Effective Kotlin: Item 25 — Limit source files to a single top-level class

Highlighted in item 25 of the fantastic Effective Java by Joshua Bloch is the inner workings of the javac compiler and how it silently compiles source files not mentioned on the command line but referenced by those that are. In the book’s example, the main class references a Utensil class and so calling javac Main.java looks for a Utensil.java file automatically which on executing outputs “pancake”. If instead, you use the command javac Dessert.java Main.java it will print “potpie”.

Although it appears as though the Kotlin compiler doesn’t automatically search for source files it will search the classpath if you ask it and so can still lead to undesirable behaviour. Consider the following three files:

// Main.kt
fun main(args: Array<String>) {
System.out.println(Utensil.NAME + Dessert.NAME)
// Utensil.kt
object Utensil {
const val NAME = "pan"
object Dessert {
const val NAME = "cake"
// Dessert.kt
Utensil {
const val NAME = "pot"
object Dessert {
const val NAME = "pie"

When you execute kotlinc Utensil.kt, the compiler will generate two top-level class files, Utensil.class and Dessert.class. When you then execute kotlinc Dessert.kt, the compiler will also generate two top-level class files overriding the ones from the previous command. Compiling Main.kt with kotlinc -include-runtime -d out.jar Main.kt and executing the resulting jar will print the output of whichever of Utensil.kt or Dessert.kt you compiled last.

Although the example may seem contrived, it still highlights the complexities of compilation and a good reason why you should “never put multiple top-level classes or interfaces in a single source file”.

Kotlin removes Java’s restriction around multiple public classes in a file, but I still believe it to be good practice to keep classes and interfaces separate. For one it makes finding definitions by browsing the source folders much easier without having to check-out the code base into an IDE and also better enforces the idea of single responsibility.

Each week I am looking at “items” from Joshua Bloch’s well-respected book, Effective Java to see how it applies to Kotlin. You can find the rest of the items I’ve covered at Effective Kotlin. Please let me know your thoughts.

Matt Dolan has been eating doughnuts and developing with Android since the dark days of v1.6.

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