Visibility and encapsulation

A register should allow readers to see its attendance count while keeping changes behind an intentional method. Access control states where declarations may be used. It helps maintain an API contract rather than relying on every caller to avoid changing internal fields.

What you’ll learn

Expose reads while restricting writes.
Choose a module-appropriate visibility boundary.

Visibility and encapsulation in the example

1
Readable state
A register should allow readers to see its attendance count while keeping changes behind an intentional method.
```
Readable state
```
→2
Controlled operation
Kotlin public is the default visibility.
```
Controlled operation
```
→3
Restricted setter
The controlled method increments count to one.
```
Restricted setter
```

The controlled method increments count to one. A caller can read it but cannot directly assign a different count through the private setter. This allows update rules to be centralized inside the register.

Understand visibility and encapsulation

Kotlin public is the default visibility. Internal limits access to a module, private restricts an implementation boundary, and protected applies to suitable subclass access. A private setter can expose a property for reading while directing writes through a deliberate method.

Follow visibility and encapsulation

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

class Register {
  var count: Int = 0
    private set
  fun recordArrival() { count++ }
}
fun main() {
  val register = Register()
  register.recordArrival()
  println(register.count)
}

Expected output

Follow the execution

The controlled method increments count to one. A caller can read it but cannot directly assign a different count through the private setter. This allows update rules to be centralized inside the register.

Try this in the Kotlin compiler by pasting the complete program.

Practice: Visibility and encapsulation

Your task: Where should you enforce a maximum count?

Kotlin/JVM · complete program

class Register {
  var count: Int = 0
    private set
  fun recordArrival() { count++ }
}
fun main() {
  val register = Register()
  register.recordArrival()
  println(register.count)
}

Worked answer

In the operation that changes count, with an explicit result when the limit is reached. Restricted writes help keep callers from bypassing that policy.