Event Market

Fernando Urbano Final Project

Video Explanation

https://www.youtube.com/watch?v=38eP5bMDDyA

Table of Contents

• Event Market - Final Project
• Video Explanation
• Project Structure
• Postgres - Create Database and Database User
• Postgres - Create Tables
• Run Camel File
• Create a New Application
• Run All Applications
• Other Commands
• Project Overview and Explanation
• Project Separation
• Applications
• Design Patterns
• EIP Patterns
• Design Patterns Code

Project Structure

Each of the following is a separate application:

.
├── README.md
├── design_patterns.md
├── eip_patterns.md
├── event-market
├── event-market-abstract
├── event-market-event-creation
├── event-market-financial-validation
├── event-market-notification-engine
├── event-market-recommendation-engine
├── event-market-ticket-purchase
├── event-market-ticket-validation
├── event-market-creator-creation
├── event-market-user-creation
└── uml

249 directories, 372 files

• event-market: main application containing the database and classes.
• event-market-creator-creation, event-market-user-creation, and event-market-event-creation: applications to create instances of User, Creator and Event. Communicates with ActiveMQ via Camel.
• event-market-ticket-purchase: application to purchase ticket. Communicates with ActiveMQ via Camel.
• event-market-notification-engine: application to notify Users and Creators of new events, ticket purchase, financial validation of ticket purchase, recommendations, etc... Communicates with ActiveMQ via Camel.
• event-market-financial-validation, event-market-ticket-validation: applicatons to validate ticket purchase. Communicates with ActiveMQ via Camel.
• event-market-recommendation-engine: application to create recommendation for User given recent purchases. Communicates with ActiveMQ via Camel.
• event-market-abstract: blueprint application to create new applications with default files used for EventMarketInput.java and AbstractCamel.java.

Postgres - Create Database and Database User

Download postgres:

brew install postgres

Inside terminal, enter postgres terminal:

psql postgres

Check the existing databases:

\l

If the eventmarket_db does not exist, create it:

CREATE DATABASE eventmarket_db;

Enter in the newly created database:

\c eventmarket_db

Check that user fernandorochacorreaurbano exists:

\du

If the user does not exist, create the user:

CREATE USER fernandorochacorreaurbano WITH PASSWORD 'fernando123';

Grant all the privileges to this user:

GRANT ALL PRIVILEGES ON DATABASE eventmarket_db TO fernandorochacorreaurbano;

Quit the database:

\q

If the eventmarket_test_db does not exist, create it:

CREATE DATABASE eventmarket_test_db;

Enter in the newly created database:

\c eventmarket_db

Check that user fernandorochacorreaurbano exists:

\du

The user should exist, If the user does not exist, check again if the user permissions were applied to the first database.

CREATE USER fernandorochacorreaurbano WITH PASSWORD 'fernando123';

Grant all the privileges to this user:

GRANT ALL PRIVILEGES ON DATABASE eventmarket_db TO fernandorochacorreaurbano;

Quit the database:

\q

It is recommended that you download an extension to be able to visualize the databases. I recommend the "Database" extension from VSCode.

Postgress - Create Tables

The "DatabaseInitializer" can be used to create the tables that the applications have.

cd event-market
mvn clean install
mvn exec:java -Dexec.mainClass="eventmarket.main.DatabaseInitializer"

If you cannot run the DatabaseInitializer, it means there are relationship between classes that are not working well and should be changed. You can see the errors in the terminal.

Java Version

For this project, we use Java 21.0.2 and ActiveMQ 6.1.2.

Run Camel File

In a general way, running camel file can be achieved by the following command when inside a maven project that contains a pom referencing it:

mvn exec:java -Dexec.mainClass=camelinaction.FileCopierWithCamel

For our applications that communicate with Camel or that take inputs, we create the following shortcuts to run the files:

mvn exec:java -P run-event-market-input

mvn exec:java -P run-camel

Those can be viewed in the pom of each of the applications.

Create a New Application

1. Copy-paste event-market-abstract

2. If you need an input file, write it at the src/main/java/eventmarketinput/EventMarketInput.java. Else, keep it clean.

3. Define the name of your camel class which is located at src/main/java/camelinaction/AbstractCamel.java.

4. Inside the pom.xml, change the artifactId and name:

  <artifactId>event-market-{change the id here!}</artifactId>
  <groupId>com.camelinaction</groupId>
  <name>Event Market - {Change the name here!}</name>
  <version>1.0.0</version>

Change the name of the class used in Camel in the profiles part: <mainClass>camelinaction.UserCreationProducer</mainClass>.

  <profiles>
    <profile>
        <id>run-event-market-input</id>
        <properties>
            <mainClass>eventmarketinput.EventMarketInput</mainClass>
        </properties>
    </profile>
    <profile>
        <id>run-camel</id>
        <properties>
            <mainClass>camelinaction.{Change to the name of your Camel file here!}</mainClass>
        </properties>
      </profile>
  </profiles>

5. To run the input file:

mvn exec:java -P run-event-market-input

To run camel:

mvn exec:java -P run-camel

Run All Applications

0. SET UP

0.0 DATABASE

Create the database as specified before.

0.1 EVENT-MARKET

cd event-market
mvn clean install
cd ..

You can now close this terminal. It is important to run the event-market application before running camel applications.

0.1. ACTIVEMQ

activemq console

It is important to keep this terminal running at all times.

1. ENTITIES CREATION: user-creation & event-creation & creator-creation

1.1. TERMINAL notification-engine:

cd event-market-notification-engine
mvn clean install exec:java -P run-camel
cd ..

1.2. user-creation

1.2.1 TERMINAL user-creation Camel

cd event-market-user-creation
mvn clean install exec:java -P run-camel
cd ..

1.2.2 TERMINAL user-creation Input

cd event-market-user-creation
mvn clean install exec:java -P run-event-market-input
cd ..

1.3. creator-creation

1.3.1 TERMINAL creator-creation Camel

cd event-market-creator-creation
mvn clean install exec:java -P run-camel
cd ..

1.3.2 TERMINAL creator-creation Input

cd event-market-creator-creation
mvn  clean install exec:java -P run-event-market-input
cd ..

1.4. event-creation

1.4.1 TERMINAL event-creation Camel

cd event-market-event-creation
mvn  clean install exec:java -P run-camel
cd ..

1.4.2 TERMINAL event-creation Input

cd event-market-event-creation
mvn clean install exec:java -P run-event-market-input
cd ..

2. TICKET PURCHASE: ticket-purchase & financial-validation & ticket-validation & recommendation-engine

2.1. TERMINAL notification-engine:

cd event-market-notification-engine
mvn clean install exec:java -P run-camel
cd ..

2.2. TERMINAL financial-validation:

cd event-market-financial-validation
mvn clean install exec:java -P run-camel
cd ..

2.3. TERMINAL ticket-validation:

cd event-market-ticket-validation
mvn clean install exec:java -P run-camel
cd ..

2.4. TERMINAL recommendation-engine:

cd event-market-recommendation-engine
mvn clean install exec:java -P run-camel
cd ..

2.5. ticket-purchase

2.5.1 TERMINAL ticket-purchase Camel

cd event-market-ticket-purchase
mvn clean install exec:java -P run-camel
cd ..

2.5.2 TERMINAL ticket-purchase Input

cd event-market-ticket-purchase
mvn clean install exec:java -P run-event-market-input
cd ..

Other commands

Update puml files

In case modificatons are made in the classes of the event-market, it is necessary to update the puml files inside the uml folder.

After having updated them, run the following script:

chmod +x update_uml_images.sh # make the script executable in mac
./update_uml_images.sh

Transform latex in markdown

pandoc -s reports/main.tex -o reports/main.md

Project Overview and Explanation

We aim to build an Event Ticket Purchase Platform, including event creation, purchase app, notification system, payment validation, event recommender engine.

In order to facilitate the understanding we highlight the following with specific formatting:

• [Application Names]

• [Design Patterns]

• [Classes]

Project Separation

Creation Processes

Ticket Purchase Processes

Applications

• Main Application with Database connection:

  • [Event Market: event-market]

• Creation Camel Applications:

  • [Event Creation: event-market-event-creation]

  • [User Creation: event-market-user-creation]

  • [Creator Creation: event-market-event-creation]

• Ticket Purchase Camel Applications:

  • [Ticket Purchase: event-market-ticket-purchase]

  • [Financial Validation: event-market-financial-validation]

  • [Ticket Validation: event-market-ticket-validation]

  • [Event Recommender Engine: event-market-recommendation-engine]

  • [Notification Engine: event-market-notification-engine]

Design Patterns

Iterator

• Inside the [Ticket Purchase] app.

• Iterator pattern is used inside the [TicketTypeIterator] class to iterate over the ticket types of an Event.

• Provides a convenient way for users to browse through various ticket options, helping them understand the minimum age, prices, and types of tickets available for an event.

Singleton

• Inside the [Financial Validation] app.

• The [FinancialValidator] class is a Singleton, ensuring that a single instance validates ticket requests and manages their statuses.

• The Singleton pattern allows the [FinancialValidator] to track which requests it has validated recently, avoiding redundant checks and improving efficiency.

Strategy

• Inside the [Financial Validation] app.

• The [PaymentMethod] class acts as a Strategy pattern, providing an interface implemented by different payment classes such as [CheckingsAccountPayment] and [CardPayment].

• This pattern enables the [Financial Validation] app to support various payment methods flexibly, facilitating future additions or modifications by simply extending the interface.

Facade

• Inside the [Event Recommender Engine].

• The [EventRecommender] class acts as a Facade, simplifying the complexity of underlying classes derived from the [AbstractRecommenderModel].

• This pattern allows for interactions with different recommendation models, making it easier to generate recommendations for users based on recent ticket requests.

Template Method

• Inside the [Event Recommender Engine].

• The Template Method pattern is used by [AbstractRecommenderModel] and its derived classes, implementing some functionalities while leaving the heavy lifting to derived classes.

• This pattern provides a clear structure for creating different recommendation models, allowing derived classes to implement the specific logic needed to generate suggestions for users.

Applications and UML Drawings

Event Creation

App creates a new instances of [Event].

The [Event] class has:

• at least one [TicketType] instance: for instance, premium, normal, free, etc... which specifies the price, minimum age, total tickets for the type.

• one [Location] instance.

• one [Creator] (each [Creator] can have multiple events).

THe application communicates with [Notification Engine].

User Creation

Similar to the creation of Event. THe application communicates with [Notification Engine].

Creator Creation

Similar to the creation of Event. THe application communicates with [Notification Engine].

Ticket Purchase

App allows purchase of tickets from [User].

The [TicketRequest] is the main class in this process.

Every [TicketRequest] instance has an instance of:

• [TicketType]: the type of ticket the user is buying. A single event allows for multiple [TicketType]'s with different prices, descriptions, etc. A [TicketRequest] instance only has one [TicketType] instance among the ones available for the event.

• [Financial Information]: the information regarding the payment of the ticket. Each instance of it has one instance of a class derived from [PaymentMethod], which can be [CheckingsAccountPayment] or [CardPayment]. In case the ticket price is 0, the [FinancialInformation] is not necessary.

• [User]: the person purchasing the ticket.

• [Recipient]: the person who will enter the event. An instance of class [User] can be transformed into an instance of the class [Recipient] in case the user is trying to buy a ticket for himself/herself.

Once the [TicketRequest] is completed, it is sent to:

• the [Financial Validation] in case a payment is necessary.

• the [Ticket Validation] in case no payment is necessary.

• the [Event Recommender Engine] regardless of payment options.

• the [Notification Engine] to say that the ticket is being processed.

Inside the [Ticket Purchase] we use the [Iterator] pattern with [TicketTypeIterator], which receives an [Event] and iterates over the ticket types. The iterator is considerably useful for the [User] to understand the minimum age among all the ticket types, the minimum and maximum price of the event, etc...

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Financial Validation

App gets messages from the distributed message queue saying that a [TicketRequest] needs payment validation.

Inside the app, The [TicketRequest] is sent to the [FinancialValidator] which ask for a payment process.

The [FinancialValidator] is a [Singleton] because it aims to track which instances of [TicketRequest] it has checked recently: this allows the [FinancialValidator] to avoid wasting time checking twice the same [TicketRequest].

The [TicketRequest] instance has an instance of [FinancialInformation], which has one instance of a class derived from [PaymentMethod].

The available classes are:

• [CardPayment]

• [CheckingsAccountPayment]

[PaymentMethod] is a [Strategy] pattern.

The [FinancialValidator] must be able to handle both classes.

Therefore, if there is an addition/change in those classes, the [FinancialValidator] must be changed/incremented as well.

Finally, the [FinancialValidator] sends a request to the desired bank to approve the payment (here we simplified the implementation and just randomly approve or reject the payment).

The application sends message to the

• the [Ticket Validation] in case the payment is accepted.

• the [Notification Engine] in case the payment is rejected.

Ticket Validation

App gets messages from the distributed message queue saying that the ticket for that [User] has been approved.

The [Ticket] instance is created with a [TicketRequest] instance inside. The [TicketRequest] contains all the information about the [Event] inside it.

The [Ticket] instance is added to the [User]. More specifically, the [Ticket] instance is added to a list of owned tickets and remove the instance of [TicketRequest] from the list of requested tickets.

Finally, a message is sent to the [Notification Engine].

Event Recommender Engine

App creates recommendations to the [User] based on recent purchases.

The event recommender engine receives a message everytime a [User] uses the [Ticket Purchase] application.

The [EventRecommender] class takes [User] and a model name (the one that is currently being used) and creates a recommendation for the user based on recent ticket requests.

The [EventRecommender] works as a [Facade] as it simplifies the underlying complexity of classes derived from the [AbstractRecommenderModel], like [NlpRecommenderModel] and [PopularityRecommenderModel].

We have a [Template Method] with the [AbstractRecommenderModel] and its derived classes. The [AbstractRecommenderModel] implements some of the functionalities of the model, but leaves the heavy lifting of creating suggestions for the derived classes.

It sends messages to the [Notification Engine].

Notification Engine

The app translates internal information of the system into proper notifications for the users, allowing the market department to modify the content to better address the [User] or [Creator].

It sends message to the Notification Sender (which could later just send the notification to the user or creator.)

EIP Patterns

1. Message Channel and Endpoint: How we connect an application to the channel. It is used in every single application.

2. Content-Based Router: Routes each message to the correct recipient based on message content.

3. Dead-Letter Channel: The messaging system determines that it cannot or should not deliver a message. It is used (in our case and generally) for messages that cannot be processed successfully after a certain amount of tries.

4. Invalid-Message Channel: Channel for messages that are malformed or fail validation checks and cannot be processed as expected.

5. Message Translator: Used between other filters or applications to translate one data format into another.

6. Wire Tap: Inserts a simple Recipient List into the channel that publishes each incoming message to the main channel and a secondary channel.

7. Normalizer: Routes each message type through a custom Message Translator so that the resulting messages match a common format.

8. Content Enricher: Access an external data source in order to augment a message with missing information.

9. Multicast: Allows a message to be sent to multiple recipients or destinations simultaneously (not in the book, but included in Camel: https://camel.apache.org/components/4.4.x/eips/multicast-eip.html).

event-market-user-creation: User Creation Producer

1. Message Channel

   to("jms:queue:notification.producer.user.creation.queue")

2. Point-to-Point Channel: Inherited and implemented by this type of queue.

event-market-creator-creation: Creator Creation Producer

1. Message Channel

   to("jms:queue:notification.producer.creator.creation.queue")

2. Point-to-Point Channel: Inherited and implemented by this type of queue.

event-market-event-creation: Event Creation Producer

1. Message Channel

   to("jms:queue:notification.producer.event.creation.queue")

2. Point-to-Point Channel: Inherited and implemented by this type of queue.

event-market-notification-engine: Notification Engine Producer

1. Message Channel

   from("jms:queue:notification.producer.user.creation.queue")
   from("jms:queue:notification.producer.creator.creation.queue")
   from("jms:queue:notification.producer.event.creation.queue")
   from("jms:queue:notification.producer.ticketpayment.inprocess.queue")
   from("jms:queue:notification.producer.ticketpayment.rejected.queue")
   from("jms:queue:notification.producer.ticketpayment.approved.queue")
   from("jms:queue:notification.producer.recommendations.queue")

2. Content-Based Router

   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")
       .otherwise()
           .to("jms:queue:notification.builder.user.creation.queue")
   
   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")
       .otherwise()
           .to("jms:queue:notification.builder.creator.creation.queue")
   
   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")
       .otherwise()
           .to("jms:queue:notification.builder.event.creation.queue")
   
   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")
       .otherwise()
           .to("jms:queue:notification.builder.ticketpayment.inprocess.queue")
   
   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")
       .otherwise()
           .to("jms:queue:notification.builder.ticketpayment.rejected.queue")
   
   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")
       .otherwise()
           .to("jms:queue:notification.builder.ticketpayment.approved.queue")
   
   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")
       .otherwise()
           .to("jms:queue:notification.builder.recommendations.queue")

3. Point-to-Point Channel: Inherited and implemented by this type of queue.

4. Content Enricher

   if (user != null) {
       notificationJson.put("receivesEmails", user.getReceivesEmails());
       notificationJson.put("receivesPushs", user.getReceivesPushs());
   } else if (creator != null) {
       notificationJson.put("receivesEmails", true);
       notificationJson.put("receivesPushs", true);
   }

5. Dead-Letter Channel

   choice()
       .when(header("validMessage").isEqualTo(false))
           .to("jms:queue:notification.deadletter.queue")

event-market-ticket-purchase: Ticket Purchase Producer

1. Message Channel

   to("jms:queue:ticketvalidation.deadletter.queue")
   to("jms:queue:ticketvalidation.financial.queue")
   to("jms:queue:ticketvalidation.completion.queue")
   to("jms:queue:notification.producer.ticketpayment.inprocess.queue")
   to("jms:queue:recommender.builder.queue")

2. Content-Based Router

   .when(simple("${body[validPrice]} == false"))
       .to("jms:queue:ticketvalidation.deadletter.queue")
   .otherwise()
       .multicast().stopOnException()
           .parallelProcessing()
           .to("direct:processTicket", "direct:inProcessNotification", "direct:inProcessRecommender")
       .end()
   .end()

3. Invalid-Message Channel

   .choice()
       .when(simple("${body[validPrice]} == false"))
           .log("Sending to ticketvalidation.invalidmessage.queue")
           .to("jms:queue:ticketvalidation.invalidmessage.queue")

4. Wire Tap

   .to("direct:processTicket", "direct:inProcessNotification", "direct:inProcessRecommender")

5. Message Translator

   .marshal().json(JsonLibrary.Jackson)  // Convert Map to 

6. Multicast

   .otherwise()
       .multicast().stopOnException()
           .parallelProcessing()
           .to("direct:processTicket", "direct:inProcessNotification", "direct:inProcessRecommender")
       .end()
   .end()

event-market-financial-validation: Financial Validation Producer

1. Message Channel

   from("jms:queue:ticketvalidation.financial.queue")

2. Content-Based Router

   choice()
       .when(simple("${body[financiallyValid]} == true"))
           .to("jms:queue:ticketvalidation.completion.queue")
       .otherwise()
           .to("jms:queue:notification.producer.ticketpayment.rejected.queue")

event-market-ticket-validation: Ticket Validation Producer

1. Message Channel

   from("jms:queue:ticketvalidation.completion.queue")

2. Point-to-Point Channel: Inherited and implemented by this type of queue.

3. Content-Based Router

   choice()
       .when(simple("${body[message]} == 'ticket-payment-approved'"))
           .to("jms:queue:notification.producer.ticketpayment.approved.queue")
       .otherwise()
           .to("jms:queue:ticketvalidation.deadletter.queue")

4. Message Translator

   .marshal().json(JsonLibrary.Jackson)
   .marshal().json(JsonLibrary.Jackson)

5. Normalizer

   .process(exchange -> {
       Object body = exchange.getIn().getBody();
       if (body instanceof String) {
           String jsonBody = (String) body;
           @SuppressWarnings("unchecked")
           Map<String, Object> messageData = objectMapper.readValue(jsonBody, Map.class);
           exchange.getIn().setBody(messageData);
       } else if (body instanceof Map) {
           // No conversion needed, body is already a Map
       } else {
           throw new IllegalArgumentException("Unsupported message format: " + body.getClass());
       }
   })

6. Dead-Letter Channel

   .choice()
       .when(simple("${body[message]} == 'ticket-payment-approved'"))
           .to("jms:queue:notification.producer.ticketpayment.approved.queue")
       .otherwise()
           .to("jms:queue:ticketvalidation.deadletter.queue")

event-market-recommendation-engine: Recommendation Engine Producer

1. Message Channel

   from("jms:queue:recommender.builder.queue")

2. Message Translator

   .unmarshal().json(JsonLibrary.Jackson, Map.class)  // Unmarshal 

3. Content Enricher

   .process(new Processor() {
       @Override
       public void process(Exchange exchange) throws Exception {
           EntityManager em = emf.createEntityManager();
           em.getTransaction().begin();
   
           try {
               @SuppressWarnings("unchecked")
               Map<String, Object> messageData = exchange.getIn().getBody(Map.class);
               int userId = (int) messageData.get("userId");
   
               // Fetch User from the database
               User user = em.find(User.class, userId);
               if (user == null) {
                   throw new Exception("User not found for id: " + userId);
               }
   
               // Create the recommender model and set EntityManager
               NlpRecommenderModel model = new NlpRecommenderModel(user);
               model.setEntityManager(em);
               
               // Or use PopularityRecommenderModel if needed
               // PopularityRecommenderModel model = new PopularityRecommenderModel(user);
               // model.setEntityManager(em);
   
               EventRecommender eventRecommender = new EventRecommender(model);
               eventRecommender.requestRecommendation();
               List<Event> recommendations = eventRecommender.sendRecommendation();
   
               // Extract event IDs from the recommendations
               List<Integer> eventIds = recommendations.stream()
                   .map(Event::getId)
                   .collect(Collectors.toList());
   
               // Prepare recommendation message
               Map<String, Object> recommendationMessage = new HashMap<>();
               recommendationMessage.put("eventIds", eventIds);
               recommendationMessage.put("userId", userId);
               recommendationMessage.put("message", "event-recommendation");
   
               String jsonMessage = objectMapper.writeValueAsString(recommendationMessage);
               exchange.getIn().setBody(jsonMessage);
   
               em.getTransaction().commit();
           } catch (Exception e) {
               if (em.getTransaction().isActive()) {
                   em.getTransaction().rollback();
               }
               throw e;
           } finally {
               em.close();
           }
       }
   })

Design Patterns Code

Singleton Pattern

public class FinancialValidator {
    private static FinancialValidator instance;

    private FinancialValidator() {
        recentlyCheckedRequestIds = new ArrayList<>();
    }

    public static FinancialValidator getInstance() {
        if (instance == null) {
            instance = new FinancialValidator();
        }
        return instance;
    }
}

Iterator Pattern

public abstract class AbstractIterator<T> {
    public abstract T next();
    public abstract int getSteps();
    public abstract void setSteps(int step);
    public abstract boolean isDone();
    public abstract T current();
    public abstract T first();
}

public class TicketTypeIterator extends AbstractIterator<TicketType> {
    private Event event;
    private int currentIndex;
    private List<TicketType> ticketTypes;
    
    public TicketTypeIterator(Event event) {
        this.event = event;
        this.ticketTypes = event.getTicketTypes();
        this.currentIndex = 0;
    }

    @Override
    public TicketType next() {
        if (isDone()) {
            throw new IllegalStateException("No more ticket types");
        }
        TicketType ticketType = ticketTypes.get(currentIndex);
        currentIndex++;
        return ticketType;
    }

    // other overridden methods...
}

Template Method Pattern

public abstract class AbstractRecommenderModel {
    public abstract List<Event> recommend(int n);
    public abstract List<Event> recommend();
}

public class NlpRecommenderModel extends AbstractRecommenderModel {
    @Override
    public List<Event> recommend(int n) {
        // implementation...
    }

    @Override
    public List<Event> recommend() {
        // implementation...
    }
}

public class PopularityRecommenderModel extends AbstractRecommenderModel {
    @Override
    public List<Event> recommend(int n) {
        // implementation...
    }

    @Override
    public List<Event> recommend() {
        // implementation...
    }
}

Facade Pattern

public class EventRecommender {
    private int id;
    private User user;
    private AbstractRecommenderModel model;
    private int numberRecommendations;
    private List<Event> eventRecommendations;

    public EventRecommender(AbstractRecommenderModel model) {
        this.user = model.getUser();
        this.model = model;
        this.numberRecommendations = 3;
        this.eventRecommendations = new ArrayList<>();
    }

    public List<Event> sendRecommendation() {
        return eventRecommendations;
    }

    public void setNumberRecommendations(int numberRecommendations) {
        this.numberRecommendations = numberRecommendations;
    }

    // ...
}

Strategy Pattern

public abstract class PaymentMethod {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    protected int id;

    // common fields if any

    // Getters and Setters
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}

public class CardPayment extends PaymentMethod {
    @Column(nullable = false)
    private String cardNumber;

    @Column(nullable = false)
    private String cardHolder;

    @Temporal(TemporalType.DATE)
    @Column(nullable = false)
    private Date expirationDate;

    @Column(nullable = false)
    private String cvv;

    public CardPayment() {
    }

    public CardPayment(String cardNumber, String cardHolder, Date expirationDate, String cvv) {
        this.cardNumber = cardNumber;
        this.cardHolder = cardHolder;
        this.expirationDate = expirationDate;
        this.cvv = cvv;
    }

    // ...
}

public class CheckingAccountPayment extends PaymentMethod {
    @Column(nullable = false)
    private String accountNumber;

    @Column(nullable = false)
    private String routingNumber;

    public CheckingAccountPayment() {
    }

    public CheckingAccountPayment(String accountNumber, String routingNumber) {
        this.accountNumber = accountNumber;
        this.routingNumber = routingNumber;
    }
    
    // ...
}