Category: Elixir

In Kotlin, a sealed class or interface is a special type of class or interface that can only be extended or implemented by a specific set of subclasses or implementing classes that are defined in the same file. This is useful for creating restricted class hierarchies that provide better type safety and easier code maintenance.

Here’s an example of how you might use a sealed interface in Kotlin:

sealed interface Shape {
  fun area(): Double
}

data class Circle(val radius: Double): Shape {
  override fun area() = Math.PI * radius * radius
}

data class Rectangle(val width: Double, val height: Double): Shape {
  override fun area() = width * height
}

data class Triangle(val base: Double, val height: Double): Shape {
  override fun area() = base * height / 2
}

In this example, the Shape interface is sealed, so it can only be implemented by the Circle, Rectangle, and Triangle classes, which are defined in the same file. Any attempt to implement the Shape interface in another file will result in a compilation error.

You can then use the sealed interface and its implementing classes in the following way:

val shapes = listOf(Circle(10.0), Rectangle(5.0, 10.0), Triangle(5.0, 10.0))
val areas = shapes.map { it.area() }

This code creates a list of Shape objects, and then calculates the area of each shape using the area() method defined in the sealed interface.

Example Kotlin Sealed Interface and MySQL database

To use a sealed interface with MySQL database records in Kotlin, you can define the sealed interface and its implementing classes as described in the previous example, and then use a library like JDBC or MySQL Connector/J to query the database and retrieve the records.

Here’s an example of how you might do this using JDBC:

Add the necessary dependencies to your Kotlin project:

implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
implementation("org.jetbrains.kotlinx:kotlinx-coroutines-jdk8:1.4.2")
implementation("com.h2database:h2")
implementation("org.postgresql:postgresql")

Define the sealed interface and its implementing classes:

sealed interface Entity {
  val id: Int
}

data class User(override val id: Int, val name: String, val email: String): Entity
data class Order(override val id: Int, val product: String, val quantity: Int, val price: Double): Entity

Create a function to retrieve records from the database and map them to the appropriate implementing class:

suspend fun  query(sql: String, mapper: (ResultSet) -> T): List {
  val connection = DriverManager.getConnection("jdbc:h2:mem:test")
  connection.

Kotlin Sealed Interface with RestApi SpringBoot

To use a sealed interface with a Spring Boot REST API in Kotlin, you can define the sealed interface and its implementing classes as described in the previous example, and then create a Spring Boot application to expose the sealed interface and its implementing classes as REST endpoints.

Here’s an example of how you might do this:

Add the necessary dependencies to your Kotlin project:

dependencies {
  implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
  implementation("org.jetbrains.kotlinx:kotlinx-coroutines-jdk8:1.4.2")
  implementation("org.springframework.boot:spring-boot-starter-web")
  implementation("org.springframework.boot:spring-boot-starter-data-jpa")
  runtimeOnly("com.h2database:h2")
  runtimeOnly("org.postgresql:postgresql")
}

Define the sealed interface and its implementing classes:

sealed interface Entity {
  val id: Int
}

data class User(override val id: Int, val name: String, val email: String): Entity
data class Order(override val id: Int, val product: String, val quantity: Int, val price: Double): Entity

Create a repository interface for each of your implementing classes, using the Spring Data JPA library:

interface UserRepository : JpaRepository
interface OrderRepository : JpaRepository

Create a REST controller to expose the implementing classes as HTTP endpoints:

@RestController
@RequestMapping("/api")
class EntityController(private val

Kotlin Sealed Interface with Read File

To use a sealed interface with a file reading operation in Kotlin, you can define the sealed interface and its implementing classes as described in the previous example, and then use the Kotlin File and BufferedReader classes to read the file and map the contents to the appropriate implementing class.

Here’s an example of how you might do this:

Define the sealed interface and its implementing classes:

sealed interface Entity {
  val id: Int
}

data class User(override val id: Int, val name: String, val email: String): Entity
data class Order(override val id: Int, val product: String, val quantity: Int, val price: Double): Entity

Create a function to read the file and map its contents to the appropriate implementing class:

fun readFile(file: File): List {
  val entities = mutableListOf()
  val reader = BufferedReader(FileReader(file))
  try {
    var line = reader.readLine()
    while (line != null) {
      val parts = line.split(",")
      when (parts[0]) {
        "user" -> entities += User(parts[1].toInt(), parts[2], parts[3])
        "order" -> entities += Order(parts[1].toInt(), parts[2], parts[3].toInt(), parts[4].toDouble())
      }
      line = reader.readLine()
    }
  } finally {
    reader.close()
  }
  return entities
}

Use the function to read the file and print the contents:

val file = File("data.txt")
val entities = readFile(file)
for (entity in entities) {
  println(entity)
}

This code reads a file called data.txt, which is expected to contain rows of data in the following format:

user,1,Alice,alice@example.com
order,1,Product A,5,19.99
order,2,Product B,10,29.99

The function reads each line of the file, splits it into parts using the comma delimiter, and maps the contents to the appropriate implementing class using the when expression. The resulting entities list will contain a mixture of User and Order objects, depending on the data in the file.

I hope this example helps clarify how to use a sealed interface with a file reading operation in Kotlin. Let me know if you have any questions or need further assistance.