Observer and Observable

Using JAVA Implementation

Message Board (Observable)

package com.alkathirikhalid.demo; import java.util.Observable; /**   * <p>   * Message Board as Observable.</p>   * <p>   * It can be observed for new message changes, it can also have one or more   * observers, when an observable object is newly created, its set of observers   * is always empty at first.</p>   *   * @author alkathirikhalid   */ public class MessageBoard extends Observable {     /**       * The message on the Message Board       */      private String message;     /**       * Getter       *       * @return message Board       */      public String getMessage() {          return message;      }     /**       * Setter       *       * @param message Board       */      public void setMessage(String message) {          this.message = message;          // Marks this Observable object as having been changed          setChanged();          // notify all of Message Board observers          notifyObservers();      } }

User (Observer)

package com.alkathirikhalid.demo; import java.util.Observable; import java.util.Observer; /**   * <p>   * User as Observer.</p>   * <p>   * Gets updated messages based on changes in observable Message Board.</p>   *   * @author alkathirikhalid   */ public class User implements Observer {     /**       * Observable Message Board       */      private final MessageBoard messageBoard;      /**       * User In box message       */      private String inbox;     /**       * Constructor       *       * @param messageBoard the user is Observing       */      public User(MessageBoard messageBoard) {          this.messageBoard = messageBoard;      }     /**       * Getter       *       * @return in box message       */      public String getInbox() {          return inbox;      }     /**       * Setter       *       * @param inbox message       */      public void setInbox(String inbox) {          this.inbox = inbox;      }     @Override      public void update(Observable o, Object arg) {          // Confirms the Observable is a Message Board Object          if (o == messageBoard) {              // Sets the user in box message as the incoming new message board              setInbox(messageBoard.getMessage());              // Prints to screen              System.out.println(getInbox());          }      } }

Test Application

package com.alkathirikhalid.demo; /**   *   * @author alkathirikhalid   */ public class TestApp {     /**       * @param args the command line arguments       */      public static void main(String[] args) {          // New Message Board Instance          MessageBoard messageBoard = new MessageBoard();          // New User Instance Observing Message Board          User user = new User(messageBoard);          // Add User as an Observer of Message Board          messageBoard.addObserver(user);          // Update the Message Board with new Message          messageBoard.setMessage(“New Message!”);      } }

Writing Your Own Implementation

Message Board Listener Interface

package com.alkathirikhalid.demo; /**   *   * @author alkathirikhalid   */ public interface MessageBoardListener {     void update(String message); }

Message Board Class

package com.alkathirikhalid.demo; public class MessageBoard {     private MessageBoardListener messageBoardListener;      /**       * The message on the Message Board       */      private String message;     public void setMessageBoardListener(MessageBoardListener messageBoardListener) {          this.messageBoardListener = messageBoardListener;      }     /**       * Getter       *       * @return message Board       */      public String getMessage() {          return message;      }     /**       * Setter       *       * @param message Board       */      public void setMessage(String message) {          this.message = message;          // notify all of Message Board listeners          messageBoardListener.update(message);      } }

User

package com.alkathirikhalid.demo; public class User implements MessageBoardListener {     private String inbox;     public String getInbox() {          return inbox;      }     public void setInbox(String inbox) {          this.inbox = inbox;      }     @Override      public void update(String message) {          setInbox(message);          // Prints to screen          System.out.println(getInbox());      } }

Test Application

package com.alkathirikhalid.demo; /**   *   * @author alkathirikhalid   */ public class TestApp {     /**       * @param args the command line arguments       */      public static void main(String[] args) {          // New Message Board Instance          MessageBoard messageBoard = new MessageBoard();          // New User Instance          User user = new User();          // Add User as a Listening to the Message Board          messageBoard.setMessageBoardListener(user);          // Update the Message Board with new Message          messageBoard.setMessage(“New Message!”);      } }

Result

The same for both implementations

New Message!