feat(HashMapChainingDemo): implement custom HashMap with chaining demo

What
- Added `HashMapChainingDemo` with an inner `SimpleHashMap` implementation.
- Uses `LinkedList` arrays as buckets to simulate **chaining** for handling collisions.
- Implements basic operations:
  - `put(String, Integer)` → insert/update key-value pairs
  - `get(String)` → retrieve value by key
  - `printBuckets()` → visualize internal bucket structure with chains
- Includes `Entry` class for storing key-value pairs.
- Demonstrates collisions by using a deliberately simple hash function (`sum of chars % capacity`).

Why
- To illustrate **how hash-based collections handle collisions internally**.
- Shows the difference between open addressing vs. chaining.
- Helps developers understand how Java’s `HashMap` internally uses buckets and chains for collision resolution.

How
1. Created an array of `List<Entry>` to serve as buckets.
2. Implemented `simpleHash(String)` that computes `sum(chars) % capacity`.
3. On `put`, if key exists in the bucket → update value, else add new `Entry`.
4. On `get`, linearly search the bucket for matching key.
5. `printBuckets()` displays each bucket index and its chain.
6. In `main()`, inserted colliding keys (`ABC`, `CBA`, `BAC`, `HELLO`, `OLLEH`) to demonstrate chaining.

Logic
- Keys that hash to the same bucket index are stored in a **LinkedList chain**.
- Lookup traverses the chain until the matching key is found.
- Update modifies the existing entry instead of inserting a duplicate.

Real-life applications
- Explains how Java’s `HashMap`, `Hashtable`, and `ConcurrentHashMap` resolve hash collisions.
- Useful in teaching DS/Algo concepts like hashing, collision handling, and chaining.
- Foundation for building custom hash-based caches, symbol tables, or indexing systems.

Notes
- HashMap in Java uses **bucket array + linked list (or tree from Java 8+)** for collisions.
- The demo’s hash function is intentionally weak to cause frequent collisions.
- Demonstrates `null` buckets, updated values, and retrieval from chains.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section 25 Collections Frameworks/Map Interface/HashMap/src/HashMapChainingDemo.java

@@ -1,8 +1,7 @@
 import java.util.LinkedList;
 import java.util.List;
 
-/**
- * Simple custom HashMap-like demo to show chaining using LinkedList.
+/* Simple custom HashMap-like demo to show chaining using LinkedList.
  * Uses a very simple hash function (sum of chars % capacity) to force collisions.
 
  * chaining (LinkedList) inside a custom HashMap so you can see collisions and how entries chain inside a bucket.