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  1. // ==================================================
  2. // Observer Design Pattern
  3. // ==================================================
  4. //
  5. // What is Observer Pattern?
  6. // Observer Pattern defines a one-to-many dependency between objects
  7. // so that when one object (Subject) changes state, all its dependents
  8. // (Observers) are notified and updated automatically.
  9. //
  10. // It is also known as the Publisher-Subscriber model.
  11. //
  12. // When to use:
  13. // - When a change in one object requires changing others
  14. // - When you want loose coupling between the event source and its listeners
  15. // - When you want to add or remove listeners without modifying the source
  16. //
  17. // Key parts:
  18. // 1. Subject (Publisher)     → maintains list of observers and notifies them
  19. // 2. Observer (Subscriber)   → interface with an update method
  20. // 3. Concrete Observers      → classes that react to the notification
  21. //
  22. // Example used here: Student registration event
  23. // When a student registers, multiple actions should happen:
  24. // - Assign batch
  25. // - Send email
  26. // - Send Slack invite
  27. //
  28. // --------------------------------------------------
  29. // Folder Structure (if kept as separate files):
  30. //
  31. // observer/
  32. // ├── Observer.java
  33. // ├── RegistrationSubject.java
  34. // ├── BatchCreator.java
  35. // ├── EmailService.java
  36. // ├── SlackNotification.java
  37. // └── ObserverDemo.java
  38. // ==================================================
  39.  
  40. import java.util.ArrayList;
  41. import java.util.List;
  42.  
  43. // ==================================================
  44. // File: Observer.java
  45. // Observer Interface
  46. // All subscribers must implement this
  47. // ==================================================
  48. interface Observer {
  49.     void update(String studentName, String eventType);
  50. }
  51.  
  52. // ==================================================
  53. // File: RegistrationSubject.java
  54. // Subject (Publisher)
  55. // Maintains list of observers and notifies them when an event occurs
  56. // ==================================================
  57. class RegistrationSubject {
  58.     private final List<Observer> observers = new ArrayList<>();
  59.     private String studentName;
  60.  
  61.     public void addObserver(Observer observer) {
  62.         observers.add(observer);
  63.     }
  64.  
  65.     public void removeObserver(Observer observer) {
  66.         observers.remove(observer);
  67.     }
  68.  
  69.     private void notifyObservers(String eventType) {
  70.         for (Observer observer : observers) {
  71.             observer.update(studentName, eventType);
  72.         }
  73.     }
  74.  
  75.     public void registerStudent(String studentName) {
  76.         this.studentName = studentName;
  77.         System.out.println("Student registered: " + studentName);
  78.         notifyObservers("STUDENT_REGISTERED");
  79.     }
  80. }
  81.  
  82. // ==================================================
  83. // File: BatchCreator.java
  84. // Concrete Observer 1
  85. // ==================================================
  86. class BatchCreator implements Observer {
  87.     @Override
  88.     public void update(String studentName, String eventType) {
  89.         if ("STUDENT_REGISTERED".equals(eventType)) {
  90.             System.out.println("[BatchCreator] Assigned batch to " + studentName);
  91.         }
  92.     }
  93. }
  94.  
  95. // ==================================================
  96. // File: EmailService.java
  97. // Concrete Observer 2
  98. // ==================================================
  99. class EmailService implements Observer {
  100.     @Override
  101.     public void update(String studentName, String eventType) {
  102.         if ("STUDENT_REGISTERED".equals(eventType)) {
  103.             System.out.println("[EmailService] Sent welcome email to " + studentName);
  104.         }
  105.     }
  106. }
  107.  
  108. // ==================================================
  109. // File: SlackNotification.java
  110. // Concrete Observer 3
  111. // ==================================================
  112. class SlackNotification implements Observer {
  113.     @Override
  114.     public void update(String studentName, String eventType) {
  115.         if ("STUDENT_REGISTERED".equals(eventType)) {
  116.             System.out.println("[SlackNotification] Sent Slack invite to " + studentName);
  117.         }
  118.     }
  119. }
  120.  
  121. // ==================================================
  122. // File: ObserverDemo.java
  123. // Client / Usage
  124. // (public removed so it can compile in the same file)
  125. // ==================================================
  126. class ObserverDemo {
  127.     public static void main(String[] args) {
  128.         RegistrationSubject registration = new RegistrationSubject();
  129.  
  130.         Observer batchCreator = new BatchCreator();
  131.         Observer emailService = new EmailService();
  132.         Observer slack = new SlackNotification();
  133.  
  134.         registration.addObserver(batchCreator);
  135.         registration.addObserver(emailService);
  136.         registration.addObserver(slack);
  137.  
  138.         registration.registerStudent("Sai Harsha");
  139.  
  140.         System.out.println("\n--- Removing Email Service ---");
  141.         registration.removeObserver(emailService);
  142.  
  143.         registration.registerStudent("Another Student");
  144.     }
  145. }
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Student registered: Sai Harsha
[BatchCreator] Assigned batch to Sai Harsha
[EmailService] Sent welcome email to Sai Harsha
[SlackNotification] Sent Slack invite to Sai Harsha

--- Removing Email Service ---
Student registered: Another Student
[BatchCreator] Assigned batch to Another Student
[SlackNotification] Sent Slack invite to Another Student
https://ideone.com/trspZJ
language:
Java (HotSpot 12)
created:
5 days ago
visibility:
public

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