Kotlin Help

Add a data class to Spring Boot project

In this part of the tutorial, you'll add some more functionality to the application and discover more Kotlin language features, such as data classes. It requires changing the MessageController class to respond with a JSON document containing a collection of serialized objects.

Update your application

  1. In the DemoApplication.kt file, create a Message data class with two properties: id and text:

    data class Message(val id: String?, val text: String)

    Message class will be used for data transfer: a list of serialized Message objects will make up the JSON document that the controller is going to respond to the browser request.

    Data classes – data class Message

    The main purpose of data classes in Kotlin is to hold data. Such classes are marked with the data keyword, and some standard functionality and some utility functions are often mechanically derivable from the class structure.

    In this example, you declared Message as a data class as its main purpose is to store the data.

    val and var properties

    Properties in Kotlin classes can be declared either as:

    • mutable, using the var keyword

    • read-only, using the val keyword

    The Message class declares two properties using val keyword, the id and text. The compiler will automatically generate the getters for both of these properties. It will not be possible to reassign the values of these properties after an instance of the Message class is created.

    Nullable types – String?

    Kotlin provides built-in support for nullable types. In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that cannot (non-nullable references).
    For example, a regular variable of type String cannot hold null. To allow nulls, you can declare a variable as a nullable string by writing String?.

    The id property of the Message class is declared as a nullable type this time. Hence, it is possible to create an instance of Message class by passing null as a value for id:

    Message(null, "Hello!")
  2. In the same file, amend the index() function of a MessageController class to return a list of Message objects:

    @RestController class MessageController { @GetMapping("/") fun index() = listOf( Message("1", "Hello!"), Message("2", "Bonjour!"), Message("3", "Privet!"), ) }
    Collections – listOf()

    The Kotlin Standard Library provides implementations for basic collection types: sets, lists, and maps.
    A pair of interfaces represents each collection type:

    • A read-only interface that provides operations for accessing collection elements.

    • A mutable interface that extends the corresponding read-only interface with write operations: adding, removing, and updating its elements.

    The corresponding factory functions are also provided by the Kotlin Standard Library to create instances of such collections.

    In this tutorial, you use the listOf() function to create a list of Message objects. This is the factory function to create a read-only list of objects: you can't add or remove elements from the list.
    If it is required to perform write operations on the list, call the mutableListOf() function to create a mutable list instance.

    Trailing comma

    A trailing comma is a comma symbol after the last item of a series of elements:

    Message("3", "Privet!"),
    This is a convenient feature of Kotlin syntax and is entirely optional – your code will still work without them.

    In the example above, creating a list of Message objects includes the trailing comma after the last listOf() function argument.

The response from MessageController will now be a JSON document containing a collection of Message objects.

Here is a complete code of the DemoApplication.kt:

package com.example.demo import org.springframework.boot.autoconfigure.SpringBootApplication import org.springframework.boot.runApplication import org.springframework.data.annotation.Id import org.springframework.web.bind.annotation.GetMapping import org.springframework.web.bind.annotation.RestController @SpringBootApplication class DemoApplication fun main(args: Array<String>) { runApplication<DemoApplication>(*args) } @RestController class MessageController { @GetMapping("/") fun index() = listOf( Message("1", "Hello!"), Message("2", "Bonjour!"), Message("3", "Privet!"), ) } data class Message(val id: String?, val text: String)

Run the application

The Spring application is ready to run:

  1. Run the application again.

  2. Once the application starts, open the following URL:


    You will see a page with a collection of messages in JSON format:

    Run the application

Next step

In the next part of the tutorial, you'll add and configure a database to your project, and make HTTP requests.

Proceed to the next chapter

Last modified: 14 December 2023