What is the access modifier 'internal' in Kotlin and how is it used?

The access modifier internal in Kotlin means that the declaration is visible within the same module. A module can be a set of Kotlin files compiled together, for example:

• Gradle module (source set).
• Maven module.
• Ant source set.

This is useful for hiding implementation details within a module, making them inaccessible to code from other modules.

internal can be used with:

• Classes
• Interfaces
• Objects
• Functions
• Properties
• Constructors

Example of usage:

// Module "module1"
package com.example.module1

internal class InternalClass {
    internal fun internalFunction() {
        // This code is visible only within module1
    }
}

// This object is also visible only within module1
internal object InternalObject {
    internal val internalProperty = "Internal value"
}

// Module "module1"
package com.example.module1

fun someFunctionInModule1() {
    val internalCls = InternalClass() // Accessible
    internalCls.internalFunction()     // Accessible
    println(InternalObject.internalProperty) // Accessible
}

// Module "module2"
package com.example.module2

import com.example.module1.InternalClass // Compilation error
import com.example.module1.InternalObject // Compilation error

fun someFunctionInModule2() {
    // Cannot create an instance of InternalClass or access InternalObject
    // val internalCls = InternalClass() // Not accessible
    // println(InternalObject.internalProperty) // Not accessible
}

The main difference from public (visible everywhere), private (visible only within the class), and protected (visible within the class and its subclasses) is precisely the level of visibility by modules.