How does HashMap work internally?

A HashMap is an array of buckets. Each bucket holds a linked list of nodes, and since Java 8 a bucket can become a red-black tree. The default initial capacity is 16 and the default load factor is 0.75, so the table resizes when the number of entries exceeds 12.

The lookup path

• Compute the key hash and spread the high bits with hash XOR hash unsigned-shifted by 16
• Select the bucket with (capacity - 1) AND spreadHash
• Compare the hash, then the key by reference or equals
• On collision, walk the bin list or tree

static int bucketIndex(int hash, int capacity) {
    int spread = hash ^ (hash >>> 16);
    return (capacity - 1) & spread;
}

When a bin grows past eight entries and the table has at least 64 buckets, the bin treeifies. Tree bins give O(log n) worst-case lookup within a collision chain instead of O(n). Resizing doubles capacity and redistributes entries. A bad hashCode that sends everything to one bucket degrades performance badly.

HashMap is not thread-safe. Concurrent writes can corrupt the table or lose entries. Use ConcurrentHashMap for shared access. Also keep keys immutable and consistent with equals, because a changed hash makes an entry unreachable.