Algunos ejemplos de como usar Kotlin de una mejor manera

apply.kt

    fun getDataSource(): MysqlDataSource {
        val fileInputStream = FileInputStream("myfile.properties")

        val properties = Properties()
        properties.load(fileInputStream)

        val mySqlDataSource = MysqlDataSource()?.apply {
            setURL(properties.getProperty("MYSQL_DB_URL"))
            user = properties.getProperty("MYSQL_DB_USERNAME")
            setPassword(properties.getProperty("MYSQL_DB_PASSWORD"))
            port = properties.getProperty("MYSQL_DB_PORT").toInt()
            databaseName = properties.getProperty("MYSQL_DB_NAME")
            maxQuerySizeToLog = properties.getProperty("MYSQL_DB_QUERYSIZE").toInt()
        }
        
        return mySqlDataSource
    }

    fun makeDir(path: String) = File(path).apply { mkdirs() }

defaultarguments.kt

    fun findPhoneNumber(number: String, locale: String = "MX"): String {
        TODO("Implementa tu búsqueda")
    }

extensionfunction.kt

fun String.countAmountOfX(): Int {
    return this.length - this.replace("x", "").length
}

let.kt

        db?.let {
            try {
                db.beginTransaction()
                comicValues.forEach { db.insert(ComicContract.ComicEntry.TABLE_NAME, null, it) }
                db.setTransactionSuccessful()
            } catch (e: SQLException) {
                Log.e(javaClass.simpleName, "Ha ocurrido un error durante la inserción.", e)
            } finally {
                db.endTransaction()
            }
        }

whenexpression.kt

fun getDefaultLocale(deliveryArea: String) = when (deliveryArea.toLowerCase()) {
    "mexico", "peru" -> MyLocale.SPANISH
    "brazil" -> MyLocale.PORTUGUESE
    "usa" -> MyLocale.ENGLISH
    else -> MyLocale.SPANISH
}