Observer Design Pattern in java:

               This is the third design pattern which deals with behavioral design.It has a subject and observers. Subject is the object and observers are the dependents. When a change occurs it reflects the observers.

Let us write a java program to implement this design pattern.

Steps:

• ·       Create an interface observer with a method updateIt with a string msg.
• ·       Write a code for subject class with observer array and four member functions. One for insertObserver, removing observer,notify the observer and display the state information.
• ·       Implement a concreteObserver to implement observer.
• ·       Override the updateIt() function.
• ·       Finally, create the main() function for creating objects and function calling.

#Java Program to implement Observer Design Pattern

import java.util.ArrayList;

import java.util.List;

interface Observer {

    void updateIt(String msg);

}

 class Subject {

    private List<Observer> obs = new ArrayList<>();

    private String stateInfo;

     public void InsertObserver(Observer observer) {

        obs.add(observer);

    }

     public void Observer(Observer observer) {

        obs.remove(observer);

    }

     public void setState(String stateInfo) {

        this.stateInfo = stateInfo;

        notifyObservers();

    }

     public void notifyObservers() {

        for (Observer observer : obs) {

            observer.updateIt(stateInfo);

        }

    }

}

 class ConcreteObserver implements Observer {

    private String name;

     ConcreteObserver(String name) {

        this.name = name;

    }

     @Override

    public void updateIt(String stateInfo) {

        System.out.println(name + " received state change: " + stateInfo);

    }

}

 public class Main19 {

    public static void main(String[] args) {

        Subject subject = new Subject();

        Observer observer1 = new ConcreteObserver("Observer 1");

        Observer observer2 = new ConcreteObserver("Observer 2");

        subject.InsertObserver(observer1);

        subject.InsertObserver(observer2);

        subject.setState("State 1");

        subject.setState("State 2");

    }

}

While executing this program, you will get the below output.

C:\raji\blog>javac Main19.java

C:\raji\blog>java Main19

Observer 1 received state change: State 1

Observer 2 received state change: State 1

Observer 1 received state change: State 2

Observer 2 received state change: State 2

Thus the program to implement the observer design pattern is executed. Happy Coding!!!!