feat(HashCodeDemo): show hashCode() behavior for String, Integer, and Double

What
- Added `HashCodeDemo` class in `HashCode` package.
- Demonstrates the use of `.hashCode()` on:
  - `String` (`"ABC"` and `"CBA"`)
  - `Integer` (`100`)
  - `Double` (`12.34`)
- Prints each value alongside its computed hash code.

Why
- To illustrate how Java computes and uses hash codes across different object types.
- Shows that different objects generally produce different hash codes, but collisions are possible.
- Reinforces the idea that `hashCode()` is consistent with `equals()` contract.

How
- Step 1: Create two strings (`"ABC"`, `"CBA"`) and print their hash codes.
- Step 2: Create an `Integer` with value `100` and print its hash code.
- Step 3: Create a `Double` with value `12.34` and print its hash code.

Logic
- Inputs: fixed literals (`"ABC"`, `"CBA"`, `100`, `12.34`).
- Outputs: console logs of their hash codes.
- Flow:
  1. Instantiate or assign values.
  2. Call `.hashCode()` on each.
  3. Print results.

Real-life applications
- Hash codes are critical for performance in hash-based collections (`HashMap`, `HashSet`, `Hashtable`).
- Used in caching, lookups, and equality checks.
- Helps developers understand how Java’s hashing mechanism works internally.

Notes
- `String.hashCode()` is computed from character values with polynomial accumulation.
- `Integer.hashCode()` simply returns the int value.
- `Double.hashCode()` returns a hash derived from its IEEE 754 bit representation.
- Hash codes are not guaranteed to be unique; collisions can occur.

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

Author: Someshdiwan

Section 25 Collections Frameworks/Map Interface/HashMap/src/HashCode/HashCodeDemo.java

@@ -0,0 +1,17 @@
+package HashCode;
+
+public class HashCodeDemo {
+    public static void main(String[] args) {
+        String s1 = "ABC";
+        String s2 = "CBA";
+
+        System.out.println("HashCode of " + s1 + " = " + s1.hashCode());
+        System.out.println("HashCode of " + s2 + " = " + s2.hashCode());
+
+        Integer num = 100;
+        System.out.println("HashCode of Integer 100 = " + num.hashCode());
+
+        Double d = 12.34;
+        System.out.println("HashCode of Double 12.34 = " + d.hashCode());
+    }
+}