Java Watcher pattern

Observer pattern

Intent: define a one-to-many dependency between objects, and when the state of an object changes, all objects that depend on it are notified and automatically updated.

Workaround: An object state changes to other object notification issues, and to take into account the ease of use and low coupling, to ensure a high degree of collaboration.

use: the state of an object (the target object) is changed, and all dependent objects (Observer objects) are notified of the broadcast notification.

The key: There is a ArrayList in the abstract class that holds the observers.

Advantages: 1, the observer and the observer are abstract coupled.

2, set up a set of trigger mechanism.

Cons: 1, if an observer object has a lot of direct and indirect observers, all observers are notified to spend a lot of time.

2. If there is a cyclic dependency between the observer and the observation target, observing the target triggers a cyclic call between them, which can cause the system to crash.

3. The observer pattern does not have a mechanism for the observer to know how the target object is changing, but only to know that the observation target has changed.

code example:

1. Define the Observer abstract class

/**@author*/publicabstract class  Observer {    publicabstractvoid  update (String msg);}

2. Define multiple observers

 Public class extends observer{    @Override    publicvoid  update (String msg) {        // TODO auto-generated Method stub        System.out.println (Observer1.  Class+ ":" +msg);}    }

 public  class  Observer2 extends   observer{@Override   void   update (String msg) { 
    
     //
      TODO auto-generated method stub  System.out. println (Observer2.    Class  + ":" +msg); }}

 Public class extends observer{    @Override    publicvoid  update (String msg) {        // TODO auto-generated Method stub        System.out.println (Observer3.  Class+ ":" +msg);}    }

3. Definition of the person being observed

Importjava.util.ArrayList;Importjava.util.List;/*** Observer mode-the person being observed *@author[email protected] **/ Public classSubject {PrivateList<observer> obs=NewArraylist<observer>();  Public voidAdd (Observer ob) {obs.add (OB); }         Public voidsetmsg (String msg) { for(Observer ob:obs) {ob.update (msg); }    }         Public Static voidMain (string[] args) {Observer1 O1=NewObserver1 (); Observer2 O2=NewObserver2 (); Observer3 O3=NewObserver3 (); Subject s=NewSubject ();        S.add (O1);        S.add (O2);        S.add (O3); S.setmsg (Information); S.setmsg (Change); }}

Java Watcher pattern