Shulou(Shulou.com)06/02 Report--

Today, I will talk to you about the observer pattern in the Java design pattern, which may not be well understood by many people. in order to make you understand better, the editor has summarized the following content for you. I hope you can get something according to this article.

I. the definition and characteristics of the observer model

The definition of the Observer pattern:

It means that there is an one-to-many dependency between multiple objects. when the state of an object changes, all objects that depend on it are notified and updated automatically. This pattern is sometimes called publish-subscribe pattern and model-view pattern, and it is an object behavior pattern.

Features:

It reduces the coupling relationship between the target and the observer, which is an abstract coupling relationship. In line with the principle of dependency inversion.

A trigger mechanism is established between the target and the observer.

Second, the structure of the observer model

When implementing the observer pattern, we should pay attention to that there is no direct call between the specific target object and the specific observer object, otherwise they will be closely coupled, which violates the object-oriented design principle. The main roles of the observer mode are as follows.

Subject class: it stores all references to the observer object in an aggregation, each topic can have any number of observers, and the abstract topic provides an interface to add and delete any observer object.

Observer class: abstract observers, define an interface for all concrete observers, and update themselves when notified of the topic

ConcreteSubject: a specific topic that stores the relevant state in a specific observer object and notifies all registered observers when the internal state of the specific topic changes

ConcreteObserver: a concrete observer that implements the update interface required by the abstract observer role in order to reconcile its own state with that of the subject

Third, code examples

Now there is a demand, each website needs to subscribe to the weather demand, we need to update and send the weather information in time, and we are free to register or remove the website we want to send, using observer mode.

If we use the traditional model to implement this case, then there will be a problem, that is, if we want to modify the website, we may go back to change the code of the website class, and the code that we operate to update the data, which is not in line with our opening and closing principles. therefore, we use the observer mode to implement, because it is also an one-to-many dependency, there are numerous cases in life, such as subscribing to magazines, and so on.

The structure diagram is as follows

Code example

Abstract target class Subject

Package com.observerPattern.weatherCase / * @ author wang * @ version 1.0 * @ packageName com.observerPattern.weatherCase * @ className Subject * @ date 15:49 on 2021-12-28 * @ Description Subject Abstract Target Class Achieve * / public interface Subject {/ * @ Date 16:20 on 2021-12-28 * @ Param * @ param o * @ Return void * @ MetodName registerObserver * @ Author wang * @ Description registered Observer method * / void registerObserver (Observer o) / * @ Date 16:20 on 2021-12-28 * @ Param * @ param o * @ Return void * @ MetodName removeObserver * @ Author wang * @ Description remove the observer * / void removeObserver (Observer o) / * @ Date 16:20 on 2021-12-28 * @ Param * @ Return void * @ MetodName notifyObservers * @ Author wang * @ Description notify the observer * / void notifyObservers ();}

Specific target WeatherDate class

Package com.observerPattern.weatherCase;import java.util.ArrayList;/** * @ author wang * @ version 1.0 * @ packageName com.observerPattern.weatherCase * @ className WeatherDate * @ date 16:00 on 2021-12-28 * @ Description contains the latest weather data, is a specific goal, and achieves the abstract goal subject * this class contains observer sets and is managed using ArrayLis collections. * when the data is updated, actively call the ArrayList collection to notify each observer * * / public class WeatherDate implements Subject {private float temperature; private float pressure; private float humidity; private ArrayList observers / * * @ Date 16:10 on 2021-12-28 * @ Param * @ Return null * @ MetodName WeatherDate * @ Author wang * @ Description initializes the observer collection * / public WeatherDate () {this.observers = new ArrayList ();} public float getTemperature () {return temperature;} public float getPressure () {return pressure } public float getHumidity () {return humidity;} / * @ Date 16:10 on 2021-12-28 * @ Param * @ Return void * @ MetodName dateChange * @ Author wang * @ Description calls the notification method to push the updated data to each observer * / public void dateChange () {notifyObservers () } / * @ Date 16:11 on 2021-12-28 * @ Param * @ param temperature * @ param pressure * @ Return void * @ MetodName setDate * @ Author wang * @ Description Update data * / public void setDate (float temperature,float pressure,float humidity) {this.temperature = temperature; this.pressure = pressure This.humidity = humidity; dateChange ();} / * * @ Date 16:11 on 2021-12-28 * @ Param * @ param o * @ MetodName registerObserver * @ Author wang * @ Description z register an observer * / @ Override public void registerObserver (Observer o) {observers.add (o) } / * @ Date 16:11 on 2021-12-28 * @ Param * @ param o * @ Return void * @ Author wang * @ Description remove an observer * / @ Override public void removeObserver (Observer o) {if (observers.contains (o)) {observers.remove (o) }} / * @ Date 16:12 on 2021-12-28 * @ Param * @ Return void * @ MetodName notifyObservers * @ Author wang * @ Description notify the observer * / @ Override public void notifyObservers () {for (int I = 0 observers.size I < observers.size ()) Observers.get +) {update (I) .update (this.temperature,this.pressure,this.humidity);}

Abstract Observer Observer:

Package com.observerPattern.weatherCase / * @ author wang * @ version 1.0 * @ packageName com.observerPattern.weatherCase * @ className Observer * @ date 15:50 on 2021-12-28 * @ Description Observer interface, method to update temperature, pressure, humidity * / public interface Observer {/ * @ Date 16:18 on 2021-12-28 * @ Param * @ param temperature * @ param pressure * @ param humidity * @ Return void * @ MetodName update * @ Author wang * @ Description * / void update (float temperature,float pressure,float humidity) }

Specific Observer 1

Package com.observerPattern.weatherCase;/** * @ author wang * @ version 1.0 * @ packageName com.observerPattern.weatherCase * @ className CurrentCondition * @ date 15:54 on 2021-12-28 * @ Description a specific observer class, indicates the current weather conditions, and implements the observer interface * / public class CurrentCondition implements Observer {private float temperature; private float pressure; private float humidity / * @ Date 15:58 on 2021-12-28 * @ Param * @ param temperature * @ param pressure * @ param humidity * @ Return void * @ MetodName update * @ Author wang * @ Description this method pushes the updated data to the observer, and the observer prints * / @ Override public void update (float temperature, float pressure, float humidity) {this.temperature = temperature This.pressure = pressure; this.humidity = humidity; display () } / * * @ Date 15:59 on 2021-12-28 * @ Param * @ Return void * @ MetodName display * @ Author wang * @ Description this method displays updated data * / public void display () {System.out.println ("Test shows current temperature:" + temperature + "degrees") System.out.println ("Test shows current pressure: + pressure +" Pa "); System.out.println (" Test shows current humidity: "+ humidity +" Rh ");}}

Specific Observer 2:

Package com.observerPattern.weatherCase;/** * @ author wang * @ version 1.0 * @ packageName com.observerPattern.weatherCase * @ className SinaNet * @ date 2021-12-28 16:21 * @ Description Sina website as an observer * / public class SinaNet implements Observer {private float temperature; private float pressure; private float humidity / * @ Date 15:58 on 2021-12-28 * @ Param * @ param temperature * @ param pressure * @ param humidity * @ Return void * @ MetodName update * @ Author wang * @ Description this method pushes the updated data to the observer, and the observer prints * / @ Override public void update (float temperature, float pressure, float humidity) {this.temperature = temperature This.pressure = pressure; this.humidity = humidity; display ();} / * * @ Date 15:59 on 2021-12-28 * @ Param * @ Return void * @ MetodName display * @ Author wang * @ Description this method displays updated data * / public void display () {System.out.println ("= Sina website =") System.out.println ("Sina shows current temperature:" + temperature + "degrees"); System.out.println ("Sina shows current pressure:" + pressure + "Pa"); System.out.println ("Sina shows current humidity:" + humidity + "Rh");}}

Client test class

Package com.observerPattern.weatherCase;/** * @ author wang * @ version 1.0 * @ packageName com.observerPattern.weatherCase * @ className ClientTest * @ date 16:12 on 2021-12-28 * @ Description client test code, test observer mode * / public class ClientTest {public static void main (String [] args) {/ / create a weatherDate specific target WeatherDate weatherDate = new WeatherDate () / / create an observer CurrentCondition currentCondition = new CurrentCondition (); / / register an observer weatherDate.registerObserver (currentCondition); / / register Sina SinaNet sinaNet = new SinaNet (); weatherDate.registerObserver (sinaNet); / / Test update System.out.println ("notify observers"); weatherDate.setDate (3meme 65Power12) / / Test removes weatherDate.removeObserver (currentCondition); System.out.println ("="); System.out.println ("second update"); weatherDate.setDate (6m 8je 16) } / * notify observers that the test shows current temperature: 3.0 degrees test shows current pressure: 65.0 Pa test shows current humidity: 12.0Rh= Sina website = Sina shows current temperature: 3.0 degrees Sina shows current pressure: 65.0 Pa Sina shows current humidity: 12.0RhSina website = Sina shows current temperature: 6.0 degrees Sina Show current pressure: 88.0pa Sina shows current humidity: 16.0Rh * /

This advantage is that if we have a new website, we can just add an observer class without modifying the code.

As well as deletion and registration are all independent.

After reading the above, do you have any further understanding of the Observer pattern in the Java design pattern? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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