feat: Implement Actor Model pattern

* feat: Implement Actor Model pattern #3232

* feat: Implement Actor Model pattern #3232

* feat: update Actor Model implementation with multi-actor logic #3251

* feat: update Actor Model implementation with multi-actor logic and loose coupling  #3251

* test: add unit test for actor model #3251

* test: add test for App.java to increase coverage

* docs: add complete README for Actor Model pattern also implemented changes #3251

actor-model/README.md

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+---
+title: "Actor Model Pattern in Java: Building Concurrent Systems with Elegance"
+shortTitle: Actor Model
+description: "Explore the Actor Model pattern in Java with real-world examples and practical implementation. Learn how to build scalable, message-driven systems using actors, messages, and asynchronous communication."
+category: Concurrency
+language: en
+tag:
+    - Concurrency
+    - Messaging
+    - Isolation
+    - Asynchronous
+    - Distributed Systems
+    - Actor Model
+---
+
+## Also Known As
+
+- Message-passing concurrency
+- Actor-based concurrency
+
+---
+
+## Intent of Actor Model Pattern
+
+The Actor Model pattern enables the construction of highly concurrent, distributed, and fault-tolerant systems by using isolated components (actors) that interact exclusively through asynchronous message passing.
+
+---
+
+## Detailed Explanation of Actor Model Pattern with Real-World Examples
+
+### 📦 Real-world Example
+
+Imagine a customer service system:
+- Each **customer support agent** is an **actor**.
+- Customers **send questions (messages)** to agents.
+- Each agent handles one request at a time and can **respond asynchronously** without interfering with other agents.
+
+---
+
+### 🧠 In Plain Words
+
+> "Actors are like independent workers that never share memory and only communicate through messages."
+
+---
+
+### 📖 Wikipedia Says
+
+> [Actor model](https://en.wikipedia.org/wiki/Actor_model) is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent computation.
+
+---
+
+### 🧹 Architecture Diagram
+
+![UML Class Diagram](./etc/Actor_Model_UML_Class_Diagram.png)
+
+---
+
+## Programmatic Example of Actor Model Pattern in Java
+
+### Actor.java
+
+```java
+public abstract class Actor implements Runnable {
+
+    @Setter @Getter private String actorId;
+    private final BlockingQueue<Message> mailbox = new LinkedBlockingQueue<>();
+    private volatile boolean active = true; 
+
+
+    public void send(Message message) {
+        mailbox.add(message); 
+    }
+
+    public void stop() {
+        active = false; 
+    }
+
+    @Override
+    public void run() {
+        
+    }
+
+    protected abstract void onReceive(Message message);
+}
+
+```
+
+### Message.java
+
+```java
+
+@AllArgsConstructor
+@Getter
+@Setter
+public class Message {
+    private final String content;
+    private final String senderId;
+}
+```
+
+### ActorSystem.java
+
+```java
+public class ActorSystem {
+    public void startActor(Actor actor) {
+        String actorId = "actor-" + idCounter.incrementAndGet(); // Generate a new and unique ID
+        actor.setActorId(actorId); // assign the actor it's ID
+        actorRegister.put(actorId, actor); // Register and save the actor with it's ID
+        executor.submit(actor); // Run the actor in a thread
+    }
+    public Actor getActorById(String actorId) {
+        return actorRegister.get(actorId); //  Find by Id
+    }
+
+    public void shutdown() {
+        executor.shutdownNow(); // Stop all threads
+    }
+}
+```
+
+### App.java
+
+```java
+public class App {
+  public static void main(String[] args) {
+    ActorSystem system = new ActorSystem();
+      Actor srijan = new ExampleActor(system);
+      Actor ansh = new ExampleActor2(system);
+
+      system.startActor(srijan);
+      system.startActor(ansh);
+      ansh.send(new Message("Hello ansh", srijan.getActorId()));
+      srijan.send(new Message("Hello srijan!", ansh.getActorId()));
+
+      Thread.sleep(1000); // Give time for messages to process
+
+      srijan.stop(); // Stop the actor gracefully
+      ansh.stop();
+      system.shutdown(); // Stop the actor system
+  }
+}
+```
+
+---
+
+## When to Use the Actor Model Pattern in Java
+
+- When building **concurrent or distributed systems**
+- When you want **no shared mutable state**
+- When you need **asynchronous, message-driven communication**
+- When components should be **isolated and loosely coupled**
+
+---
+
+## Actor Model Pattern Java Tutorials
+
+- [Baeldung – Akka with Java](https://www.baeldung.com/java-akka)
+- [Vaughn Vernon – Reactive Messaging Patterns](https://vaughnvernon.co/?p=1143)
+
+---
+
+## Real-World Applications of Actor Model Pattern in Java
+
+- [Akka Framework](https://akka.io/)
+- [Erlang and Elixir concurrency](https://www.erlang.org/)
+- [Microsoft Orleans](https://learn.microsoft.com/en-us/dotnet/orleans/)
+- JVM-based game engines and simulators
+
+---
+
+## Benefits and Trade-offs of Actor Model Pattern
+
+### ✅ Benefits
+- High concurrency support
+- Easy scaling across threads or machines
+- Fault isolation and recovery
+- Message ordering within actors
+
+### ⚠️ Trade-offs
+- Harder to debug due to asynchronous behavior
+- Slight performance overhead due to message queues
+- More complex to design than simple method calls
+
+---
+
+## Related Java Design Patterns
+
+- [Command Pattern](../command)
+- [Mediator Pattern](../mediator)
+- [Event-Driven Architecture](../event-driven-architecture)
+- [Observer Pattern](../observer)
+
+---
+
+## References and Credits
+
+- *Programming Erlang*, Joe Armstrong
+- *Reactive Design Patterns*, Roland Kuhn
+- *The Actor Model in 10 Minutes*, [InfoQ Article](https://www.infoq.com/articles/actor-model/)
+- [Akka Documentation](https://doc.akka.io/docs/akka/current/index.html)
+