"How to use" observer mode

This paper

Define one to many dependencies between objects , thus , When an object changes state , All objects that depend on it will be notified , And automatically update .

The theme 【 Publisher 】

import java.util.Observable;

@Data
public class SubjectData extends Observable {
    

    /** *  Subject data subscribed by observers  */
    private String lookData;

    /** *  Call this method when the subject data changes , At the same time, time subscription publishing  * @param changeData ： Data of subject changes  */
    public void lookDataChanged(String changeData) {
    
        // Modify subject data 
        lookData = changeData;
        // Set the change state of the subject object data 
        this.setChanged();
        // Notify all observers 
        this.notifyObservers(lookData);
    }
}

subscriber

import java.util.Observable;
import java.util.Observer;

public class ObserverOne implements Observer {
    

    /** *  Observers need to hold references to topics ,  The function of this reference is to enable the observer to actively pull the subject data , Instead of relying solely on the full volume push of the topic  */
    private Observable observable;

    /** *  Through a construction method , Implement instantiation of topic object  */
    public ObserverOne(Observable observable) {
    
        this.observable = observable;
    }

    /** *  The observer passively updates the data  *  When the subject object data changes, call lookDataChanged() After method , This method serves as the subscriber's method  *  Will handle update logic  * @param o * @param arg */
    @Override
    public void update(Observable o, Object arg) {
    
        if (o instanceof SubjectData) {
    
            String lookData = (String) arg;
            System.out.println("ObserverOne get lookData = " + lookData);
        }
    }

    /** *  The observer actively pulls data  *  Because the observer already contains the reference of the subject object through the construction method , Therefore, it can actively pull data  */
    public String getData() {
    
        if (observable instanceof SubjectData) {
    
            SubjectData subjectData = ((SubjectData) observable);
            return subjectData.getLookData();
        }
        return "instance error";
    }
}

test

public class TestDrive {
    

    public static void main(String[] args) {
    
        // Instantiate the topic object 
        SubjectData subjectData = new SubjectData();
        // Instantiate the observer , And build references to topic objects 
        ObserverOne observerOne = new ObserverOne(subjectData);
        ObserverTwo observerTwo = new ObserverTwo(subjectData);

        // Add an observer to the subscription list of the topic 
        subjectData.addObserver(observerOne);
        subjectData.addObserver(observerTwo);

        // The topic tries to modify its state 
        subjectData.lookDataChanged(" Today, Wednesday ");

        // Because the observer has the quotation of the topic , Therefore, you can actively pull data by yourself 
        String observerOneData = observerOne.getData();
        System.out.println("observerOneData = " + observerOneData);
    }
}

Output ：

ObserverTwo get lookData =  Today, Wednesday 
ObserverOne get lookData =  Today, Wednesday 
observerOneData =  Today, Wednesday