What is the internal working of HashMap in Java and how does collision resolution work in Java 8?

In Java, a HashMap works on the principle of hashing. It inte ally uses an array of Node<K,V> buckets. When put(key, value) is called:

1. The key's hashCode() is computed, then spread via an inte al bitwise hash function.
2. The array index is determined by (n - 1) & hash, where n is the bucket table length.
3. If the bucket is empty, a new Node is placed.
4. If collision occurs (multiple keys map to the same bucket), Java resolves it via linked list chaining.

Java 8 Optimization: When the number of nodes in a bucket exceeds TREEIFY_THRESHOLD = 8 and table capacity is at least 64, the linked list is converted into a Red-Black Tree. This changes worst-case lookup from O(n) to O(log n).