A simple socket-server written in Kotlin

Server.kt

package dk.im2b

import java.io.OutputStream
import java.net.ServerSocket
import java.net.Socket
import java.nio.charset.Charset
import java.util.*
import kotlin.concurrent.thread


fun main(args: Array<String>) {
    val server = ServerSocket(9999)
    println("Server is running on port ${server.localPort}")

    while (true) {
        val client = server.accept()
        println("Client connected: ${client.inetAddress.hostAddress}")

        // Run client in it's own thread.
        thread { ClientHandler(client).run() }
    }

}

class ClientHandler(client: Socket) {
    private val client: Socket = client
    private val reader: Scanner = Scanner(client.getInputStream())
    private val writer: OutputStream = client.getOutputStream()
    private val calculator: Calculator = Calculator()
    private var running: Boolean = false

    fun run() {
        running = true
        // Welcome message
        write("Welcome to the server!\n" +
                "To Exit, write: 'EXIT'.\n" +
                "To use the calculator, input two numbers separated with a space and an operation in the ending\n" +
                "Example: 5 33 multi\n" +
                "Available operations: 'add', 'sub', 'div', 'multi'")

        while (running) {
            try {
                val text = reader.nextLine()
                if (text == "EXIT"){
                    shutdown()
                    continue
                }

                val values = text.split(' ')
                val result = calculator.calculate(values[0].toInt(), values[1].toInt(), values[2])
                write(result)
            } catch (ex: Exception) {
                // TODO: Implement exception handling
                shutdown()
            } finally {

            }

        }
    }

    private fun write(message: String) {
        writer.write((message + '\n').toByteArray(Charset.defaultCharset()))
    }

    private fun shutdown() {
        running = false
        client.close()
        println("${client.inetAddress.hostAddress} closed the connection")
    }

}

class Calculator {

    fun calculate(a: Int, b: Int, operation: String): String {
        when (operation) {
            "add" -> return calc(a, b, ::add).toString()
            "sub" -> return calc(a, b, ::sub).toString()
            "div" -> return calc(a.toDouble(), b.toDouble(), ::div).toString()
            "multi" -> return calc(a, b, ::multi).toString()
            else -> {
                return "Something whent wrong"
            }
        }
    }

    // A Calculator (functional programming)
    private fun <T> calc(a: T, b: T, operation: (T, T) -> T): T {
        return operation(a, b)
    }

    private fun add(a: Int, b: Int): Int = a + b
    private fun sub(a: Int, b: Int): Int = a - b
    private fun div(a: Double, b: Double): Double = a / b
    private fun multi(a: Int, b: Int): Int = a * b


}

tysir

HI! Do we really need to create copy of the socket connection?
...
class ClientHandler(client: Socket) {
-> private val client: Socket = client <-
private val reader: Scanner = Scanner(client.getInputStream())
...

@Airog this is a late reply, but that line doesn't copy anything.

In Kotlin, as in Java, C# and many other languages, variables contain references, not their actual value. Here, the line copies the reference, which is not its content. Copying a reference is virtually free (it's very light); the Socket object here is not modified nor copied.

This could've been written:

class ClientHandler(val client: Socket) {
...

and would've been exactly identical

could you please give a client-example as well? Thanks!

could you please give a client-example as well? Thanks!

Sorry for the late replay (That goes for everyone). I've made a simple Client.kt for you as well.
https://gist.github.com/Silverbaq/1fdaf8aee72b86b8c9e2bd47fd1976f4

First of all: Thanks, this helps me a lot.
Second: If you change Calculator into an object iso a class you don't have to initialize it (there is nothing initialized anyway in it's constructor)

tysir

meaning?

tysir

meaning?

Thank You Sir ? 🤷‍♂️

Thanks for this useful codes. its help me a lot and i developed a simple chat app for android devices . I'll be happy if you visit it
https://github.com/saeedrznr/Chat-application-client
https://github.com/saeedrznr/Chat-application-server