Java Learning Notes-observer patterns

The Observer pattern (Observer) defines a one-to-many dependency that allows multiple observer objects to listen to a Subject object at the same time. When the subject object changes in state, all observer objects are notified so that they can automatically update themselves.

For example, when a button has multiple listeners, when the button is clicked, multiple observers are notified and perform the appropriate action.

The viewer consists of four parts:

• Abstract Theme Role: Save references to all observer objects in a collection, and each abstract theme role can have any number of observers. Abstract topics provide an interface that can add and remove observer roles. Typically implemented with an abstract class or interface.
• Abstract Observer role: Define an interface for all specific observers and update yourself when you get notifications for a topic.
• Specific topic role: Notifies all registered observers when the internal state of a specific topic changes. A specific theme role is typically implemented with a subclass.
• Specific observer role: This role implements the update interfaces required by the abstract observer role in order to reconcile the state of itself with the state of the topic. If necessary, the specific observer role can save a reference to a specific topic role. Typically implemented with a subclass.

The simple implementation of the observer pattern is as follows.

Abstract Theme Roles:

 Public Interface watched{    publicvoid  Addwatcher (     Watcher Watcher);//Increase observer public   void  Removewatcher (Watcher Watcher);//move    out observer public void  Notifywatchers (String str);//Notify Viewer}

Abstract Observer Role:

 Public Interface watcher{    publicvoid  update (String str);

Specific theme roles:

 Public classConcretewatchedImplementswatched{Privatelist<watcher> list =NewArraylist<watcher>(); @Override Public voidAddwatcher (Watcher Watcher) {List.add (watcher); } @Override Public voidRemovewatcher (Watcher Watcher) {list.remove (watcher); } @Override Public voidnotifywatchers (String str) { for(Watcher watcher:list) {watcher.update (str); }    }}

Specific Observer roles:

 Public class Implements watcher{    @Override    publicvoid  update (String str)    {        System.out.println (str);}    }

Test class:

 public  class   observertest{ public  static  void   main (string[] args) {                Watched girl  = new   concretewatched ();        Watcher W1  = new   Concretewatcher ();        Watcher W2  = new   Concretewatcher ();                Watcher W3  = new   Concretewatcher ();        Girl.addwatcher (W1);        Girl.addwatcher (W2);        Girl.addwatcher (W3);    Girl.notifywatchers ( "I ' m Happy" 

Java Learning Note-Observer mode