feat(HashTable): add demo showing legacy synchronized Hashtable behavior

What
- Added `HashTable` class to demonstrate basic usage of `java.util.Hashtable`.
- Created a `Hashtable<Integer, String>` and inserted several key–value pairs.
- Printed the hashtable, retrieved a value by key, and tested `containsKey`.
- Attempted to insert a null key and null value to trigger `NullPointerException`.

Why
- To illustrate the differences between `Hashtable`, `HashMap`, and `ConcurrentHashMap`.
- Shows that `Hashtable` is synchronized, does not allow null keys or null values, and is considered a legacy class.
- Demonstrates what happens when null key/value is inserted.

How
- Step 1: Instantiate `Hashtable<Integer, String>`.
- Step 2: Put sample entries: (1 → "Apple"), (2 → "Banana"), (3 → "Cherry").
- Step 3: Print entire table and access by key (`get(2)`).
- Step 4: Use `containsKey(2)` to verify key presence.
- Step 5: Call `hashtable.put(null, "value")` and `hashtable.put(4, null)` to demonstrate null handling (throws `NullPointerException`).

Logic
- Inputs: integer keys and string values.
- Outputs:
  - Console log of the hashtable contents.
  - Specific value retrieval (`Banana` for key 2).
  - Boolean for containsKey check.
  - Runtime exception when inserting null key or null value.
- Flow:
  1. Insert entries → stored in synchronized hash buckets.
  2. Printing uses `toString()`, showing contents in `{}`.
  3. Null insertion triggers exception.

Real-life applications
- Rarely used in modern code; kept for backward compatibility.
- Historically used for thread-safe maps before `ConcurrentHashMap`.
- Now mainly educational to show legacy synchronization and null restrictions.

Notes
- Hashtable is synchronized at method level, so slower under contention.
- Does not allow **null keys** or **null values** (unlike HashMap).
- Collision handling uses linked lists (pre-Java 8 style).
- Prefer `HashMap` (unsynchronized) or `ConcurrentHashMap` (concurrent) in new code.

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

Author: Someshdiwan

Section25CollectionFramework/src/HashMapDemo/HashTable.java renamed to Section 25 Collections Frameworks/Map Interface/Hash Table/src/HashTable.java

@@ -1,17 +1,12 @@
-package HashMapDemo;
-
 import java.util.Hashtable;
 
 public class HashTable {
     public static void main(String[] args) {
         Hashtable<Integer, String> hashtable = new Hashtable<>();
 
-        //HashTable is Synchronized
-        //No Null key or value
-        //Legacy class, ConcurrentHashMap
-        //Slower than HashMap
-        //only linked list in case of collision
-        //all methods are synchronized
+        // HashTable is Synchronized, No Null key or value
+        // Legacy class, ConcurrentHashMap and Slower than HashMap, only linked list in case of collision
+        // all methods are synchronized
 
         hashtable.put(1,"Apple");
         hashtable.put(2,"Banana");
@@ -21,7 +16,7 @@ public static void main(String[] args) {
 
         System.out.println(hashtable.containsKey(2));
 
-        hashtable.put(null, "value"); // Throws exception
-        hashtable.put(4, null);   // Throws exception*/
+        hashtable.put(null, "value");
+        hashtable.put(4, null);
     }
-}
+}