feat(HashMapDemo): add basic HashMap operations demo

What
- Implemented a simple `HashMapDemo` showcasing common operations with `HashMap<Integer, String>`.
- Demonstrates insertion, lookup, key/value presence checks, and iteration.

Why
- To illustrate how `HashMap` provides constant-time average complexity for insert/get/contains operations.
- Helps understand differences between key-based vs. value-based lookups.
- Shows iteration over keys using `keySet()`.

How
1. Created a `HashMap<Integer, String>` to map student IDs → names.
2. Inserted three entries with `put()`.
3. Retrieved values using `get(key)` (returns `null` if key not present).
4. Checked existence with `containsKey()` and `containsValue()`.
5. Iterated over all keys via `map.keySet()` inside an enhanced for-loop.

Logic
- `put(31,"Smith")`, `put(11,"Shubham")`, `put(2,"John")` inserts mappings.
- `map.get(3)` returns `null` since key `3` does not exist.
- `map.containsKey(2)` returns `true`.
- `map.containsValue("John")` returns `true`.
- Iteration prints the entire `map` object for each key (demonstrates `toString()` representation).

Real-life applications
- `HashMap` is one of the most widely used collections in Java.
- Used for implementing caches, lookup tables, dictionaries, and frequency counters.
- O(1) average-time operations make it efficient for quick key-based access.

Notes
- `HashMap` allows **null keys and multiple null values** (unlike `Hashtable`).
- It is **not synchronized** — for concurrent use, prefer `ConcurrentHashMap`.
- Iteration order is **not guaranteed** (use `LinkedHashMap` if order matters).

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

Author: Someshdiwan

Section25CollectionFramework/src/HashMapDemo/HashMapDemo.java renamed to Section 25 Collections Frameworks/Map Interface/HashMap/src/HashMapDemo.java

@@ -1,4 +1,3 @@
-package HashMapDemo;
 import java.util.HashMap;
 import java.util.Set;
 
@@ -9,14 +8,12 @@ public static void main(String[] args) {
         map.put(31,"Smith");
         map.put(11,"Shubham");
         map.put(2,"John");
-
         System.out.println(map);
 
         String student = map.get(3);
         System.out.println(student);
 
         map.get(6);
-
         System.out.println(map.containsKey(2));
 
         System.out.println(map.containsValue("John"));
@@ -26,4 +23,4 @@ public static void main(String[] args) {
             System.out.println(map);
         }
     }
-}
+}