internal

Learn module visibility with internal.

internal: Module visibility

The internal visibility modifier states that a definition is visible only inside the same module. A module is a Gradle or Maven unit that is compiled together. public is reachable from anywhere, internal only from inside the module.

internal class Helper {
    fun assist() = "help"
}

Here the Helper class can be used only within its own module; it cannot be reached from another module. This is useful to hide the internal implementation of a library from its users. private works inside a class, protected in subclasses; internal works at the module level.

The module concept

A module is a unit of code that is compiled together; usually a Gradle subproject. A project can have several modules: an app module, a library module, a test module and so on. internal definitions are visible to all files in the same module; they are invisible to other modules.

// module-a: Helper internal
class Api {
    internal fun helper() = "secret"
}

Here the Api class is public but the helper method is internal. So Api can be used from another module but helper cannot. This is useful to narrow the surface of an API; external users see only the public parts.

Exercise Define an internal class and use it

Define internal class Helper; inside it add fun greet(name: String): String = "Hello, $name!". In main create an instance with val h = Helper(); print the result with println(h.greet("Alex")).

Show example solution

internal class Helper {
    fun greet(name: String): String = "Hello, $name!"
}

fun main() {
    val h = Helper()
    println(h.greet("Alex"))
}

Test yourself

Where does internal visibility apply?
- Only in the same module
- Only in the same package
- Only in the same file
- Everywhere
Show answer

Only in the same module — internal restricts a definition to the same module.
Match the visibility modifiers

public

Accessible everywhere
internal

Same module
private

Same class
protected

Subclasses
Restrict a class to the module
```
____ class Helper
```
Show answer
```
internal class Helper
```

Next lesson: Gradle

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