash = hash(key.hashCode());
    int i = indexFor(hash, table.length);
    // i is the index where we want to insert the new element
    addEntry(hash, key, value, i);
    return null;
}

void addEntry(int hash, K key, V value, int bucketIndex) {
    // take the entry that's already in that bucket
    Entry e = table[bucketIndex];
    // and create a new one that points to the old one = linked list
    table[bucketIndex] = new Entry<>(hash, key, value, e);
}

For those who are curious how the Entry class in HashMap comes to behave like a list, it turns out that HashMap defines its own static Entry class which implements Map.Entry. You can see for yourself by viewing the source code:

GrepCode for HashMap