Java HashMap: Internal Working, Buckets, TreeBins & LinkedHashMap

HashMap is an implementation of the Map interface that stores associations between unique Keys and Values. It provides near-instantaneous O(1) average time complexity for get() and put() operations.

How HashMap Works Internally (The 4-Step Lifecycle)

1. 1. Hash Calculation: When calling map.put(key, value), the JVM executes a hash function: int hash = (h = key.hashCode()) ^ (h >>> 16);.
2. 2. Bucket Index Calculation: The array bucket index is calculated using bitwise AND: index = (n - 1) & hash, where n is array capacity (default 16).
3. 3. Collision Handling: If two different keys produce the same bucket index (a Hash Collision), elements are chained together as a singly-linked list of Node<K,V> objects.
4. 4. Java 8 Treeification: If collisions in a single bucket reach 8 nodes and table capacity is ≥ 64, the bucket linked list automatically converts into a balanced Red-Black Tree (TreeBin). This guarantees worst-case lookup improves from O(n) to O(log n)!

Practical Example: Managing User Sessions

import java.util.HashMap;
import java.util.Map;

public class HashMapDemo {
    public static void main(String[] args) {
        HashMap<String, String> userRoles = new HashMap<>();

        // 1. put(K, V) - Insert or overwrite
        userRoles.put("aarav101", "ADMIN");
        userRoles.put("sneha99", "EDITOR");
        userRoles.put("vikram5", "VIEWER");

        // 2. get(K) - O(1) lookup
        System.out.println("Aarav's Role: " + userRoles.get("aarav101")); // ADMIN

        // 3. getOrDefault() - Safe lookup without null checks
        System.out.println("Unknown user: " + userRoles.getOrDefault("guest", "GUEST_ROLE"));

        // 4. Iterating over Map.Entry sets:
        System.out.println("
--- All Users ---");
        for (Map.Entry<String, String> entry : userRoles.entrySet()) {
            System.out.println("User: " + entry.getKey() + " → Role: " + entry.getValue());
        }
    }
}