The state pattern deals with altering an object’s behaviour when its state changes.

Imagine the case of a class responsible for generating user interface based on the state. You got anonymous, logged-in and admin users.

We shall create an interface called GreetingState which defines the action of drawing a html text with a welcome message to the user. There is going to be a different implementation according to the states that we have.

package com.gkatzioura.design.behavioural.state;

public interface GreetingState {

    String create();

}

We shall implement the GreetingState for the anonymous user.

package com.gkatzioura.design.behavioural.state;

public class AnonymousGreetingState implements GreetingState {

    private static final String FOOTER_MESSAGE = "<p><Hello anonymous user!</p>";

    @Override
    public String create() {
        return FOOTER_MESSAGE;
    }

}

Then we shall implement the GreetingState for the logged in user. This one would create a personalised message.

package com.gkatzioura.design.behavioural.state;

public class LoggedInGreetingState implements GreetingState {

    private static final String FOOTER_MESSAGE = "<p><Hello %s!</p>";

    private final String username;

    public LoggedInGreetingState(final String username) {
        this.username = username;
    }

    @Override
    public String create() {
        return String.format(FOOTER_MESSAGE,username);
    }

}

And at last the admin Footer.

package com.gkatzioura.design.behavioural.state;

import java.util.Date;

public class AdminGreetingState implements GreetingState {

    private static final String FOOTER_MESSAGE = "<p><Hello %s, last login was at %s</p>";

    private final String username;
    private final Date lastLogin;

    public AdminGreetingState(final String username, Date lastLogin) {
        this.username = username;
        this.lastLogin = lastLogin;
    }


    @Override
    public String create() {
        return String.format(FOOTER_MESSAGE,username,lastLogin);
    }

}

The we shall create the stateui context.

package com.gkatzioura.design.behavioural.state;

import java.io.PrintWriter;

public class StateUIContext {

    private GreetingState greetingState;

    public void setGreetingState(GreetingState greetingState) {
        this.greetingState = greetingState;
    }

    public void create(PrintWriter printWriter) {
        printWriter.write(greetingState.create());
    }
}

Let’s put them all together.

package com.gkatzioura.design.behavioural.state;

import java.io.PrintWriter;
import java.util.Date;

public class StateMain {

    public static void main(String[] args) {

        StateUIContext stateUIContext = new StateUIContext();

        try(PrintWriter printWriter = new PrintWriter(System.out)) {
            stateUIContext.setGreetingState(new AnonymousGreetingState());
            stateUIContext.create(printWriter);
            printWriter.write("\n");
            stateUIContext.setGreetingState(new LoggedInGreetingState("someone"));
            stateUIContext.create(printWriter);
            printWriter.write("\n");
            stateUIContext.setGreetingState(new AdminGreetingState("admin",new Date()));
            stateUIContext.create(printWriter);
            printWriter.write("\n");
        }
    }
}

You can find the sourcecode on github.

Observer is one of the most popular design patterns. It has been used a lot on many software use cases and thus many languages out there provide it as a part of their standard library.

By using the observer pattern we can tackle the following challenges.

• Dependency with objects defined in a way that avoids tight coupling
• Changes on an object changes its dependent objects
• An object can notify all of its dependent objects

Imagine the scenario of a device with multiple sensors. Some parts of the code will need to get notified when new sensor data arrive and thus act accordingly. We will start by a simple class which represents the json data.

package com.gkatzioura.design.behavioural.observer;

public class SensorData {

    private final String sensor;
    private final Double measure;

    public SensorData(String sensor, Double measure) {
        this.sensor = sensor;
        this.measure = measure;
    }

    public String getSensor() {
        return sensor;
    }

    public Double getMeasure() {
        return measure;
    }
}

The we shall create the observer interface. Every class that implements the observer interface shall be notified once a new object is created.

package com.gkatzioura.design.behavioural.observer;

public interface Observer {

    void update(SensorData sensorData);

}

Next step is to create the observable interface. The observable interface will have methods in order to register the observers that need to get notified.

package com.gkatzioura.design.behavioural.observer;

public interface Observable {

    void register(Observer observer);

    void unregister(Observer observer);

    void updateObservers();

}

Now let us put some implementations.
The sensor listener will receive data from the sensors and notify the observers about the presence of data.

package com.gkatzioura.design.behavioural.observer;

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

public class SensorReceiver implements Observable {

    private List data = new ArrayList();
    private List observers = new ArrayList();

    @Override
    public void register(Observer observer) {
        observers.add(observer);
    }

    @Override
    public void unregister(Observer observer) {
        observers.remove(observer);
    }

    public void addData(SensorData sensorData) {
        data.add(sensorData);
    }

    @Override
    public void updateObservers() {

        /**
         * The sensor receiver has retrieved some sensor data and thus it will notify the observer
         * on the data it accumulated.
         */

        Iterator iterator = data.iterator();

        while (iterator.hasNext()) {

            SensorData sensorData = iterator.next();

            for(Observer observer:observers) {
                observer.update(sensorData);
            }

            iterator.remove();
        }
    }

}

The we will create an observer which shall log the sensor data received to database, it might be an influxdb or an elastic search you name it.

package com.gkatzioura.design.behavioural.observer;

public class SensorLogger implements Observer {

    @Override
    public void update(SensorData sensorData) {

        /**
         * Persist data to the database
         */

        System.out.println(String.format("Received sensor data %s: %f",sensorData.getSensor(),sensorData.getMeasure()));
    }

}

Let’s put the all together.

package com.gkatzioura.design.behavioural.observer;

public class SensorMain {

    public static void main(String[] args) {

        SensorReceiver sensorReceiver = new SensorReceiver();
        SensorLogger sensorLogger = new SensorLogger();
        sensorReceiver.register(sensorLogger);
        sensorReceiver.addData(new SensorData("temperature",1.2d));
        sensorReceiver.updateObservers();
    }
}

You can find the source code on github.