feat: add ImmutableUser example demonstrating immutability and defensive copying

Implemented ImmutableUser.java to illustrate creation of immutable objects in Java.
Showcases best practices:
– final class and final fields
– no setters
– defensive copying of mutable fields (Date)
– safe getter design preventing external mutation
Includes demonstration of how original mutable references do not affect internal state.

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

Author: Someshdiwan

Section16StaticFinal/src/ImmutableUser.java

@@ -0,0 +1,40 @@
+import java.util.Date;
+
+// ImmutableUser class example. Demonstrates how to create an immutable class in Java.
+public final class ImmutableUser {
+    // 1. Make class final so it cannot be subclassed
+
+    // 2. Make all fields private and final
+    private final String username;
+    private final Date dateOfBirth; // Date is mutable, so handle with care
+
+    // Constructor performs defensive copying for mutable fields.
+    public ImmutableUser(String username, Date dateOfBirth) {
+        this.username = username;
+        // 3. Perform a defensive copy for mutable objects
+        this.dateOfBirth = new Date(dateOfBirth.getTime());
+    }
+
+    // 4. No setters — immutability maintained
+    public String getUsername() {
+        return username;
+    }
+
+    // 5. Return a copy instead of the original reference to prevent external modification.
+    public Date getDateOfBirth() {
+        return new Date(dateOfBirth.getTime());
+    }
+
+    public static void main(String[] args) {
+        Date dob = new Date();
+        ImmutableUser user = new ImmutableUser("Alice", dob);
+
+        System.out.println("Username: " + user.getUsername());
+        System.out.println("Date of Birth: " + user.getDateOfBirth());
+
+        // Trying to modify original Date object won't affect the ImmutableUser
+        dob.setTime(0);
+        System.out.println("Modified Original Date: " + dob);
+        System.out.println("Immutable User Date of Birth: " + user.getDateOfBirth());
+    }
+}