Add comprehensive documentation for Hashing, HashSet, and HashMap in Java

Author: gourabofficial

Hashing/notes.md

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+# 📘 Hashing, HashSet & HashMap in Java
+
+---
+
+## 1. 🔑 Hashing
+
+**Definition**
+Hashing is a technique to convert data into a fixed-size integer (hash code) using a **hash function**. This hash code decides where data will be stored in memory.
+
+* Used in `HashMap`, `HashSet`, `Hashtable`, etc.
+* Provides **O(1) average time complexity** for search, insert, and delete.
+
+**How it works:**
+
+1. Object → Hash Function → Hash Code
+2. Hash Code → Index in an array (bucket)
+3. If collision happens → handled using chaining (LinkedList/Tree)
+
+**Why use Hashing?**
+
+* Faster search compared to arrays or lists.
+* Used in databases, caches, compilers, and cryptography.
+
+**Example**
+
+```java
+String s = "Hello";
+System.out.println(s.hashCode()); 
+```
+
+---
+
+## 2. 🌿 HashSet
+
+**Definition**
+
+* Stores **unique elements** only.
+* **Unordered collection** (no guarantee of order).
+* Internally uses **HashMap** (values are stored as `Object` constants).
+
+**Important Methods**
+
+* `add(element)`
+* `remove(element)`
+* `contains(element)`
+* `size()`
+* `clear()`
+
+**Example**
+
+```java
+import java.util.HashSet;
+
+HashSet<Integer> set = new HashSet<>();
+set.add(10);
+set.add(20);
+set.add(20); // duplicate ignored
+System.out.println(set); // [20, 10]
+```
+
+**Use Cases**
+
+* Removing duplicates
+* Storing unique items (roll numbers, IDs)
+
+---
+
+## 3. 🗂️ HashMap
+
+**Definition**
+`HashMap<K, V>` stores **key-value pairs**.
+
+* Keys must be unique.
+* Values can be duplicated.
+* Provides O(1) average time for operations.
+
+**Important Methods**
+
+* `put(key, value)`
+* `get(key)`
+* `remove(key)`
+* `containsKey(key)`
+* `containsValue(value)`
+* `keySet()`
+* `values()`
+* `entrySet()`
+
+**Example**
+
+```java
+import java.util.HashMap;
+
+HashMap<Integer, String> map = new HashMap<>();
+map.put(1, "Gourab");
+map.put(2, "Rahul");
+map.put(3, "Ananya");
+
+System.out.println(map.get(2)); // Rahul
+map.remove(1);
+
+for (Integer key : map.keySet()) {
+    System.out.println(key + " -> " + map.get(key));
+}
+```
+
+**Use Cases**
+
+* Dictionary (word → meaning)
+* Student ID → Name
+* Cache storage
+* Configuration storage
+
+---
+
+## 4. ⚖️ HashSet vs HashMap
+
+| Feature           | HashSet                     | HashMap                            |
+| ----------------- | --------------------------- | ---------------------------------- |
+| Stores            | Unique elements             | Key-Value pairs                    |
+| Duplicate Keys?   | Not applicable              | ❌ Not allowed                      |
+| Duplicate Values? | ❌ Not allowed (only unique) | ✅ Allowed                          |
+| Nulls             | One `null` allowed          | One `null` key, many `null` values |
+| Use Case          | Unique roll numbers, IDs    | Roll number → Student name         |
+
+---
+
+## ✅ Recap
+
+* **Hashing**: Converts object → hash code → bucket index (fast operations).
+* **HashSet**: Stores only unique values (internally uses `HashMap`).
+* **HashMap**: Stores key-value pairs (unique keys, duplicate values allowed).
+
+---