Working With Files

Part of the Object Oriented Programming section of Coddy's Kotlin journey. Lesson 1 of 57.

Real programs are split into several files. All the .kt files of a Kotlin program are compiled together, so a function or class declared in one file can be used in any other file of the same package without an import:

// Greeting.kt
fun greeting(name: String) = "Hello, $name!"

// Main.kt
fun main() {
    println(greeting("Ada"))
}

Running this program prints Hello, Ada!: main in Main.kt calls the function that lives in Greeting.kt.

A Kotlin file holds declarations only: classes, functions, properties. Statements such as println(...) must be inside a function. The program starts at fun main(), which by convention lives in Main.kt:

// Greeting.kt
fun greeting(name: String) = "Hello, $name!"
// println("hi")            // error: statements must be inside a function

// Main.kt
fun main() {
    val name = readLine() ?: "friend"
    println(greeting(name))
}

The usual layout is one class per file, named after the class: Dog.kt holds Dog, Account.kt holds Account. In this section's challenges, your code goes in those files and Main.kt holds the supplied input/output code, which you usually cannot edit. The editor shows each file as a tab.

Dog.kt        // class Dog(...) { ... }
Account.kt    // class Account { ... }
Main.kt       // fun main(): reads the input and prints the results

Visibility follows files too. A top-level declaration marked private is visible only inside its own file, while the default, public, makes it visible in every file:

// Secret.kt
private const val SALT = "x7"        // only this file can see it
fun hashed(s: String) = s + SALT

// Main.kt
fun main() {
    println(hashed("pw"))            // fine: hashed is public
    // println(SALT)                 // error: SALT is private in Secret.kt
}

Challenge

Easy

The program has two files. main in Main.kt reads names and prints the result of greeting(name). Complete greeting in Greeting.kt so it returns Hello, Ada! for the name Ada, or Hello, friend! when the name is empty.

Your code goes in Greeting.kt. Main.kt holds the supplied input/output code and cannot be edited.

Try it yourself

fun main() {
    // Supplied input/output code: keep it as it is
    val input = generateSequence(::readLine).toList()
    for (name in input) println(greeting(name))
}