The observer pattern for learning Java Design Patterns _java

• Observer pattern: A one-to-many dependency between objects that allows multiple observer objects to simultaneously monitor a Subject object (observed).
  

So that when the state of an object changes, all objects that depend on it are notified and changed accordingly.
There are many ways to implement the Observer pattern: The pattern must contain two roles of the observer and the object being observed. There is a logical relationship between the observer and the observed, and when the observed changes, the observer observes the change and sends a corresponding change.

/**
 * Observer Interface: Observer, the class that needs to use the observer pattern should implement this interface/
 public
interface Observer
{public
  void update (Object obj);
}

/**
 * Observed (generally abstract, easy to expand): The method of affirming, a certain change has taken place, and the observer is informed of the change.
 * * Public
interface beenobserved
{public
  void Addobserver (Observer obs);//Add Watcher Object
  public void Removeobserver (Observer obs);//Observer object public
  void Notifyobservers (String changed);//notify observer object for corresponding change 
}

/** * Target observed: Implements the interface of the observed, corresponds to the Observer object/public class Concretewatched implements
  erved {//Viewer object Set private list<observer> List = new arraylist<observer> ();
    @Override public void Addobserver (Observer obs)//Add the Observer {if (!list.contains (OBS)) {List.add (OBS); @Override public void Removeobserver (Observer OBS)//observer tells the observed to undo the observation, and the observed removes the observer from the container {if (list.contains) (OBS
    ) {List.remove (OBS);  @Override public void Notifyobservers (String change) {//Traversal object, call method for Update Notification action for (Observer obs:list)
    {obs.update (change); }
  }
}

/**
 * Target observer (specific observer)/public
class Specificwatcher implements Observer
{
  @Override
  public void update (Object obj)
  {
    System.out.println (obj.tostring ());//changes occurred 
  }
}
/**
 * Test code
 * @description: */public
class Test {public
   static void Main (string[] args)
    {
      beenobserved bobs = new concretewatched ();
      Observer OBS1 = new Specificwatcher ();
      Observer OBS2 = new Specificwatcher ();
      Observer OBS3 = new Specificwatcher ();

      Bobs.addobserver (OBS1);//Add Observer Object
      Bobs.addobserver (OBS2);
      Bobs.addobserver (OBS3);
      Bobs.notifyobservers ("* * * received the notice * * *);

      " System.out.println ("----------Dividing Line--------------");

      Bobs.removeobserver (OBS2)//Remove observer
      bobs.notifyobservers ("* * * * ONE less observer * * * *)";
    }

Final Print Result:

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