Observer

Real world example

A good example would be the job seekers where they subscribe to some job posting site and they are notified whenever there is a matching job opportunity.

In plain words

Defines a dependency between objects so that whenever an object changes its state, all its dependents are notified.

Wikipedia says

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

Programmatic example

Translating our example from above. First of all we have job seekers that need to be notified for a job posting

package com.prady00.dp.behavioral.observor;
class JobPost
{
    private String title;

    public JobPost(String title)
    {
        this.title = title;
    }

    public String getTitle()
    {
        return this.title;
    }
}

package com.prady00.dp.behavioral.observor;
class JobSeeker
{
    private String name;

    public JobSeeker(String name)
    {
        this.name = name;
    }

    public void onJobPosted(JobPost jobPost)
    {
        // Do something with the job posting
        System.out.println("Hi " + this.name + "! New job posted: "+ jobPost.getTitle());
    }
}

Then we have our job postings to which the job seekers will subscribe

package com.prady00.dp.behavioral.observor;

import java.util.ArrayList;
import java.util.List;

class EmploymentAgency
{
    private List<JobSeeker> observers = new ArrayList<>();

    protected void notify(JobPost jobPost)
    {
        for(JobSeeker jobSeeker :  observers) {
            jobSeeker.onJobPosted(jobPost);
        }
    }

    public void attach(JobSeeker observer)
    {
        observers.add(observer);
    }

    public void addJob(JobPost jobPost)
    {
        this.notify(jobPost);
    }
}

Then it can be used as

package com.prady00.dp.behavioral.observor;

public class ObservorRunner {

    public static void main(String[] args) {
        // Create subscribers
        JobSeeker pradeep = new JobSeeker("pradeep kumar");
        JobSeeker gaurav = new JobSeeker("gaurav varshney");

        // Create publisher and attach subscribers
        EmploymentAgency employmentAgency = new EmploymentAgency();
        employmentAgency.attach(pradeep);
        employmentAgency.attach(gaurav);

        // Add a new job and see if subscribers get notified
        employmentAgency.addJob(new JobPost("Software Engineer"));

        // Output
        // Hi pradeep kumar! New job posted: Software Engineer
        // Hi gaurav varshney! New job posted: Software Engineer
    }

}