Understanding HashMap Collision Resolution and Internal Implementation in Java

HashMap uses a hash algorithm to determine the storage location of each element; when map.put(key, value) is called, the key's hashCode() is obtained and transformed by the hash(int h) function to compute a hash value.

The hash value is then mapped to an array index using indexFor(hash, table.length) , which efficiently replaces a modulo operation with a bitwise AND, requiring the table size to be a power of two for uniform distribution.

If multiple keys produce the same index, HashMap resolves the conflict with the linked‑list method: each bucket holds a singly‑linked list of Entry objects, where each Entry stores a key, value, hash, and a reference to the next entry.

static int hash(int h) {
    h ^= (h >>> 20) ^ (h >>> 12);
    return h ^ (h >>> 7) ^ (h >>> 4);
}

static int indexFor(int h, int length) {
    return h & (length - 1);
}

The put method first checks for a null key, handling it specially, then computes the hash and index, traverses the bucket's list to replace an existing value if the key matches, or appends a new Entry at the head of the list.

public V put(K key, V value) {
    if (key == null)
        return putForNullKey(value);
    int hash = hash(key.hashCode());
    int i = indexFor(hash, table.length);
    for (Entry
e = table[i]; e != null; e = e.next) {
        Object k;
        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
            V oldValue = e.value;
            e.value = value;
            e.recordAccess(this);
            return oldValue;
        }
    }
    modCount++;
    addEntry(hash, key, value, i);
    return null;
}

When the number of entries exceeds threshold = (int)(capacity * loadFactor) (default load factor 0.75), the map resizes by doubling the table size, rehashing all existing entries, and updating the threshold.

void resize(int newCapacity) {
    Entry[] oldTable = table;
    int oldCapacity = oldTable.length;
    if (oldCapacity == MAXIMUM_CAPACITY) {
        threshold = Integer.MAX_VALUE;
        return;
    }
    Entry[] newTable = new Entry[newCapacity];
    transfer(newTable);
    table = newTable;
    threshold = (int)(newCapacity * loadFactor);
}

The get method mirrors put by computing the hash, locating the bucket, and traversing the linked list to find the entry whose key equals the requested key.

public V get(Object key) {
    if (key == null)
        return getForNullKey();
    int hash = hash(key.hashCode());
    for (Entry
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;
}

Overall, the article provides a comprehensive walkthrough of HashMap's internal data structures, collision resolution strategy, load factor trade‑offs, and performance considerations, making it a valuable resource for Java developers and anyone studying hash‑based collections.