## Behavioral Patterns
- [Observer/Listener](#observerlistener)
- [Strategy](#strategy)
- [Command](#command)
- [State](#state)
- [Chain of Responsibility](#chain-of-responsibility)
- [Visitor](#visitor)
- [Mediator](#mediator)
- [Memento](#memento)

### Observer/Listener
The Observer pattern, also known as the Listener pattern, is a 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.

Event-driven programming is a common use case for the Observer pattern, where the subject is an event source and the observers are event handlers.

**Example**

```java
package nl.itzitzo.design.patterns.observer

import kotlin.properties.Delegates

interface EventListener {
    fun onEvent(event: String)
}

class EventSubscriber(val name: String) : EventListener {
    override fun onEvent(event: String) {
        println("$name received event: $event")
    }
}

/**
 *  The Broker class acts as the subject in the Observer pattern. It maintains a list of subscribers (listeners) and notifies them of any events that occur.
 *  The event property is observable, and whenever it changes,
 *  the notify method is called to inform all subscribers of the new event.
 */
class Broker {
    private val listeners = mutableListOf<EventListener>()

    fun subscribe(listener: EventListener) {
        listeners.add(listener)
    }

    fun unsubscribe(listener: EventListener) {
        listeners.remove(listener)
    }

    var event: String by Delegates.observable("nope") { _, old, new ->
        if (old != new) {
            notify(new)
        }
    }

    private fun notify(event: String) {
        for (listener in listeners) {
            listener.onEvent(event)
        }
    }
}

/**
 *  The EventPubSub object demonstrates the usage of the Broker and EventSubscriber classes.
 *  It creates a broker, subscribes two listeners, and then changes the event property to trigger notifications to the subscribers.
 *  It also demonstrates unsubscribing a listener and showing that it no longer receives events.
 */
object EventPubSub {

    @JvmStatic
    fun main(args: Array<String>) {
        val broker = Broker()

        val subscriber1 = EventSubscriber("Subscriber 1")
        val subscriber2 = EventSubscriber("Subscriber 2")

        with(broker) {
            subscribe(subscriber1)
            subscribe(subscriber2)

            event = "Event 1"
            event = "Event 2"

            unsubscribe(subscriber1)

            event = "Event 3"
        }

    }

}
```

### Strategy
The Strategy pattern is a design pattern that enables selecting an algorithm's behavior at runtime.

**Example**

```java
package nl.itzitzo.design.patterns.strategy

data class Event(val name: String, val data: String) {
    override fun toString(): String {
        return "Event(name='$name', data='$data')"
    }
}

/**
 * The Formatter class represents a strategy for formatting events.
 * It takes a formatter function as a parameter, which defines how the event should be formatted.
 */
class Formatter(val formatter: (Event) -> String) {
    fun format(event: Event): String {
            return formatter(event)
        }
}

/**
 * The EventFormatter object demonstrates the usage of the Formatter class with different formatting strategies.
 * It defines a map of formatters for JSON, XML, and plain text formats.
 * In the main function, it creates an event and uses each formatter to format the event accordingly.
 */
object EventFormatter {
    /**
     *  Each formatter is a function that takes an event string and returns a formatted string.
     */
    val formatters: Map<String, (Event) -> String> = mapOf(
        "json" to { event -> """{"name": "${event.name}, "data: ${event.data}"}""" },
        "xml" to { event -> """<name>${event.name}</name><data>${event.data}</data>""" },
        "plain" to { event -> event.toString() }
    )

    @JvmStatic
    fun main(args: Array<String>) {
        val event = Event("UserLoggedIn", "User with ID 123 has logged in.")

        // Using different formatters from the map
        val jsonFormatter = Formatter(formatters["json"] ?: error("JSON formatter not found"))
        println(jsonFormatter.format(event))

        val xmlFormatter = Formatter(formatters["xml"] ?: error("XML formatter not found"))
        println(xmlFormatter.format(event))

        val plainFormatter = Formatter(formatters["plain"] ?: error("Plain formatter not found"))
        println(plainFormatter.format(event))
    }
}
```

### Command

The command pattern is used to encapsulate a request as an object with all the information needed to perform the action,
thereby allowing for parameterization of clients with queues, requests, and operations.
 
**Example**

```java
package nl.itzitzo.design.patterns.command

data class Event(val name: String, val data: String)

val events = mutableListOf<Event>()

sealed interface EventCommand {
    fun execute(event: Event)
}

class LogCommand() : EventCommand {
    override fun execute(event: Event) {
        println("Log event: $event");
    }
}

class SaveCommand() : EventCommand {
    override fun execute(event: Event) {
        println("Saving event: $event")
        events.add(event)
    }
}

/**
 * The EventProcessor object demonstrates the usage of the Command pattern to process events.
 * It defines a queue of commands (LogCommand and SaveCommand) that will be executed for each event.
 * In the main function, it creates a list of events and processes each event through the command queue,
 * logging and saving the events accordingly.
 */
object EventProcessor {
private val queue = listOf<EventCommand>(LogCommand(), SaveCommand())

    @JvmStatic
    fun main(args: Array<String>) {
        val events = listOf(
            Event("UserLoggedIn", "User with ID 123 has logged in."),
            Event("UserLoggedIn", "User with ID 234 has logged in."),
            Event("UserLoggedIn", "User with ID 345 has logged in.")
        )

        events.forEach {e ->
            for (command in queue) {
                command.execute(e)
            }
            for (savedEvent in events) {
                println("Saved event: $savedEvent")
            }
        }

    }
}
```

### State
The state pattern is a design pattern that allows an object to alter its behavior when its internal state changes. 
The object will appear to change its class.

**Example**

```java
package nl.itzitzo.design.patterns.state

sealed class EventState(state: Int) {
    object Created : EventState(1)
    object Scheduled : EventState(2)
    object Running : EventState(3)
    object Completed : EventState(4)
    object Failed : EventState(5)
}

/**
 * The EventStateManager class manages the state of an event using the State design pattern.
 * It maintains a current state and provides methods to set the state based on an integer input
 * and to display the current state. The setState method updates the state based on predefined integer values,
 * while the displayState method prints out the current state of the event.
 */
class EventStateManager() {
    private var state: EventState = EventState.Created

    fun setState(int: Int) {
        when (int) {
            1 -> state = EventState.Created
            2 -> state = EventState.Scheduled
            3 -> state = EventState.Running
            4 -> state = EventState.Completed
            5 -> state = EventState.Failed
            else -> println("Invalid state: $int")
        }
    }

        fun displayState() {
            when (state) {
                is EventState.Created -> println("Event is in Created state")
                is EventState.Scheduled -> println("Event is in Scheduled state")
                is EventState.Running -> println("Event is in Running state")
                is EventState.Completed -> println("Event is in Completed state")
                is EventState.Failed -> println("Event is in Failed state")
            }
        }
}

/**
 * The EventStater object demonstrates the usage of the State pattern to manage the state of an event.
 * It creates an instance of EventStateManager and changes its state through a series of method calls,
 * displaying the current state after each change. This illustrates how the state of an event can be
 * managed and displayed using the State design pattern.
 */
object EventStater {
    @JvmStatic
    fun main(args: Array<String>) {
        val stateManager = EventStateManager()

        stateManager.setState(1)
        stateManager.displayState()

        stateManager.setState(2)
        stateManager.displayState()

        stateManager.setState(3)
        stateManager.displayState()

        stateManager.setState(4)
        stateManager.displayState()

        stateManager.setState(5)
        stateManager.displayState()
    }
}
```

### Chain of Responsibility

The Chain of Responsibility pattern is a design pattern that allows an object to send a command without knowing which object will handle the request.

**Example**
```java
package nl.itzitzo.design.patterns.chain

data class Event(val name: String, val data: String)

val events = mutableListOf<Event>()

interface EventHandler {
    fun handle(event: Event): Boolean
}

class LogHandler(val next: EventHandler) : EventHandler {
    override fun handle(event: Event): Boolean {
        println("Log event: $event")
        return next.handle(event)
    }
}

class SaveHandler(val next: EventHandler? = null) : EventHandler {
    override fun handle(event: Event): Boolean {
        println("Saving event: $event")
        events.add(event)
        return next?.handle(event) ?: true
    }
}


object EventChain {
    @JvmStatic
    fun main(args: Array<String>) {
        val handlerChain = LogHandler(SaveHandler())

        val event = Event("UserLoggedIn", "User with ID 123 has logged in.")
        handlerChain.handle(event)
    }
}
```

### Visitor
The Visitor pattern is a design pattern that allows you to separate an algorithm from the objects on which it operates. 

**Example**

```java
package nl.itzitzo.design.patterns.visitor

sealed class EventContract(val name: String, val data: String) {
    abstract fun accept(visitor: EventVisitorResult): String
}

class SqsEventContract(data: String) : EventContract("sqs", data) {
    override fun accept(visitor: EventVisitorResult): String {
        return visitor.visit(this)
    }
}
class KafkaEventContract(data: String) : EventContract("kafka", data) {
    override fun accept(visitor: EventVisitorResult): String {
        return visitor.visit(this)
    }
}
class RabbitEventContract(data: String) : EventContract("rabbit", data) {
    override fun accept(visitor: EventVisitorResult): String {
        return visitor.visit(this)
    }
}

val eventContracts = mutableListOf<EventContract>()

interface EventVisitorResult {
    fun visit(event: SqsEventContract): String
    fun visit(event: KafkaEventContract): String
    fun visit(event: RabbitEventContract): String
}

class LogVisitor : EventVisitorResult {
    override fun visit(event: SqsEventContract): String {
        return "Processing SQS event: ${event.data} \n"
    }

    override fun visit(event: KafkaEventContract): String {
        return "Processing Kafka event: ${event.data} \n"
    }

    override fun visit(event: RabbitEventContract): String {
        return "Processing RabbitMQ event: ${event.data} \n"
    }
}

/**
 * The SaveVisitor class implements the EventVisitorResult interface to save event contracts to a list.
 * It overrides the visit methods for each type of event contract (SQS, Kafka, RabbitMQ) and adds the event to the eventContracts list.
 * It also returns a string indicating that the event is being saved, along with the event data.
 */
class SaveVisitor : EventVisitorResult {
    override fun visit(event: SqsEventContract): String {
        eventContracts.add(event)
        return "Saving SQS event: ${event.data} \n"
    }

    override fun visit(event: KafkaEventContract): String {
        eventContracts.add(event)
        return "Saving Kafka event: ${event.data} \n"
    }

    override fun visit(event: RabbitEventContract): String {
        eventContracts.add(event)
        return "Saving RabbitMQ event: ${event.data} \n"
    }
}

/**
 * The EventVisitor object demonstrates the usage of the Visitor pattern to process different types of event contracts.
 * It defines a list of event contracts (SQS, Kafka, RabbitMQ) and applies two visitors (LogVisitor and SaveVisitor) to each event contract.
 * The LogVisitor processes the events by logging their data, while the SaveVisitor saves the events to a list.
 * The results of both visitors are printed to the console.
 */
object EventVisitor {

    @JvmStatic
    fun main(args: Array<String>) {
        val events = listOf(
            SqsEventContract("User with ID 123 has logged in."),
            KafkaEventContract("User with ID 234 has logged in."),
            RabbitEventContract("User with ID 345 has logged in.")
        )
        events.map { it.accept(LogVisitor()) }.reduce { acc, s -> acc + s }.also { println(it) }
        events.map { it.accept(SaveVisitor()) }.reduce { acc, s -> acc + s }.also { println(it) }
    }
}
```

### Mediator
The Mediator pattern is a design pattern that allows you to reduce the dependencies between objects 
by introducing a mediator object that handles the communication between them.

**Example**

```java
package nl.itzitzo.design.patterns.mediator

class EventUser(val mediator: EventMediator, val name: String) {
    fun send(message: String) {
        println("$name sends message: $message")
        mediator.notify(this, message)
    }

    fun receive(message: String) {
        println("$name receives message: $message")
    }

}

class EventMediator() {
    private val users = mutableListOf<EventUser>()

    fun addUser(user: EventUser) {
        users.add(user)
    }

    fun notify(sender: EventUser, message: String) {
        for (user in users) {
            if (user != sender) {
                user.receive(message)
            }
        }
    }
}

/**
 * The Mediator object demonstrates the usage of the Mediator pattern to facilitate communication between EventUser instances.
 * It creates an EventMediator and two EventUser instances, adds the users to the mediator, and then sends messages between the users.
 * The mediator handles the communication, ensuring that messages are sent to all users except the sender.
 */
object Mediator {
    @JvmStatic
    fun main(args: Array<String>) {
    EventMediator().apply {
        val user1 = EventUser(this, "User1")
        val user2 = EventUser(this, "User2")

        addUser(user1)
        addUser(user2)

        user1.send("Hello,  User2!")
        user2.send("World!")
    }

    }
}
```

### Memento
The Memento pattern is a design pattern that allows you to capture and externalize an object's internal state without violating encapsulation, 
so that the object can be restored to this state later (rollback).

**Example**

```java
package nl.itzitzo.design.patterns.memento

data class Event(val name: String, val data: String)

class EventCreator(var event: Event) {
        fun createMemento(): Event {
            return Event(event.name, event.data)
        }

        fun restore(memento: Event) {
            event = memento
        }
}

class EventHistory {
    private val history = mutableListOf<Event>()

    fun save(event: Event) {
        history.add(event)
    }

    fun restore(index: Int): Event? {
        return if (index in history.indices) history[index] else null
    }
}

object Memento {
    @JvmStatic
    fun main(args: Array<String>) {
        val eventCreator = EventCreator(Event("UserLoggedIn", "User with ID 123 has logged in."))
        val history = EventHistory()

        // Save the current state of the event
        history.save(eventCreator.createMemento())

        // Change the event
        eventCreator.event = Event("UserLoggedOut", "User with ID 123 has logged out.")

        println("Current event: ${eventCreator.event}")

        // Restore the previous state of the event
        val memento = history.restore(0)
        if (memento != null) {
            eventCreator.restore(memento)
            println("Restored event: ${eventCreator.event}")
        } else {
            println("No memento found at index 0")
        }
    }
}
```

## Creational Patterns
- [Builder](#builder)
- [Factory](#factory)
- [Singleton](#singleton)

### Builder
The Builder pattern is a design pattern that allows you to create complex objects step by step.

**Example**

```java
package patterns.creational.builder

data class Event(val name: String, val data: String)

class EventBuilder {
    private var name: String? = null
    private var data: String? = null

    fun name(name: String): EventBuilder {
        this.name = name
        return this
    }

    fun data(data: String): EventBuilder {
        this.data = data
        return this
    }

    fun build(): Event {
        return Event(
            requireNotNull(name) { "Event name must not be null" },
            requireNotNull(data) { "Event data must not be null" }
        )
    }
}

object Builder {
    @JvmStatic
    fun main(args: Array<String>) {
        val event = EventBuilder()
            .name("UserLoggedIn")
            .data("User with ID 123 has logged in.")
            .build()

        println(event)
    }
}
```

### Factory
The Factory pattern is a design pattern that allows you to create objects without specifying the exact class of object that will be created.

**Example**

```java
package patterns.creational.factory

sealed class Event(
    val name: String,
    val data: String,
)

object KafkaEvent : Event("KafkaEvent", "Data from Kafka")

object RabbitEvent : Event("RabbitEvent", "Data from RabbitMQ")

object SqsEvent : Event("SqsEvent", "Data from SQS")

data class EventContract(
    val name: String,
    val data: String,
)

class EventFactory {
    fun contractByType(event: Event): EventContract =
        when (event) {
            is KafkaEvent -> EventContract(KafkaEvent.name, KafkaEvent.data)
            is RabbitEvent -> EventContract(RabbitEvent.name, RabbitEvent.data)
            is SqsEvent -> EventContract(SqsEvent.name, SqsEvent.data)
        }
}

object Factory {
    @JvmStatic
    fun main(args: Array<String>) {
        val factory = EventFactory()

        val kafkaContract = factory.contractByType(KafkaEvent)
        println(kafkaContract)

        val rabbitContract = factory.contractByType(RabbitEvent)
        println(rabbitContract)

        val sqsContract = factory.contractByType(SqsEvent)
        println(sqsContract)
    }
}
```

### Singleton
The Singleton pattern is a design pattern that restricts the instantiation of a class to a single.

**Example**

```java
package patterns.creational.singleton

class EventSingleton {
    init {
        println("EventSingleton instance created: $this" )
    }
    fun log(msg: String) {
        println("Log msg: $msg for instance $this")
    }
}

object EventInstantiator {
    // default lazy initialization (SYNCHRONIZED), thread-safe and efficient
    val instance: EventSingleton by lazy { EventSingleton() }

    @JvmStatic
    fun main(args: Array<String>) {
        val singleton = instance
        singleton.log("log 1")
        singleton.log("log 2")
        singleton.log("log 3")
    }

}
```


Design Patterns in Kotlin


Kotlin/Java Software Engineer with many years of experience and a proven track-record in Java, Kotlin JVM languages [[...]](/aboutme/)

Soon I'll share some of my pet projects in this blog.
