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This article will explain in detail how to implement the observer mode in JAVA. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.

Concept

The observer pattern, also known as the publish-subscribe pattern, is an one-to-many relationship between objects. When the state of an object changes, all objects that depend on it are notified and updated automatically.

Composition

Abstract topic roles (Subject)

Also known as abstract target classes, abstract topics know what their observers are, provide methods to delete and add observers, and abstract methods to notify observers, implemented by abstract classes or interfaces

Abstract Observer role (Observer)

Contains an updated abstract method that is called when an update notification of a specific topic is received and is implemented by an abstract class or interface

Specific subject roles (Concrete Subject)

Also known as the specific target class, implements the notification method of the abstract target class, notifying all observers who subscribe to it when a specific topic changes.

Specific observer role (Concrete Observer)

Implement an abstract method that abstracts the role of the observer, changing its own state after receiving a notification of a subject change

Realize

Abstract Observer Observer Interface

Contains a method for the observer to receive notification to change his or her state

Public interface Observer {/ / received notification to change its status void update ();}

Specific observers ObserverOne and ObserverTwo

Implement the update method in Observer interface

Public class ObserverOne implements Observer {@ Override public void update () {System.out.println ("first observer received notification, status updated");}} public class ObserverTwo implements Observer {@ Override public void update () {System.out.println ("second observer received notification, status updated");}}

Subject interface

Contains methods to add, delete, and notify the observer, implemented by the abstract target class, and a method that operates itself, implemented by the specific target class

Public interface Subject {/ / add observer void add (Observer observer); / / delete observer void remove (Observer observer); / / notify all observers void notifyObservers (); / / self-operation void operation ();}

Abstract target class AbstractSubject

Abstract class, because the number of observers is not fixed, we use Vector dynamic array to store observers and implement the add,remove,notifyObservers method in the Subject interface. The operation method is implemented by the specific target class.

Public abstract class AbstractSubject implements Subject {Vector vector = new Vector (); @ Override public void add (Observer observer) {vector.add (observer);} @ Override public void remove (Observer observer) {vector.remove (observer);} @ Override public void notifyObservers () {for (Observer observer: vector) {observer.update ();}

Specific target class MySubject

Inherit the AbstractSubject class, implement the operation method, and call the notifyObservers method in the operation method to notify the observer after the state changes.

Public class MySubject extends AbstractSubject {@ Override public void operation () {System.out.println ("specific target status change"); System.out.println ("notifying observer.") ; notifyObservers (); System.out.println ("Notification is over!") ;}}

Test class Test

Public class Test {public static void main (String [] args) {MySubject mySubject = new MySubject (); mySubject.add (new ObserverOne ()); mySubject.add (new ObserverTwo ()); mySubject.operation ();}}

Summary

What is the main problem solved by the observer model?

Under the condition of low coupling, the state of one object changes and other objects are notified.

When will the observer mode be used

When the state of an object changes, all dependent objects will receive a broadcast notification

Advantages of the observer model

The observer and the target are abstractly coupled and low coupled, which is a set of trigger mechanisms.

The shortcomings of the observer model

① if there are many direct and indirect observers dependent on a target, it takes a lot of time to notify all observers.

② if there is a circular dependency between the observer and the observed, a circular call may occur, causing the system to crash

The ③ observer cannot know how the target state has changed, only that the target state has changed

Considerations for the Observer Model

There are already supporting classes for the Observer pattern in ① JAVA, and you can directly use the

② avoids circular calls

If ③ executes the observer's update method sequentially, an observer error will cause the system to get stuck, usually in an asynchronous way.

This is the end of the article on "how JAVA implements the Observer Mode". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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